extending human presence into the solar system human presence into the solar system an independent...

Extending Human Presence

into the Solar SystemAn Independent Study for The Planetary Society on

Strategy for the Proposed U.S. Space Exploration Policy

July 2004

The Planetary Society**65 N. Catalina Avenue, Pasadena, CA 91106-2301**(626) 793-5100** Fax (626) 793-5528**E-mail: [email protected]** Web: http://planetary.org

Study Team

William Claybaugh

Owen K. Garriott (co-Team Leader)

John Garvey

Michael Griffin (co-Team Leader)

Thomas D. Jones

Charles Kohlhase

Bruce McCandless II

William O’Neil

Paul A. Penzo


Table of Contents

Study Team 2Executive Summary 4Overview of Exploration Plan 5Introduction 6Approach to Human Space Flight Program Design 9

Destinations for the Space Exploration Enterprise 9International Cooperation 13

1. Roles 132. Dependence on International Partners 143. Regulatory Concerns 15

Safety and Exploration Beyond LEO 15The Shuttle and the International Space Station 17

Attributes of the Shuttle 17ISS Status and Utility 18

Launch Vehicle Options 18U.S. Expendable Launch Vehicles 19Foreign Launch Vehicles 20Shuttle-Derived Vehicles 21New Heavy-Lift Launcher 21Conclusions and Recommendations 22

Steps and Stages 22Departing Low Earth Orbit 22Electric Propulsion 24Nuclear Thermal Propulsion 25Interplanetary Cruise 27

Human Factors 27Gravitational Acceleration 27Radiation 28Social and Psychological Factors 28System Design Implications 29

The Cost of Going to Mars 30Development Costs 30Production Costs 30First Mission Cost 31Subsequent Mission Cost 31Total 30-Year Cost 31Sensitivity Analysis 31Cost Summary 32

Policy Implications and Recommendations for Shuttle Retirement 32Overview, Significant Issues, and Recommended Studies 33References 35


Executive Summary(modified 08/17/04)

We propose here a staged approach tohuman exploration beyond low Earth orbit(LEO). We believe such a plan must beadopted if the overall funding profile is to bekept within the bounds that are likely to beacceptable to the many future Congressesand Administrations that must “sign on” tothe Exploration Initiative if it is to succeed.

Stage 1 features the development of anew crew exploration vehicle (CEV), thecompletion of the International SpaceStation (ISS), and an early retirement of theShuttle Orbiter. Orbiter retirement would bemade as soon as the ISS U.S. Core iscompleted (perhaps only 6 or 7 flights) andthe smallest number of additional flightsnecessary to satisfy our internationalpartners’ ISS requirements. Money saved byearly Orbiter retirement would be used toaccelerate the CEV development schedule tominimize or eliminate any hiatus in U.S.capability to reach and return from LEO.

Stage 2 requires the development ofadditional assets, including an uprated CEVcapable of extended missions of manymonths in interplanetary space. Habitation,laboratory, consumables, and propulsionmodules, to enable human flight to thevicinities of the Moon and Mars, theLagrange points, and certain near-Earthasteroids. Development of human-ratedplanetary landers is completed in Stage 3,allowing human missions to the surface ofthe Moon and Mars beginning around 2020.The overall plan is summarized in Table 1.

A key to this vision is the requirement tocomplete assembly of the ISS and to retirethe Shuttle Orbiter, without in the processincurring another lengthy hiatus in theability of the United States to conductcrewed spaceflight operations. To this end,

we recommend phased development of thenew CEV, with the “Block 1” versiondesigned for LEO access and return only,with a later “Block 2” version suited to therequirements of interplanetary missions. TheCEV would be launched on a new human-rated vehicle, possibly based on the existingShuttle solid rocket motor (SRM),augmented with a new liquid upper stage.Such a system could be available before2010. With Orbiter retired after U.S. Corecomplete and with international agreementto proceed, any remaining assembly taskscan be completed by the heavy-lift launchvehicle (HLLV) that must be developed tosupport later stages of the ExplorationInitiative, by use of expendable launchvehicles (EELVs) as appropriate, or onsuitable international vehicles such asAriane or Proton.

Stages 2 and 3 of the proposedExploration architecture will require heavy-lift launch capability well in excess of the20–25 metric ton capacity of the presentevolved EELV fleet. We believe theserequirements can best be met, at leastinitially, by means of designs that utilizeexisting Space Shuttle components (e.g., theSRM and External Tank). Some proposedShuttle-derived HLLVs have a payloadcapacity in excess of 100 metric tons andoffer a near-term approach to meetingExploration requirements with a minimumof non-recurring investment.

Prompt studies to confirm ourrecommendations are needed in areas ofearly CEV design for Block 1 capability toand from LEO, to establish the minimumnumber of Shuttle flights necessary to meetinternational requirements, to find the bestlaunch vehicle for the CEV, and to performtrade studies for HLLV needs andconfiguration.


Overview of Exploration Plan

Stage One, access to LEO, through 2010• Shuttle-Orbiter return to flight (RTF), complete the ISS through at least “US CoreComplete”

• Select and demonstrate launch vehicle for CEV

• Demonstrate early CEV use for crew transfer at the ISS

• Negotiate with international partners to obtain best way to transport remaining

heavy modules to the ISS

• Retire Orbiter as soon as above steps are completed

• Costs distributed across full Exploration window

Stage Two, interplanetary cruise, through 2015 and beyond• Develop interplanetary cruise capability; uprated CEV, and necessary additionalmodules for the destination selected

• Ensure HLLV available, probably a Shuttle-derived HLLV

• Enable lunar orbit missions, remote sensing, Rovers with sample return

• Enable visits to Sun-Earth-Lagrange #2, astronomy, etc.

• Enable visit and study of near-earth objects (NEOs)

• Enable visits to Mars vicinity, including moons Phobos and Deimos. Includeremote sensors and Rover with return samples. Begin infrastructure placement.Select sites.

• Select destinations as appropriate: science, public, other interests

Stage Three, human surface landings, 2020 and beyond• Prepare infrastructure for moon and/or Mars bases

• Build on thorough preparation in preceding stages

• Initiate human landings at selected destinations

• Plan for future solar system exploration


Introduction

The recent Presidential Directive to focusNASA’s future on exploration via "theMoon and on to Mars" has invigorated thespace community and many of the generalpublic. Meeting these goals while remainingwithin realistic funding expectations isforeseen as the major difficulty in meetingthis challenge. The Planetary Society hascommissioned this Report to encouragesupport for this new venture and to suggest aworkable strategy for human exploration ofthe solar system, with the specific goal ofplacing humans on the Martian surface atthe earliest possible moment, while allowingcosts to be managed at reasonable levels.

It will be suggested below that theexploration can be conducted in three stages.Stage 1 is the early development of a newCrew Exploration Vehicle (CEV), as thePresident has directed, accompanied by thedevelopment of a launch vehicle to transportthe CEV to and from low Earth orbit (LEO).Success with the CEV will lead to missionsbeyond LEO. We are recommending thatstrong consideration be given to a specificdesign using the Shuttle solid rocket motor(SRM), together with a new liquidpropellant upper stage, for this role. Webelieve that this evolutionary developmentwill be the quickest and least expensive pathto realizing a U.S. capability to send humansto LEO, and beyond, without the use of theShuttle Orbiter. This capability should beavailable well before 2010, the date bywhich the Orbiter is to be retired. By thistime, the International Space Station (ISS)should have reached at least the “U.S. CoreComplete” stage, and NASA should havereached an agreement with our internationalpartners about how best to complete ourobligations to them.

The CEV should be designed explicitlyto have sufficient on-orbit life that it can beresident at the ISS for extended periods, thus

providing the emergency crew returncapability that, at present, is available onlyvia the Russian Soyuz spacecraft. The long-duration requirement is an obvious necessityin an exploration vehicle. In addition, thecrew-return vehicle (CRV) function requiresthat it be capable of remaining stable andquiescent, with minimal power drain, forlong periods.

We believe that there are significantadvantages, both for the United States andfor the ISS partners, associated withdeveloping the new LEO transportationcapability as early as possible. All partnerswould benefit from an earlier beginning ofthe benefits of having larger multinationalcrews on the ISS. The remaining heavymodules for the ISS might be bettertransported to the ISS by means of a Shuttle-derived HLLV, to be discussed below. Itshould again be emphasized that theproposed early development of a new LEOtransport system is intended to achieveearlier and more frequent access to the ISSfor all partners. The Orbiter would then beretired promptly to save the high costs ofmaintaining Orbiter operations, with the costsavings making funds available for Stages 2and 3. The ISS can be used not only by ourinternational partners but also by U.S. crewsfor tasks associated with solar systemexploration, including qualifying personnelfor long-duration missions and for studyingsocial-psychological interaction withinlarger crews. Remaining cargo can bedelivered to the ISS by other vehicles,including non-U.S. launchers.

A key Space Shuttle capability thatcannot be provided in a CEV designed for


voyages beyond LEO, or by the RussianSoyuz or Progress systems, is the so-calleddowncargo capacity of the Shuttle. TheShuttle’s large cargo bay enables the returnfrom the ISS of rack-sized experiments,samples, and equipment needing to beanalyzed, refurbished, or upgraded on theground. (Shuttle downcargo typically alsoincludes crew laundry, other recyclableitems, and various waste and trash, but suchuse is largely opportunistic rather thanreflective of a fundamental need.) However,these items, while valuable, need not bereturned in a vehicle designed to meethuman-rating safety standards. It should bepossible, indeed relatively easy, to design anautomated semiballistic vehicle, possiblyexpendable, for the purpose of ferryingstandard ISS rack-sized cargo up to the ISSand returning other items safely to Earth.The basic technology was first proven in theCorona program, during which literallyhundreds of film-carrying capsules werereturned to Earth from reconnaissancesatellites. It may be noted further that therecently cancelled Alternate Access to Spaceprogram could also accommodatesubstantial downmass capability.

Stage 2 initiates human exploration ofthe solar system, with a variety ofdestinations including “near Earth objects”(NEOs) such as asteroids; certain uniquegravitational locations such as the Sun-EarthLagrange points, which are of specialinterest to astronomers; and the vicinities ofthe Moon and Mars. The lunar and Marsreconnaissance missions would beanalogous to the Apollo 8 and 10 missionsto the Moon more than 30 years ago, butthey would involve extensive robotic andremote sensing activity, controlled eitherfrom a manned laboratory module or fromthe Earth, whichever is found to be mostappropriate. The eventual sequence of visits

can be decided later, depending on thepublic interest in and scientific importanceof each step.

Stage 2 requires the development of aninterplanetary cruise vehicle configurationthat must include at least an extended-duration CEV or an appropriate derivativevehicle, in addition to a modest laboratoryfor surface robot control, returned sampleanalysis, and physiological experiments. Ahabitation module also is required. Thesemight be derivatives of the current ISSlaboratory and habitation module designs.Such commonality is, however, limited bythe fact that considerable differences willexist between the requirements for use onthe ISS and those for interplanetarymissions, including the need for additionalradiation shielding, upgraded avionics, andlonger-duration life support. Consumablescarriers for propellant and other crewexpendables also will be required. Human-robotic synergism is expected to play anessential role in the scientific, engineering,and new technology aspects of emerginghuman exploration of the solar system. Assuch, it is expected to become an importantcomponent of our Stage 2 program. Finally,there will be a clear need for a heavy-liftvehicle to transport these large items to theselected staging point, whether in LEO (as islikely for early missions) or at a Lagrangepoint (which may be advantageous in thelonger term).

We believe that the most suitable andleast expensive heavy-lift option, at least inthe near term, will be an unmanned Shuttle-derived heavy-lift launch vehicle (HLLV).Numerous design configurations for suchvehicles have been proposed, offeringpayload capability in the range of many tensto more than one hundred metric tons [1].Competing options include the use of heavy-lift versions of the Atlas V or Delta IV


vehicles, having payload capacity in the 20-25 metric ton range, and a variety of new,“clean sheet of paper” designs. Although wehave made a specific recommendation, therelative merits of the various options shouldbe confirmed in trade studies, with dueattention to the fact that for some options,mission-enabling hardware of provenreliability already exists. Depending on theretirement date finally chosen for the ShuttleOrbiter, the first use of the HLLV could wellbe in connection with completion of the ISS.

Implementation of Stage 2 should permitvisits to any of the destinations above by2015. Note that human landings on theMoon or Mars are not included in Stage 2,although landings on the Martian moons(Phobos or Deimos) could be made, as theyhave negligible gravitational attraction andno atmosphere. This arguably will be bothsafer and more cost-effective, early on, thangoing directly to the planetary surfaces, ashuman landing and ascent vehicles wouldnot be required.

In Stage 3, the development of humanlanders for the Moon and Mars is completed.It is conceptually attractive to envision usingthe same basic lander design for both planetsand, indeed, such commonality should bepursued where possible. However, there areseveral basic differences in the requirementsthat must be imposed upon the two systems,and for which the final design must account.The lunar lander requires a descent !v of

approximately 1.85 km/s from low lunarorbit; in the absence of an atmosphere, theascent requirement to the same orbit will besimilar, for a total !v of approximately 3.7

km/s. A Mars lander must employ anaeroshell for the entry phase, and the descentpropulsive !v will be considerably less than

for a lunar landing, while the ascent !v

alone will be of the order of 4 km/s. Furtherdesign differences will arise if in situ

Martian resources are used to provide fuelfor the ascent phase. It is likely that a landerdesigned for Mars will be an “over-design”for use at the Moon. However, the ascentstage of a Mars lander might, by itself, serveas a single-stage, fully reusable lunar lander.It may also be desirable to flight test theMars vehicle at the Moon.

The interplanetary cruise configurationneeded in Stage 3 is largely identical to thatused in Stage 2, with the addition of thelander. It is believed that by phasing the newdesigns across three stages of activity, costscan be more uniformly distributed across thefifteen-year development period. We believethat human landings on the Moon or onMars can begin about 2020.


Approach to Human Space

Flight Program Design

Destinations for the SpaceExploration EnterpriseWe believe that it is the destiny ofhumankind to explore deep spacepersonally. It is not a question of “if” butrather one of “when,” “where,” and “how.”The debate as to whether robotic spacecraftalone can adequately and more effectivelyexplore space for scientific purposes willcontinue, but it is essentially beside thepoint. Whether humans should travel andexplore is very much a societal rather than ascientific question and, historically,whenever the question has been asked, asignificant fraction of humankind hasanswered “yes.” Prior generations havethrived by exploring beyond their knownboundaries; we are all the descendants ofsuccessful past explorers. So it will be withthe reach of humanity into deep space.

“When” is now. We have madeshamefully little progress in explorationbeyond LEO in the decades since Apollo.Thirty-five years ago, men walked on theMoon and returned safely to Earth. After afew missions, that marvelous capability wasabandoned, and it no longer exists.

Apollo was very much an instrument ofthe Cold War, a peaceful solution to theproblem of how the United States couldcompete successfully with the Soviet Unionfor influence in the world. Apollo was thusenabled by particular world circumstancesthat no longer exist. No other problem ofsimilar scope facing America today isperceived to require a new space enterprisefor its solution, and an Apollo-like effort istherefore deemed irrelevant andunaffordable in terms of solving a knownproblem.

The United States, however, surely willcontinue to support a program of mannedspaceflight. To abandon the capability to puthumans in space, when other nations can,would be to consign America to the secondrank of nations, a clearly unacceptableposition. What is needed today is a steadilyprogressive, regular, and affordable programto enable the “where” and “how” to whichwe have referred. Significant new goals anddestinations must be reached on a regularbasis, but the political support necessary tosustain a “crash” program like Apollocannot expected.

The ultimate “where” for the 21stcentury is Mars. The human destiny isclearly to explore our most Earth-likeneighbor planet, and perhaps one day tocolonize it. But a huge program with Marsas the immediate and only target is neithertechnically wise nor politically sustainableat present. A stepped strategy similar to thatdeveloped by the International Academy ofAstronautics, “The Next Steps in ExploringDeep Space” [2], provides a very attractivefoundation for the “how” of initiating aprogram of human space exploration today.

What is needed to sustain funding andpublic support for human activities in deepspace is an exploration strategy with a seriesof intermediate destinations that are publiclyexciting and scientifically rewarding, andthat incrementally build the capability tosend humans to Mars. In this report, weprovide one possible approach to the designof just such a program.

Two particular destinations in near-Earthspace could serve as useful and interestingfirst steps toward exploring deep space. Theclosest is the Moon, which many consider tobe the best first destination for a humanspace exploration enterprise eventuallyleading to Mars. Numerous in situ scientificinvestigations remain to be performed on the


Moon in order to add to our understandingof the origin and evolution of the Earth, andmany have argued that these investigationswarrant the presence of human intelligence.Although the Moon lacks an atmosphere andhas only half the surface gravitationalacceleration of Mars, it may nonethelessoffer numerous advantages as a “testingground” for human missions to Mars,lessening the steepness of the learning curvefor future Mars expeditions.

Scientific interest in returning to theMoon remains high, and the Moon haspotential utility as a stepping-stone for theexploration of Mars. In addition, there willbe great interest in the Moon amongspacefaring nations that have not yet senthumans there—both for national prestigeand as a confidence-building step beforeparticipating in an international Marsexpedition. Europe presently has a lunarrobotic mission, and Japan, China, and Indiaare all developing their own such missions.The Moon remains a valid destination in itsown right, and any transportationarchitecture should be designed with this inmind.

A second useful destination in near-Earth space is approximately four times asfar away as the Moon, near the very edge ofEarth’s gravitational field—the Sun-EarthLagrange Point 2 (SEL2), located 1.5million kilometers from Earth in the anti-solar direction. The Lagrange points are fivelocations in space where an object can residein equilibrium between the gravitationalattractions of the Sun and Earth and thecentripetal acceleration due to its revolutionaround the Sun. Small, periodicstationkeeping maneuvers (a few meters persecond per year, minuscule by deep spacestandards) are required to remain at any ofthe Lagrange points.

SEL2 is an excellent location for thelarge space telescopes of the future becauseit has an unobstructed view of the universewithout interference from planets such asEarth or the Moon, offers a benign thermalenvironment without dust, provides aweightless environment for large mirrors,and encompasses a vast expanse fordistributed apertures. Travel to SEL2 isenergetically easier than landing on, or evenorbiting, the Moon. Althoughmathematically SEL2 is a “point” in space,for practical purposes it is a region largeenough to accommodate countless humanand robotic spacecraft.

Future space telescopes are planned tooperate at SEL2, including the successor tothe Hubble Space Telescope (HST), theJames Webb Space Telescope. There will bea continuing scientific impetus and publicinterest in advanced telescopes that cansearch for, study, and image Earth-likeplanets around nearby stars, as well assearching for evidence of extraterrestrialintelligent life. Any exploration architecturemust recognize the public ownership andsupport of this objective, because ultimatelythese telescopes will require servicing just asthe HST has.

The tremendously successful,scientifically productive HST has taught usan early lesson in the importance of humanservicing of these assets once they are inspace. Without the human servicing missionto correct the optics of HST, it would havebeen a disastrous failure. Withoutsubsequent missions to replace ailingsubsystems and to upgrade its instrumentcomplement, HST would not have beenremotely as productive as it has been. In theInternational Academy of Astronautics(IAA) architecture, regular servicing ofSEL2 assets is one element in the logic ofplacing the interplanetary staging node at


SEL2; the appropriate mix of human vs.robotic servicing activity is one of manyissues remaining to be addressed.

As an aside, we note that the tremendousoutcry over NASA’s decision to terminatemanned Hubble servicing missionsfollowing the loss of Columbia offers someperspective on the value placed by thepublic on world-class astronomy. Becauseeven higher-quality astronomy can beenabled from SEL2, we have someconfidence in the value, from the public’sperspective, of SEL2 as a significantdestination for the exploration program.

SEL2 is also an excellent point fromwhich to stage missions beyond Earth’sgravitational field. Such a staging node is ofno value for a single planetary expedition orfor an architecture built primarily aroundexpendable mission hardware. Assuming,however, that an interplanetary vehiclestationed at SEL2 is reused for many trips tomultiple destinations, the energy savingsachieved through the use of such a stagingnode are significant. The vehicle need belifted to this point on the edge of the Earth’sgravity field only once, and fuel andsupplies can be ferried robotically on slowerbut more energy-efficient trajectories,possibly using electric propulsion (EP)vehicles. In such an architecture, servicingof this vehicle and other SEL2 assetsbecomes a routine operation. Astronautsassembling and servicing these assets atSEL2 will be, at the same time, developingthe capability to live and work in much thesame environment as for the journey toMars.

Beyond SEL2, the next possibledestination short of Mars itself would be oneof the near-Earth asteroids. To rendezvouswith a near-Earth object (NEO) and returnwould require substantially less than a year,a somewhat less ambitious goal than a

probable three-year round trip mission toMars. Once astronauts are successfullycommuting to SEL2 and working there, theadditional capability required to visit anasteroid is essentially the ability to travel inspace for several months without logisticalsupport from Earth. A larger vehicle withmore onboard life-support resources isneeded. However, to “walk” on an asteroidand return samples requires little more thantraditional extravehicular activity (EVA)equipment for Earth orbit applications, sincethe surface gravity of the asteroid will benegligible. NEOs are of great interest for avariety of reasons, including the threat theypresent to Earth and their potential as asource of raw materials. They are alsoimportant scientifically because they areprimordial objects, essentially unchangedsince the formation of the solar system, andare thus likely to hold clues to the origins ofhumanity. A near-Earth asteroid missionwould have considerable public appeal as anexciting and potentially engaging popularadventure, and it is a potentially idealintermediate step to reaching Mars. Suchmissions can be accomplished with a total!v of as little as 4-5 km/s, less even than for

a lunar landing.Once humans have visited an asteroid,

the next step could be to orbit Mars. Toreach Mars orbit and return safely willrequire more support cargo than needed foran asteroid mission. Thus, specialized cargovehicles must be developed and utilized forthis step. Because the cargo can be sent wellahead of the crew, lengthy trip times will notmatter, allowing the use of more efficienttrajectories and low-thrust propulsionsystems that would be unsuited to humanmissions. This capability is not needed earlyin the human exploration enterprise andtherefore can be developed later and over a


longer period of time, thus requiring laterand more modest year-to-year funding.

A Mars orbit mission would provide theexperience of operations in Mars proximitywithout the challenge of safely descendingto the surface and ascending from it torendezvous with an Earth return vehicle,exactly the role of the Apollo 8 and 10 lunarorbital missions in their time. From Marsorbit, humans can command robotic vehicleson the surface in real time, a major steptoward in situ human intelligence operatingon the surface of Mars, in contrast with theoperational impedance imposed by the 10-to 20-minute round trip speed-of-light delayfor current robotic Mars missions.Investigating Mars from Mars orbit in thismanner is functionally much closer tohaving human capability on Mars’ surfacethan it is to the current Earth-based human-operated robotic missions.

To achieve orbit about Mars and todepart for Earth from such an orbit is easierand safer than an excursion to the surface. Amore ambitious alternative would be to landand operate on the surface of one of the twoMartian moons, Deimos or Phobos. Someminor additional propulsive capabilitywould be required, but it is negligible incomparison to that needed to land on andreturn from the Martian surface. Also, noatmospheric entry systems are required.Operating from a Martian moon is indeedanalogous to Apollo missions operating onour own moon’s surface, but without thelarge propulsive requirement and dangerousascent required to climb out of our moon’spotential well.

Once astronauts have workedsuccessfully in orbit around Mars or on thesurface of Deimos or Phobos, they will haveacquired the best possible prerequisites forthe final step to the Mars surface. Thehuman entry, descent, and landing (EDL)

vehicles and the Mars Ascent Vehicles(MAVs) would be developed in parallel tothe ongoing human Mars orbit or Marsmoon missions. These vehicles could bedelivered to Mars’ vicinity robotically, wellin advance of human missions to the surface.They would be placed at the staging pointfor human missions to the surface, either inorbit about Mars or on a Martian moon.

If desired, it would be possible toconduct one or more telerobotic test flightsof the descent and ascent vehicles beforeusing them to go to the surface and returnsafely. Most of the surface excursionhardware should be reusable, except(possibly) for heat shields and, if used,parachutes. Thus, the various landing craftshould remain at Mars, as they do not needto be returned to Earth. Refueling would beaccomplished either from propellantdelivered from Earth as cargo or from in situ

production by pre-emplaced processingplants delivered as cargo.

It is worth noting that once the landingphase has been initiated at either the Moonor Mars, the use of surface facilities and pre-emplaced assets as an “abort” optionbecomes viable. With the inherentadvantages provided by gravity and materialfor radiation shielding, as well as redundantsources of nuclear power and tools, optionsfor surface survival may in some cases bebetter than for an escape to Earth.

We have proposed an exploration

architecture that places humans in lunar

and Mars orbit substantially before

surface landings are contemplated. We

recognize that there will be disagreement

over whether to conduct orbital missions

at the Moon or Mars prior to landing,

and for that matter whether to return to

the Moon prior to initiating an expedition

to Mars. In our view, these questions need

not be answered at present. When


answers are made, they will depend as

much on the background and perspective

of those deciding as upon any specific

technical criteria. It is our intent here to

point out the destinations that have been

“enabled” in each Stage of the proposed

architecture, deferring for now any

consideration as to which will be pursued,

and when.

We have outlined a step-by-step plan forprogressive human exploration andexploitation of four destinations—the Moon,SEL2, near-Earth asteroids, and Mars itselfeach of great scientific importance and eachrequiring only one major new advance incapability beyond the prior destination.Following this plan, we can make steadyprogress toward placing humans on thesurface of Mars, at reasonable costs, whilemaintaining an exciting human enterprise inspace. The proposed destinations arejustified by their public interest, scientificmerit, and exploration value, and theyprovide for steady, measurable, and timelyprogress in a logical manner toward theultimate goal of Mars exploration. In thissense, the proposed architecture is analogousto the progression from Mercury to Geminito the early Apollo missions in buildingtoward the final outcome of Moon landingon July 20, 1969.

International CooperationThe authority to conduct international spaceactivities is granted to NASA under Section205 of the National Aeronautics and SpaceAct of 1958. Since its founding, NASA hasengaged in thousands of cooperativeprojects with foreign nations ranging fromtraining and experimentation to theconstruction of the International SpaceStation. Further such cooperation throughoutthe full range of previous activities appears alikely feature of any future Lunar-Mars

program and is specifically mandated in theNational Space Policy Directive for Moon-to-Mars.

1. RolesEuropean interest in “‘long-term’” roboticand human exploration of solar systembodies” is evidenced by the initiation in2001 of the Aurora program, which targets apotential human presence on Mars by 2025[3]. Potential roles for Europe as a whole,via the European Space Agency (ESA) andwith individual European countries, mightinclude launch; the development,construction, and operation of both humanand robotic spacecraft; the provision ofinstruments for U.S. spacecraft (or viceversa); and the provision of crew members.This latter point is, of course, the primaryincentive for any potential partner toparticipate in the overall enterprise. Thelong history of NASA cooperation with ESAand European countries suggests that furthersuch cooperation is potentially available.

Like Europe, Japan has a long history ofcooperation with the United States,beginning before the local development ofthe N-1 launch vehicle based on Americantechnology. Japanese interest in lunarexploration is evidenced by previous andplanned probes to the Moon, although theJapanese long-range plan developed severalyears ago specifies a robotic lunar base as agoal and does not address human activitiesbeyond Earth orbit. The Japanese arepotentially capable of providing cooperativedevelopment of spacecraft and instruments,as well as providing launch capacity andcrew members.

Russia has a long history of interest inhuman lunar and Mars exploration, a historythat actually predates the beginning of theSpace Age. This interest continues to thisday with active study of human Mars


missions in the context of an internationalprogram and development of an unmannedPhobos Sample Return project. Russia’sability to invest resources in space activitieshas, however, been significantly curtailedsince the collapse of the Soviet Union.Nonetheless, as recently as 12 April 2004(Cosmonautics Day), Russian PresidentPutin said, “everyone in the leadership of thecountry understands that space activities fallinto the category of the most importantthings” [4].

The Russian role in space explorationmay be circumscribed by the country’spresent financial circumstances.Nonetheless, Russian rocket engines areamong the world’s best in terms of price andperformance, and the use of such engines inthe Atlas V vehicle family provides a basisfor suggesting that any “heavy-lift”capability required in the future could quitelikely benefit from the use of Russianengines or engine technology. Russia hasprovided consistently reliable human spacetransportation since the beginning of theSpace Age; as this is written, Russianvehicles offer the only operational means ofhuman space transportation to the ISS. Theirarchitectural approaches are very differentfrom those of the U.S., featuring long-termuse of working systems and robotic testingof human systems. Such approaches may berelevant for international human Moon andMars mission planning. Russia also hasconsiderable experience with extendedduration human space missions and muchmore recent experience than the UnitedStates with nuclear rockets and with the useof nuclear power in space.

China is, after the United States andRussia, the third country to have developedan indigenous human space flight capability.At present, the Chinese capability is limitedboth by lift capacity and by the relative

immaturity of their technology, which has sofar achieved only one human space flight.The Chinese have, however, indicated thatthey hope to develop a Mir-like spacestation by 2010 and plan to launch roboticlunar probes in the same time frame; thislatter endeavor is potentially cooperativewith U.S. goals.

Leaving aside the issue of potentialroles, a central concern with regard tointernational cooperation on a program thatwill last decades is whether potentialpartners are even interested. In part, thismay hinge on how the ISS partners view theeventual outcome of that project. Thus, aseemingly successful outcome of that effortmay affect significantly both the willingnessand the ability of other nations to join a U.S.Moon-Mars program. If the continuing costof the ISS is a significant burden to othercountries, that cost alone may mitigateagainst participation in further humanexploration of the solar system.

We emphasize here that, in our view,successful completion of the ISS to meetinternational partner commitments isrequired. We are proposing—subject topartner agreement—an alternative means ofmeeting these commitments, based on earlyCEV development and Shuttle retirementand on the use of other launch assets todeliver the international modules to orbit.We believe such a scheme offers the severaladvantages of reducing cost, enhancing crewsize, and providing earlier partner access tothe ISS.

2. Dependence on International

PartnersNASA traditionally has organizedinternational cooperative programs so as toavoid having any partner on the “criticalpath”; that is, the foreign contribution wasadditive or complementary to the core U.S.


effort. Under direction from the NASAAdministrator, this policy changed in the1990’s with respect to the InternationalSpace Station. Russia’s contributions to theISS clearly are essential to the effort as awhole, especially in view of the previouslynoted U.S. dependence on Russian launchcapability for both crew rotation and cargoresupply following the Columbia accident.Whether this or a similar dependency wouldbe acceptable in the context of a largerprogram of solar system exploration is amatter that can be expected to provokeconsiderable debate. Some will argue thatthe United States must be prepared to “go italone” with its own core program in theevent that any given partner elects to end itsparticipation in the venture. Others willespouse the view that such a situation is nota true partnership at all, that the partnershipis in fact forged by the necessity of mutualdependence.

3. Regulatory ConcernsSignificant international partnership in theExploration Initiative will be impeded and insome cases prevented if the current U.S.regulatory environment continues to applyin the future. Export control laws applicableto commercial technology, the Iran Non-Proliferation Act, and the InternationalTraffic in Arms Regulations (ITAR) presentan interlocking web of regulation andprocedures designed to prevent technologytransfer—particularly aerospacetechnology—from the United States to othernations. In many cases, this purpose hasbecome moot; numerous internationalcompetitors have technical capabilitiescomparable to, or exceeding, those of U.S.firms and are occupying niches formerlydominated by U.S. companies.

Nonetheless, several high-profile, high-penalty cases in recent years have driven

home to U.S. firms the absolute necessity ofobserving these restrictions and, whereambiguity exists, adopting conservativeinterpretations of them. Many aerospaceengineers and space scientists can cite “warstories” highlighting the difficulty of forgingeffective international partnerships underthese circumstances. Without significantchanges in the existing regulatoryframework, it is difficult to imagine atechnically rich international partnership in anew exploration enterprise.

Safety and Exploration Beyond LEOA total of 21 astronauts and cosmonautshave been lost in the course of 43 years ofspace flight operations (on space flights orin ground tests while preparing for spaceflight), including the Columbia crew a littlemore than a year ago. This properly raisesconcerns about the level of safety thathuman explorers can expect when they onceagain venture out of LEO. How do the risksof leaving Earth orbit compare to thoseexperienced during launch or aboard theInternational Space Station? What standardsof safety should we set as we exploredestinations like the Moon, the Lagrangepoints, the near-Earth asteroids, and Mars?

Thirty-two years ago, the United Statescompleted six history-making expeditions tothe Moon. The Apollo Program was drivenby the imperatives of the Cold War andPresident Kennedy’s “before the decade isout” deadline, but engineers still were ableto design a system they thought had a highprobability of returning its crews safely toEarth. Apollo’s stated design goals were tohave a 0.999 probability of returning thecrew safely and a 0.99 probability ofmission success [5]. All system componentswere to be designed to meet those overallstandards of reliability. The launch escapesystem, for example, combined with system


redundancy gave the crew a variety of abortoptions. Although scheduling pressures, thebreakdown of management controls, and aflawed safety process led to the tragedy ofthe Apollo 1 fire, a renewed focus onleadership and safe engineering practicesproduced a redesigned Apollo spacecraftthat achieved the lunar landing without theloss of another crew. All crew members,even those aboard the crippled Apollo 13spacecraft, returned safely to Earth, and sixof seven lunar landing attempts weresuccessful. Considering the technology ofthe day, the safety results were remarkable.New systems for exploration beyond LEOshould aim for even higher levels ofreliability and safely.

Human explorers heading fordestinations beyond LEO will face moreserious space hazards—as well as some newones—than have astronauts on the Shuttleand the ISS. Those risk factors include alengthy mission duration, a high-radiationenvironment, microgravity deconditioning,limited communications, numerouspsychological stresses, and, eventually,working outside the spacecraft in harshsurface conditions. Additional risks arise asa result of the nature of the very thinlogistics train that will be available tosupport the crew. If cargo resupply missionsare included as an integral part of thearchitecture, reasonable redundancy in theirprovisioning must be planned. If it isplanned for the crew to be able to operatefor several years without support fromEarth, this too carries certain identifiablerisks. NASA should aggressively pursueparallel efforts to develop technicalexperience and countermeasures in all theseareas so that crews face a manageable levelof mission risk. The radiation hazard inparticular is poorly understood today.Concerted efforts must be made to obtain

data on the hazard and to develop effectivecountermeasures [6].

The Challenger and Columbia accidentshave underlined the dangers facing spacetravelers and the consequences of ignoringour own human shortcomings in designingand operating spacecraft. We also havelearned how resilient the American public isin facing these risks, as long as human spaceflight leaders are seen to be confronting andworking to reduce them, and as long as thepublic supports the goals of such flights. Thepublic recognizes that space flight is riskybut will not tolerate mismanagement orwillful disregard of astronaut safety.Another fatal accident caused by humannegligence or organizational shortcomingswould likely result in a lengthy Americanhiatus in human space flight. Althoughexploration is inherently hazardous, NASAmust execute its reach into the solar systemwith the highest possible attention to safecrew return.

The Columbia and Challenger crewswere committed to the advancement of oursociety’s science and exploration goals;however, our space program over the pasttwo decades has risked human crews formany missions that may have been moresafely executed by robotic means. Astronautexplorers risk their lives whenever theyventure into space. The exploration goals towhich we commit them should becommensurate in importance with theinherent and significant risks of humanspace flight. This point was made by theColumbia Accident Investigation Board(CAIB): the risks undertaken in humanspace flight should be in pursuit of a goalworth attaining. The CAIB report makes itquite clear that continuing to fly the SpaceShuttle to the ISS, with no definite purposelying beyond, is not such a goal. Somethingmore is required, and we believe that the


program of lunar, Lagrange point, NEO, andMartian exploration discussed here providesprecisely the goals needed.

Our experience with failure in humanspace flight should lead us to adhere, wherepracticable, to the following guidelines inthe approach to a new explorationarchitecture:

• Manned launch systems must provide alaunch escape/abort capabilitythroughout the flight envelope.

• Cargo should be separated from pilotedspacecraft to the extent that it ispossible and reasonable to do so.Automated systems designed to meetupcargo requirements should be capableof supporting downcargo requirementsas well.

• Robotic precursor missions should beemployed to understand theenvironment and validate technologiesand operations prior to initiating humanmissions.

• Intermediate mission milestones anddestinations should be used to buildconfidence and experience beforeundertaking deep space voyages.

• Spacecraft should retain an abortcapability to Earth or to a surface safehaven throughout their transit phase.

• Exploration infrastructure should evolveto maximize opportunities for redundantand emergency operations.

Both the astronauts and the publicunderstand the risks of space flight andrecognize that great discoveries merit theacceptance of danger. Our obligation in any

new exploration vision is to ensure that ourdestinations and goals are worthy of therisks that await us among the planets.

The Shuttle and the

International Space Station

Attributes of the ShuttleThe Space Shuttle has performed for morethan 20 years as the workhorse of America’shuman space flight effort. It is the world’sfirst reusable spaceship, and its capabilitiesfor large payload delivery and return, orbitalmaneuvering and rendezvous, and roboticand EVA operations are still unmatched byany other system. The Shuttle’s flexibilityand large cargo capacity have made it thelinchpin for assembly of the InternationalSpace Station.

Two fatal accidents and 14 deaths in 113flights have revealed weaknesses in theShuttle’s original design and raisedsignificant concerns about its ability tooperate safely for another decade or more. Inour view, major changes to the Shuttle’sdesign to improve crew safety dramatically(most significantly, a capable escapesystem) cannot be implemented easily. Inaddition, the Shuttle’s high operating costsunder continued tight NASA budgetsconsume funds that might be better devotedto new launch and exploration systems.

Given the unique capabilities of theShuttle (delivery and berthing of largepayloads, robotic and EVA capabilities,large down-mass capacity), its return toflight is imperative for rapid completion ofthe ISS. The tailoring of most completed ISShardware for Shuttle launch argues forkeeping the Shuttle operational untildelivery of international partner modules.However, most ISS logistical needs mightwell be met using partner assets like theRussian Progress and the ESA’s Automated


Transfer Vehicle (ATV). We see the Shuttleas essentially important to the ISS only fordelivery and assembly of major flighthardware. Once completed, the ISS shouldshift to simpler, cheaper, and newer systemsfor logistics and crew support.

The Shuttle budget for FY2004 wasnearly $4 billion [7]. If the Shuttle continuesto fly beyond 2010 due to delays in return toflight or in ISS assembly, these funds willnot be available for future explorationefforts. Moreover, as the CAIB report hasmade clear, if Shuttle flights are extendedbeyond 2010, recertification of the fleet willbe required, an inevitably expensive andtime-consuming process. For these reasons,we agree with the directive that once the ISSis complete or, as we have outlined here,possibly even at the “U.S. Core Complete”stage, the Space Shuttle should be retired[8]. NASA should also focus on options forsupporting and completing the ISS usingnew U.S. cargo and launch systems, orinternational systems, that may be availablearound 2010 and that also have utility forbeyond-LEO exploration (e.g., the new CEVand Shuttle-derived HLLV, Ariane, andProton, and heavy-lift versions of theEELVs).

ISS Status and Utility

Since Columbia’s loss early in 2003, the ISShas been in caretaker status, maintained andoperated by two-man crews launched andreturned on the Russian Soyuz. Assemblyhas halted, and scientific work is at a lowlevel due to manpower limitations and alimited cargo up- and down-mass capacity.NASA estimates that at least 23 and perhapscloser to 30 Shuttle launches would berequired to complete the ISS (through thedelivery of international partnerlaboratories). The agency also estimates thatISS completion will take at least five years

from the date that Shuttle flights resume(now no earlier than March 2005). Somelessons from the ISS have already madevaluable contributions to our futureexploration planning. In its sixth year oforbital operations, the ISS has demonstratedthe technical feasibility of complex orbitalassembly. It has begun to generate some ofthe human health and productivity data wewill need to plan longer voyages. Itstechnology may in many cases be adapted toexploration use (e.g., life support,pressurized volumes, logistics modeling, andon-orbit maintenance). We suggest aconcerted effort by NASA to complete justsuch an assessment: What on the ISS isreally applicable to deep-space missions?Knowing this answer before we proceed toStage 2 will be essential if the mostexpeditious and economical program plan isto be developed.

Although the ISS does not figureprominently in the Exploration Visionbeyond about 2016, it will nevertheless beimportant to the success of that effort. Itscompletion is an important milestonecapping the success of the ISS partnership;walking away from the program wouldcreate huge difficulties in garneringinternational participation in the ExplorationVision. The President has also made the ISSthe focus for at least a decade of orbitalresearch aimed at understanding and solvingthe problems posed by long-duration spaceflight: microgravity debilitation, crewmental health and productivity, limitedcommunications, the effects of partial G,and, to some extent, radiation exposure. Oneimportant capability the ISS currently lacksis that of housing larger crews of four toseven astronauts, necessary for evaluatingcrew dynamics and determining the bestcrew size and skill mix. Also, the currentlimitation of six-month crew stays should be


extended in preparation for humaninterplanetary flight.

The ISS orbit offers few, if any,advantages for orbital assembly of futureexploration vehicles due to the payloadpenalty incurred when launching to its highinclination, as well as the penalty exacted bythis orbital inclination when departing toother destinations. But the Station’sintelligent use and evolving partnershipsgreatly improve the prospects for the successof the first human expeditions beyond Earth-Moon space.

Launch Vehicle Options

The Exploration Initiative will createextensive, and in many cases unique,demands for launch services. Only a subsetof these can be satisfied with the existingglobal fleet of expendable launch vehicles(ELVs), and even then their cost-effectiveness will be a major issue.Although it might be possible to return tothe Moon using a combination of such assetstogether with on-orbit assembly, a realisticMars exploration scenario will require there-establishment of heavy-lift launchcapability or the development of greatlyadvanced in-space propulsion technologiesthat would reduce the “mass-to-LEO”delivery requirements.

Our study findings concur with thegrowing consensus that crew missions toorbit present more stringent safety andreliability requirements than bulk cargoshipments, and therefore a mixed-fleetapproach is the most appropriate path topursue. Under this scenario, an initial CEVconfiguration could and should be deliveredto LEO by highly reliable, human-ratedlaunch systems. To the extent possible, thesesame systems should be employed for high-value or critical hardware elements, such as

interplanetary transfer vehicles, surfacehabitation modules, landers, and nuclearpower systems. As discussed below, suchlaunch systems can be derived from presentand soon-to-be operational vehicles. Thiswould include Delta IV and Atlas V in thedomestic market, with the Proton and ArianeV being readily available internationalalternatives. The Long March and H-IIAupgrades will need extensive political andtechnical preparation before they can beregarded as viable alternatives for high-value cargo.

By contrast, bulk cargo such aspropellant, life support system consumables,and radiation shielding should be manifestedon significantly larger vehicles, the designsof which are intended to reduce coststhrough economies of scale andcommonsense relaxation of reliabilityrequirements. For example, the eliminationof crew emergency abort capability will byitself generate numerous cost-savingopportunities.

U.S. Expendable Launch VehiclesThe Delta IV and Atlas V families ofEvolved Expendable Launch Vehicles(EELVs), developed in part under Air Forcesponsorship, are the most obvious U.S.candidates for supporting the crew and high-value hardware launch activities. These aremodern launch systems with state-of-the artlaunch facilities as well as healthy supplierand manufacturing support infrastructures.Furthermore, the collapse of the commercialcommunications satellite market has resultedin surplus capacity for both systems. Anexploration agenda that exploits thiscapacity should attract the backing of EELVstakeholders. The biggest challenge may beto leverage the EELV capability withoutabsorbing an excessive share of the massiveEELV program overhead. We recommend


that NASA pursue multilaunchprocurements, the cost-effectiveness ofwhich was first demonstrated by the USAFin its block-buy of Delta II vehicles forlaunch of the GPS satellite constellation.

Technically, evolving the EELV fleet tocarry a capsule-like CEV would appear to bea straightforward engineering task, but notso for the earlier winged Orbital Space Planeconfiguration that would have inducedunique torques and lateral loads at thepayload interface. A more significant issuecould be the launch infrastructureenhancements that will be required toprovide crew access and emergency egressfrom a CEV during the terminal count.

As mentioned earlier, we haveconcluded that in addition to Atlas andDelta, another CEV launch option meritsfurther consideration. This option is basedon the development of a new launch systemthat combines a cryogenic upper stage witha single Shuttle SRM. This approach hasseveral attractive features. It allows us totake advantage of the existing Shuttle humanspace flight assets at the Vehicle AssemblyBuilding (VAB) and Launch Complexes39A and B that would otherwise becomeidle upon termination of Shuttle operations.Furthermore, the SRM has proven to be themost reliable launch vehicle in the history ofmanned space flight, with no failures in 176flights following the modificationsimplemented in the aftermath of theChallenger accident. Finally, the reusabilityof the SRM when operated independently ofthe Space Shuttle could result in significantcost savings relative to fully expendablevehicles. A sketch of such a new launchvehicle is provided in Fig. 1, courtesy ofATK Thiokol.

SRB, in-line, medium lift candidate for CEV launch.

Foreign Launch VehiclesSeveral foreign launch systems can provideessentially the same level of medium-liftcapability as Atlas and Delta. Under thecurrent political environment, Ariane Vlaunches from Kourou, Proton operations atBaikonur, and Sea Launch Zenit flights fromthe Odyssey platform in the Pacific are themost readily available options for CEV-classmissions. Ariane V offers the fewestregulatory impediments to U.S. users, and itis reasonable to suppose that any French orEuropean participation in the Moon-Marsinitiative will feature a role for this launchsystem. Furthermore, the Ariane TransferVehicle (ATV) should be adaptable to otherroles besides ISS servicing. Also worthnoting is the added flexibility andredundancy that could be achieved bylaunching human missions from Kourou.Such a capability could become available inseveral years with very little effort, once the


planned Kourou-based Soyuz launchoperations are underway.

Sea Launch presents an unusual situationdue to its multinational makeup. Although itis headquartered in and operated from LongBeach, California, the Sea Launchorganization must comply with ITAR andthe other related regulatory requirementsand constraints mentioned earlier. In manyways this makes Sea Launch a foreign entityas far as domestic users are concerned.Congressional action will be necessary tomodify the existing body of regulation tofacilitate use of this asset in the ExplorationInitiative.

Such participation by Sea Launch wouldrequire several technical changes to presentoperations. First, a CEV delivery to the ISSwould utilize a two-stage Zenit comparableto the original Zenit-2 instead of the Zenit-3SL that is employed for geosynchronousorbit missions. Furthermore, the potentialexists for conducting such ISS missionsmuch closer to the Sea Launch base ofoperations in California. It is also worthnoting that the Sea Launch consortium hasbeen pursuing commercial launchopportunities at Baikonur for several years.Should this capability materialize, it willprovide additional flexibility and synergywith ISS servicing missions.

If political constraints can be resolvedfavorably, several additional internationallaunch options would become available forexploration applications. The Chinese LongMarch (Chang Zheng) family has a proventrack record that now includes the safelaunch of a human space mission. Severalcurrent and future Long March vehicleconfigurations appear to have more thanadequate performance for CEV-classmissions, and it is likely that their priceswould be competitive with those of Westernlaunch providers.

Shuttle-Derived VehiclesThere are basically three options for Shuttle-derived vehicles (SDV). The CEV/SRMoption has been discussed briefly above buthas not been studied as a serious launchoption. This is likely due in part to therelatively recent (i.e., post-Columbia)emergence of capsule designs as crediblecontenders for the CEV mission.

In contrast, NASA and its associatedcontractors spent considerable energyassessing alternative heavy-lift Shuttlederivatives during earlier Space Stationredesign efforts. The most widely known sofar has been the so-called Shuttle-Cconfiguration, in which the Orbiter would bereplaced by a functionally equivalent stagefrom which crew systems have beeneliminated. Such a stage might or might notbe recoverable. In the latter case,consideration would need to be given tosubstituting expendable, lower-cost RS-68engines for the SSMEs (an option notavailable during the earlier trade studies).The most attractive aspect of this option isthe relatively small number of modificationsand non-recurring investments required toreach operational capability. However, thebasic Shuttle-C design thus retains many ofthe unattractive features of the Shuttle as apayload carrier, including unusual matingconfigurations and side-loading. Suchfeatures might be acceptable if the Shuttle-Cwere to be used for completing ISSassembly, since many of the remaining ISSpayloads are already configured andqualified for these environments.

Perhaps the worst feature of the Shuttle-C from an exploration perspective is the factthat the sidemount payload carrierconfiguration is exceptionally wasteful ofintrinsic lift capacity. The payload carrier isessentially a Shuttle Orbiter without wingsor a crew compartment, and it is therefore


quite heavy, with a very high recurring cost,on the order of $500-600 million per flight.The sidemount configuration would notevolve easily into a more capable design, atime-honored practice with most otherlaunch vehicles.

In contrast, a more conventional in-lineSDV design, in which the payload ismounted to the top of the External Tank,would require more initial effort toimplement but would provide numerousoperational and performance benefits, themost significant of which is greater payloadmass. Again, the SSMEs would be replacedby RS-68 engines (or production versions ofthe SSME) mounted to the base of theExternal Tank, as with the Energia boosterconfiguration. Although most effective forexploration cargo missions, the in-linedesign would likely require significantmodifications to existing ISS hardwareelement mounting arrangements, which, asnoted above, have been designed andqualified for the Shuttle Orbiter payloadbay.

New Heavy-Lift LauncherAn entirely new heavy-lift launch vehicle isabsolutely necessary only if the mostefficient mission architecture dictates thatpayloads on the order of 200 metric tons toLEO are required. At that point, the safety-related issues associated with such a largevehicle could render it incompatible withexisting U.S. launch ranges.

A new launcher optimized for cargo,akin to the old “big dumb booster” concept,could achieve significant operational savingsover an SDV. Comparisons are particularlyinteresting for lox/hydrocarbon vehiclesincorporating high-performance enginessimilar to the RD-170/171 and RD-180family, or all-solid configurations based onclusters of SRMs. However, the investment

in both vehicle and ground infrastructuredevelopment make this the most expensiveoption in terms of non-recurring costs.Furthermore, as there is no other perceiveduse for such a vehicle, it would have to besponsored and maintained entirely by theExploration Initiative.

Consequently, unless a trulyrevolutionary launch vehicle technology canbe identified, one that leapfrogs currentsystem capabilities, it is difficult to make thecase that an entirely new system is needed.

Conclusions and RecommendationsThe nation has three or four technicallyviable domestic launch options foralternative crew access to low Earth orbit inthe near term. The selection of one or moreon approaches ultimately may depend moreon political factors than on cost. Forexample, will it be acceptable to use a DeltaIV or a Sea Launch Zenit-2 to launchastronauts to the ISS if it means closing theVAB and Launch Complexes 39A and B?

On a global level, there are manyreasons to make the CEV compatible with asmany launch systems as possible.Technically, such redundancy will helpavoid the single-point failure vulnerabilityof the Shuttle system that is currentlyparalyzing ISS operations. Second, thoseparticipants who wish to develop and utilizetheir own human launch capabilities aremore likely to continue to be committedpartners during difficult periods. Finally,selling CEVs to the rest of the world couldbecome a notable export opportunity andwould enable the United States to retain thelead with respect to defining standards andguiding human launch vehicle operationsaround the world. The F-35 Joint StrikeFighter program may serve as a model inthis regard.


At this point, SDV designs includingboth an SRM-based vehicle for CEVservices and an in-line heavy-liftconfiguration appear to be very attractiveoptions for leveraging the investment ininfrastructure and people for a quickresponse. The manner in which the Shuttlephase-out is actually implemented and thedetermination of which infrastructureelements will then be available for otherapplications will be major determiningfactors in whether these vehicles canbecome viable options for near-termapplications.

Steps and Stages

Departing Low Earth OrbitThe launch vehicle options discussed in theprior section can do no more than deliverdesired payloads to low Earth orbit. Thedesire to go beyond LEO invitesconsideration as to what mission design, ordesigns, might be most effective, and whatthe criteria for such effectiveness might be.

Regardless of the chosen missionarchitectures, any mission to Mars (or even asubstantive return to the Moon) will requirethe use of a “staging area,” or “assemblynode,” to marshal the various vehicles andsystems that are required. Although theApollo landings were executed without suchassembly, it is noteworthy that the Saturn 5launch vehicle developed for the programwas capable of placing a payload of about140 metric tons in LEO, somewhat largerthan can be obtained from any of theapproaches discussed in section 4, savepossibly the “clean sheet of paper” design.Even the Saturn 5, using the lunar orbitrendezvous technique, was able to deliveronly about 35 metric tons to low lunar orbit,and 8 metric tons to the lunar surface. Thisprovided a bare minimum of mission

capability (e.g., six man-days on the lunarsurface for the longest missions). In alllikelihood, something more will be desiredfor future lunar missions, so it will benecessary to assemble larger payloads in anappropriate location. It is equally clear that aMars mission will require at least severalhundred metric tons at the assembly node.

Construction of the ISS has given usconsiderable experience in the modularassembly of large vehicles in LEO, and it isonly natural that a LEO assembly nodewould be considered for deep spacemissions. Assembly in LEO may wellbecome the method of choice; above all else,it offers the advantage of a staging area onlya few tens of minutes from home. Any LEOassembly node also possesses severalimportant disadvantages, and some orbitsare considerably less desirable than others.

A given rocket launched from a givensite will be able to place a larger payloadinto a lower-inclination (i.e., near-equatorial) orbit than into a higher-inclination (i.e., near-polar) orbit.Neglecting overflight exclusion zones forrange safety considerations, a rocketlaunched from any site can achieve themaximum possible inclination of ±90° (apolar orbit), but the lowest achievableinclination from a specified launch site isequal to the latitude of the site and isachieved by means of a due-east launchfrom the site. (Slightly lower inclinationsmay be attained, with some loss ofefficiency, by means of a “dogleg”maneuver, in which the vehicle first fliestoward the equator, then turns east.) Easterlylaunch from a low-latitude site is furtheradvantageous in that the rocket can takeadvantage of the Earth’s rotational velocity,which is greater for a lower-latitude site.Thus, near-equatorial launch sites such asKourou are favored in two ways: the full


range of orbital inclinations (±90°) can beattained, and more mass can be placed inorbit. Launch sites such as Baikonur, atapproximately 50° North latitude, andVandenberg, which is at approximately 35°North latitude but has significant overflightconstraints for easterly launches, are greatlydisadvantaged in this respect.

If it is politically necessary that thelaunch sites of all spacefaring nations beable to access the chosen LEO assemblynode, then the node must be in a high-inclination orbit, and the mass of anypayload delivered to that orbit will bedegraded by 10-20%, depending on the sitelatitude and the design of the launch vehicle.(Use of an equatorial launch site does noteliminate this penalty when launching to ahigh-inclination orbit.) This will have ameasurable economic impact on theExploration Initiative. During the time frameaddressed by this report—the next severaldecades—the cost of access to Earth orbitcan hardly be less than several thousanddollars per kilogram, and, as we havediscussed, even a Spartan expedition toMars will require many hundreds of metrictons of material to be delivered to LEO. It iseasily seen that the cost of using a high-inclination LEO staging area will besubstantially higher than it would be atlower inclinations. Other physicalconstraints exist as well. To depart for theMoon or Mars from a staging area in LEOrequires, among other things, that the orbitalplane of the assembly node contain thedeparture direction during the availablelaunch window. If this geometricalrequirement is not satisfied, an expensiveand quite possibly prohibitive orbital planechange will be required.

Departure from low Earth orbit to Mars(or another destination beyond the Earth-Moon system) requires that the departure

vector be very nearly tangent to Earth’s orbitabout the Sun during the launch window, aperiod of a few weeks duration every 26months for the most favorable opportunities.If the plane of the LEO assembly node is notso aligned as to contain the departure vectoras discussed above—and the probability ofsuch an alignment is low unless the node isplaced in an orbit selected, well in advance,to favor a particular opportunity—then themission cannot leave Earth orbit. The rapidnodal regression—several degrees per dayfor moderate inclination low Earthorbits—may restrict the usable portion of thewindow even further.

For travel to the Moon, or to the Earth-Moon Lagrange Points (e.g., EML1),departure opportunities from a LEO nodeoccur roughly every two weeks. For theSun-Earth Lagrange Points, opportunitieswould occur less often.

Although no LEO node can be optimallylocated for travel to both the Moon andMars, or even to either destination all thetime, near-equatorial orbits are heavilyfavored in terms of performance ascompared to higher-inclination orbitsbecause the required plane changes to reachthe desired destination will be smaller.

In summary, departing from Earth orbitcan result in plane change penalties, willsubstantially restrict the available departuretimes, and will place restrictions on otherconditions. This is not the case whenlaunching from the surface of the Earth,where the planetary rotation provides accessto any required launch plane orientation atleast once per day.

This conclusion provides the rationalefor the use of the Lagrange Points, eitherEarth- or solar-referenced, as staging areasin cislunar space for travel to the Moon orMars. Use of these locations also involvesperformance penalties, but they are typically


less than those for plane change maneuversand are consistent over time, freeing themission architecture from criticaldependence on specific launch windowconstraints.

In this vein, very high Earth orbits(HEO) may become attractive as stagingarea locations. The required plane change tothe target inclination can be accomplishedquite cheaply, at the apogee of the LEO-departure transfer orbit. Nodal regression forhigh, near-equatorial orbits is negligible, sothe staging area will remain usefulthroughout a given launch window and formultiple opportunities.

Electric PropulsionIf an HEO assembly node, including a nodeat EML1, is selected, it is possible tosubstantially improve the mass deliveryefficiency of the architecture. The assembledvehicle, or more likely sub-elements of it,can be delivered to the higher orbit perhapsseveral months before departure for theMoon, Mars, or an NEO, at a time when theorbit plane orientation will be correct for theanticipated departure.

The higher orbit can be reachedefficiently using low-thrust electricpropulsion (EP). This would require severalmonths, but once in high orbit a crew couldrendezvous with the assembled vehicle,departing either from Earth’s surface orfrom LEO. Then, chemical propulsion couldbe used for the remainder of the outboundmission at a fraction of the requirement forLEO departure, allowing a shorter trip. Fordeparture to Mars, a dual propulsion systemmay be useful, with chemical propulsionused for fast departure and arrival, andelectric propulsion used during the severalmonths of interplanetary travel. Solar-electric propulsion (SEP) has beenconsidered for operations in cislunar space,

and nuclear-electric propulsion (NEP) isuseful irrespective of proximity to the Sun.

Like all space propulsion schemes,electric propulsion requires the generationand expulsion of a directed mass flow fromthe vehicle, which is then accelerated in theopposite direction, according to Newton’s3rd Law of Motion. However, EP employs adifferent fundamental mechanism fortransferring energy to the fluid stream thando chemical rockets (or even nuclearthermal rockets, discussed below). Throughthe mechanism of the converging-divergingsupersonic nozzle, chemical rockets aredevices for converting the thermal energy ofcombustion into a highly directed propellantstream. With EP, the electrical energy isused to strip electrons from the atoms of aneasily ionized, preferably heavy, element(e.g., xenon). The heavy positive ions arethen accelerated in an electromagnetic fieldand ejected from the back of the “rocket” ina stream of high speed particles. (Thepreviously stripped electrons must beallowed to recombine with the ions as theyexit to prevent buildup of a net charge on thespacecraft.)

The major advantage of EP is the fuelefficiency it offers; a specific impulse in therange of thousands of seconds is easilyachieved. However, EP systems offer verylow thrust, several orders of magnitudebelow that of chemical propulsion systems.Moreover, a large mass of hardware isrequired to generate this thrust, nullifying tosome extent the efficiency of the basicengine; much of the presumed payloadadvantage of EP systems is used toaccelerate the mass of the powerplant.

The feasibility of large-scale nuclearelectric propulsion is likely to bedemonstrated first during the Jupiter IcyMoons Orbiter (JIMO) mission, a roboticmission currently planned for launch in 2015


[9]. The mission is anticipated to use areactor in the range of several hundredkilowatts and to require a mass of some 20metric tons in LEO, of which 10-13 metrictons will be xenon fuel, the favored choicefor such a system. The actual sciencepayload is projected to be in the range of1,500 kg; much of the remainder of theoverall mission mass is absorbed by thehardware (reactor, energy conversionsystem, radiators, structure, etc.) requiredfor the NEP system.

Human missions will require megawatt-class reactors and comparable increases inthe mass of fuel required. The availability ofxenon in such large quantities could be inquestion. Xenon is present in trace amountsin the atmosphere and is extracted in thecourse of liquid oxygen and nitrogenproduction. Current world production ofxenon is on the order of 10 million liters peryear (59 metric tons at standard temperatureand pressure), at an average price of about$10 per liter, or about $1,700 per kilogram[10]. Unless substantial progress is made inthis area, use of a less desirable fuel will benecessary.

Nuclear Thermal PropulsionOf the technologies so far proposed forradically transforming the architecture ofMoon and Mars exploration, nuclear thermalpropulsion (NTP) is among the mostcredible in terms of both fundamentalphysics and engineering developmentmaturity. NTP is based on the direct transferof fission-generated heat from a solidnuclear core (we omit here any discussion ofgaseous core nuclear reactors) to a workingfluid (hydrogen) that also serves as thepropellant. By contrast, with NEP theoriginal thermal energy undergoes severalconversions, and therefore losses in overallefficiency, before being imparted to xenon

or some comparable propellant via electricor magnetic fields.

NTP is attractive because of the highthrust level it can provide, similar to that ofconventional chemical rockets and severalorders of magnitude greater than NEP. Formaneuvers within planetary gravity wells,particularly Earth escape, such high thrustcan reduce transit times from months tohours. Although NTP is not as fuel-efficientin terms of specific impulse as NEP, thereduced trip time to and from Mars offerssignificant benefits in terms of reducing thecrew exposure to microgravity and radiation,while at the same time reducingrequirements for consumable supplies.

The major advantage of NTP comparedto chemical propulsion is that the energycontained in the exhausted propellant (a keyfactor in determining a rocket engine’smaximum potential performance) is notconstrained to the energy available from thechemical combustion of a fuel and anoxidizer. The hydrogen exhaust from anNTP engine can be hotter than for chemicalpropellants, limited only by the thermaltolerance of the engine materials themselves.Also, the lighter molecular weight of thepure hydrogen exhaust greatly improves theoverall operating efficiency. These effectstogether offer essentially double the 450seconds of specific impulse typical of ahigh-performance lox-hydrogen upper stageengine. With such enhanced performance,the amount of propellant needed for themission can be reduced by more than half,with a concomitant reduction in launchcosts.

The advantages of NTP are mitigated bynumerous material compatibility issues. Theheated hydrogen tends to erode the reactorfuel core, and as with any nuclear reactorthere is a high level of high-energy radiationemitted, which severely constrains the


design and configuration of the overallvehicle.

Also, although the higher specificimpulse does offer the capability to carrymore payload or less fuel, the improvementin overall performance as compared withchemical propellants is not as great as mightbe suggested from consideration of theimproved specific impulse. Because of theweight of the reactor and associatedstructure, the overall thrust-to-weight ratioof an NTP system will be substantiallypoorer than for a chemical system,nullifying part of the presumed payloadadvantage. Even with these reservations, thepotential of NTP as a tool in the explorationof the solar system is enormous, and it hasbeen recognized as such for decades.

Because of this potential, the U.S.Atomic Energy Commission (thepredecessor to ERDA and thence DoE), andthen NASA, invested significantly in NTPdevelopment during the 1950s and 1960s,with projects known as Rover and NERVA(Nuclear Energy for Rocket VehicleApplications). These efforts resulted in thedevelopment and testing of multiple reactorsand rocket engines at the Nevada Test Site.These tests validated the general operationalfeasibility of NTP by 1973, when the effortwas terminated as part of the overallretrenchment from human space explorationafter Apollo [11].

Arguments concerning performanceaspects of NTP relative to other options areas valid today as they were in 1972.However, the social environment forconducting technical R&D related to nuclearsystems has changed dramatically, makingany such task much more difficult than inearlier decades. International treatyobligations preclude the open air testingtechniques employed for the original NTPtesting, while public opinion is far less

tolerant of any nuclear systems. Moreover,the government and industrial nuclearinfrastructure has atrophied considerably inthe last 30 years as a result of the demise ofcommercial nuclear power and the end ofthe Cold War.

As discussed in connection with NEP,NASA is beginning to resurrect nuclearpropulsion options in general. Thecapabilities now under development throughProject Prometheus in preparation for NEPmissions such as JIMO can also help lay thefoundation for a more extensive programthat includes NTP. However, unlike NEP,NTP can be justified only for humanmissions, where there is major benefit to beobtained by reducing trip time andincreasing payload.

Our team endorses these, and stronger,efforts by NASA, because nuclear powerand propulsion is ultimately necessary forthe exploration of the solar system. At thesame time, however, very effective missionsto Mars clearly can be designed using thecombination of chemical propulsion fordeparture from Earth and for return fromMars, and aeroassist upon arrival at Marsand when returning to Earth. The missionarchitecture can be made substantially moreefficient by extracting propellant from theMartian atmosphere, which eliminates asubstantial part of the logistical burden,although of course a production plant mustbe pre-emplaced. Nuclear propulsion,though desirable, is thus not essential for theexploration of Mars.

Nuclear power is a separate and initiallymore critical issue for the exploration ofboth the Moon and Mars. Mars’ distancefrom the sun degrades the output of a givensolar array by a factor of approximately 2.2,making the generation of useful surfacesolar power a rather cumbersome affair, onethat is further compromised by the


accumulation of dust on the panels. Thetwo-week lunar night poses an even greaterdesign challenge for lunar missions if theyare to remain through the night. Thecompelling solution to these problems is thedevelopment of space nuclear powerreactors, a solution we consider to beessential to implement.

Interplanetary CruiseMany options have been proposedconcerning classes of trajectories that maybe used for travel between Earth and Mars.These include the Aldrin cycler concept, aswell as other types of cyclers, and the use ofVenus flybys. But chemical propulsion issuitable for six- to eight-month transits froma high Earth orbit to a high Mars orbit, suchas EML1 to Phobos or Deimos. This avoidsthe high !v requirements to achieve a low

circular orbit. If only chemical propulsion isused, the transfer vehicle will have to refuelat Mars. If a combination of chemical andelectric propulsion is used, a round trip maybe possible. The same is true if a chemical-aerobrake mission design is selected. Butcertainly cargo vehicles will be needed and,if missions are not time critical, thesevehicles could be operated with low thrustalone. Of course, the infrastructure forcontinuous operations, as with the variouscycler concepts, would have to be developedat Earth and placed at both planets.

Human Factors

Many of the human factors important tolong-duration space flight outside theprotection of the Earth’s magnetosphere canbe addressed in long-duration missions onthe ISS. The principal exception is in thearea of radiation exposure in interplanetaryspace, where Earth’s magnetic field is notavailable to deflect most of the energeticsolar particles and some of the lower energy

cosmic rays. Various areas of concern willbe discussed below.

Gravitational AccelerationHumans have evolved in our presentgravitational environment of “1-g,” about9.8 m/s2, for many millennia. At the dawn ofmanned spaceflight, 45 years ago, thereexisted real concern as to how the humanbody would respond to the near-weightlessenvironment of space flight. Experience inboth the U.S. and Soviet/Russian spaceprograms, for durations of up to almost 15months, has shown that most physiologicalsystems adapt quite well to low gravitywithin days or weeks and then return tonormal upon return to Earth. The principalexception is in “mineral balance,” especiallythe calcium in human bones, which relatesto their strength and resistance to fracture.So-called “bone loss” remains a seriousproblem, and NASA continues to pursueresearch in this area as a high priority.

Part of the difficulty in studying thisproblem lies in the low rate of mineral loss,comparable to that observed in bed-reststudies on Earth. The issue is furthercomplicated by the normal decrease inmineral content of adult bones with aging.The ultimate concern for some is the risk ofweight-bearing bone fracture upon return toEarth after several years in weightlessness.It is reassuring to note that, after all thelong-duration flights to date in Skylab, MIR,or the ISS, there have been no fractures ofweight-bearing bones for any astronaut orcosmonaut after returning to Earth.

Still, some argue for more positivepreventive action to eliminate the slowmineral loss that has been observed. Dietaryand pharmacological studies continue to bepursued. Exercise protocols that providecompression stress to the leg bones, such asan in-flight treadmill with crewmembers


anchored by bungee cords, have beenemployed with positive results. The ISScurrently employs the Interim ResistiveExercise Device (IRED) to enable crews toload muscle and bone with vigorous strengthtraining exercises. The IRED has shownsome initial promise in slowing bone loss.Finally, a large rotating structure has beenenvisaged that can provide variableacceleration levels by centrifugal force.Whether or not this is required will have toawait more studies on the ISS;, if required,such a structure will be a major design effortfor the interplanetary spacecraft.

In summary, numerous problems andinefficiencies result from the microgravityenvironment. Relative to the overall scopeof an expedition to Mars, however, they areinconveniences rather than show-stoppers.

RadiationThe biological effects of radiation in thespace environment currently have anestimated uncertainty factor of about four[5]. Radiation biology is clearly a veryimportant factor in the design ofinterplanetary spacecraft, but with such largeuncertainties in the effects of radiation, wemust await further research and thedevelopment of expert understanding beforedefinitive design rules can be developed.Lacking such, there is much that can bedone by designing spacecraft to contain“storm shelters” for protection againstenergetic solar protons. Hydrogen is one ofthe best materials for such shielding; likelythe storm shelter will be a central region ofthe spacecraft surrounded by water tanksand possibly some degree of magneticshielding from heavier and more energeticgalactic cosmic rays. Pharmacologicalprotection is also being considered. NASAshould expedite radiation studies atdedicated accelerator facilities, and it should

obtain accurate data about the radiation fluxbeyond the magnetosphere. At this point, wemust assume that some combination of thevarious design approaches will be found toreduce the radiation risk to humans to anacceptable level for several years ofinterplanetary travel.

Social and Psychological FactorsThe social-psychological aspects of crewselection, training, and in-flight problemresolution have been neglected for manyyears, particularly in the U.S. spaceprogram. In the early days of U.S. spaceflight, crews were small (three members orfewer, until the Space Shuttle arrived),potential selectees were few (usually fewerthan 50 candidates for a mission wereavailable in the Astronaut office), all hadtrained and worked together for many years(allowing them to become aware of thestrengths and weaknesses of theircrewmates), the selections for flight weremade principally by senior fellow-astronauts(Chief of Astronaut Office and Director ofFlight Crew Operations, who knew all flightcrew candidates well), and mission durationswere short (less than two weeks). In thesecircumstances, it was expected thatcompatible crews with little social-psychological friction should be the norm,and (with minor exceptions) this was foundto be true.

However, as crews have become larger(six or seven members on a Shuttle flight)and included both genders, as well as thoseof different nationalities and professionalbackgrounds, often with restricted timeavailable to train together, some increasedpsychological stress is to be expected. Theminimal crew interpersonal frictionencountered so far (at least publicly) istherefore a remarkable achievement. Creditshould perhaps go to the high motivation of


all crewmembers (some friction is simplyoverlooked or resolved), good selection ofcrews by senior astronauts and cosmonauts,and a command structure that makes eachindividual’s responsibilities clear.

For future missions in interplanetaryspace, it is recommended that additionaltime be given to train a number ofcandidates together (at least a year), inseveral locations, before the final crewcomposition is determined. Possiblelocations for Mars mission preparation couldinclude areas in the Arctic or Antarctic, theISS if adequate transportation is available,or lunar missions of shorter duration, withtechnical and scientific work to be done.Social interaction with families of othernationalities should be included. A Russianproverb, publicly related years ago bycosmonaut Oleg Atkov, states roughly that“individuals should never undertake adifficult and risky task until they hadconsumed together 20 kg of salt.” Theobvious interpretation is that they shouldshare many meals together, becoming wellknown to each other. Although difficult formission planners to achieve, this seems to bethe best way to ensure a smoothlyperforming, compatible mix ofcrewmembers for interplanetary missions.

System Design ImplicationsFor these longer missions, human factorsbecome increasingly important. Exercisefacilities are essential to maintaining thegood health required in flight, as well as atthe destination and eventually for return toEarth. Additional training in medical careand procedures is important, especially if anMD is not included in the crew. Individualprivate video conferencing capability shouldbe available for discussion of medicalproblems and especially for any treatmentrequired. Social-psychological problem

resolution, if and when required, should behandled on this private loop. Cross-skillstraining is mandatory because the number ofcrewpersons is expected to be rather small.Cross training ensures both availability ofcompetent personnel and betterunderstanding among crewmembers of theothers work objectives. NASA shouldconduct studies to determine the neededskills for each destination planned. The ISSmay provide a good training ground forsome of these.

Food systems should be entirely pre-packed, as was done in the Skylab programmore than 30 years ago. Although pleasantand enjoyable, fresh-frozen items can beomitted. Worth noting also is that radiation-stabilized foods can provide a very usefulalternative to frozen foods for long-termstorage. Onboard growth of foods is aresearch task appropriate to the ISS, ratherthan it being a critically needed operationalsystem. Although freshly grown foods seemto have a positive psychological effect onlong-duration flights on the ISS, theirproduction should not be a mandatoryrequirement for continuing an interplanetaryflight.

Artificial gravity via rotation of theentire space assembly is a major designconsideration. If used, some large-radius,relatively slow rotation should be used tominimize coriolis forces. However,weightlessness (free-fall) has proven quitesatisfactory on all previous long-durationmissions, and the ISS will have providedmuch more experience before theseinterplanetary missions are flown. Ittherefore seems best to rely on a “free-fall”design until it is shown to be unacceptablefor reasons presently unknown. It shouldalso be noted that, to the extent thatmicrogravity exposure is deemed a problem,faster transit times provide considerable


relief. The additional design cost andcomplexity attendant to large, rotatingspacecraft may well be better invested innuclear propulsion systems, which, as notedabove, allow significantly shorter transittimes.

The Cost of Going to Mars

Seventy years of aerospace systemdevelopment data show a strong correlationbetween the weight of aerospace hardware,properly segregated according to category,and the cost of its development. TheNASA/Air Force Cost Model (NAFCOM)reveals the median cost for development ofhuman-rated spacecraft to be about $420 Kper kilogram and the median cost for thefirst production unit of such spacecraft to beabout $29 K per kilogram. (FY 2004 dollarsare used throughout this discussion.)

However, no human-rated spacecraft hasbeen developed in the past 20 years. Further,the President’s Exploration program doesnot reach the funding levels needed for aMars program until 2014; thus, 30 years ofproductivity gains will have occurred beforeany such program begins. Assuming that itwill require 10 years to develop thehardware to launch the first human Marsexpedition, there will be another decade ofproductivity improvement before thespacecraft enters production. The aboveresults must therefore be adjusted for theproductivity improvements that haveoccurred and that will occur before the firstMars expedition could leave Earth. Becausefuture productivity gains are unknowable,we will assume 2% annually for the entireperiod as a conservative estimate; this islower than the level of productivity growthin the U.S. economy over the past 20 years.

Finally, we observe that the majority ofhuman Mars expedition studies have found

that about 500 metric tons is required in lowEarth orbit for a mission based on chemicalpropulsion.

Development CostsWe can estimate the design, development,test, and evaluation costs for a human Marsmission by noting that, of the approximately500 metric tons required in LEO at theoutset of the expedition, at least 250 metrictons consist of propellant, with theremainder being the so-called “dryspacecraft,” to which our hardwaredevelopment cost estimate will apply.Assuming traditional NASA programmanagement, but including 30 years of 2%productivity growth, we find that human-rated spacecraft should have a median costof about $230 K per kg at the beginning ofthe 10-year development program in 2014.This results in a development cost estimateof $58 B, or an average of about $5.8 B peryear over that 10-year period.

Production CostsAgain assuming 250 metric tons of human-rated spacecraft and accounting for 40 yearsof productivity gains before this spacecraftenters production, we estimate first unitproduction cost at about $13 K per kg in2024. This gives a total first unit productioncost of $3.2 B.

First Mission CostThe direct cost of the first Mars expeditionwill be the cost of the human-ratedspacecraft, the (negligible) cost of thepropellant, and the cost of launching all therequired mass to LEO. Observing that therehas been no significant change in the cost ofspace launch over the past 40 years (we notethat beginning about 1994, some sovereigncompetitors in the space launch businessstarted pricing below cost), we will assume


that the present launch costs of $9-11 K perkg will continue for the next 40 years. Onthis basis, we estimate the cost of placing500 metric tons in LEO at about $4.4-5.5 Bper mission. Summing these results, weconclude that the first human MarsExpedition will have a direct cost of around$7.6-8.7 B.

Subsequent Mission CostThe production of aerospace hardwarecharacteristically shows a “learning curve”of about 85% (i.e., each doubling of theproduction quantity results in a costreduction to about 85% of the earlier level).Thus, the second set of Mars expeditionhardware can be expected to cost about $2.7B, the fourth set about $2.3 B, and so forth.

Summing this well-established effectover 20 years (9 missions to Mars) results ina total hardware cost of $20.9 B. Adding tothis the $40-50 B for placing nine missionsworth of mass into LEO yields an averagemission cost, over 20 years, of $6.8-7.9 B.This result assumes that no new hardwaredevelopment is initiated over the two-decadeperiod between the first and ninth missions,which may be unrealistic.

Total 30-Year CostSumming the 10 years of development costsand mission execution costs for 9 missionsover 20 years, we estimate a total programcost over the 30-year period 2014-2044 of$119-129 B. For comparison, the Apolloprogram cost about $130 B in FY2004dollars spent over about a decade.

Sensitivity AnalysisBased on the above analysis, we canestimate the present value of using a nuclearthermal rocket instead of chemical rocketsfor LEO-Mars-LEO transportation. Inaddition to reducing the travel time, such a

rocket can be expected to save at least 100metric tons of propellant. This represents asavings of $0.9-1.1 B in launch costs foreach mission. Discounting at the U.S.Government’s 7% cost of money across theentire 40 year period (2004-2044) suggeststhat investment of up to $1.1 B iseconomically justified today if thedevelopment of a nuclear thermal rocket fora human Mars mission will save 100 metrictons of LEO mass, given current spacelaunch prices.

We can similarly determine theeconomic value of lower space launch costs.Halving the cost of launch to $4-6 K per kgwould result in a savings of about $2.5 B perhuman Mars mission; these savings have apresent value of $3.1 B if the first Marsexpedition starts 20 years from today. Suchsavings on a Mars exploration programbeginning in 20 years would thereforeeconomically justify Government spendingof up to $3.1 B today if that investmentresults in halving the cost of space launch.

This estimate of the economic value oflower cost space launch does not include thebenefit that such a lowering of cost wouldhave to all other government payloads overthe next 40 years, nor does it include anybenefit it might have for the nationaleconomy.

We may also note that there is asignificant discrepancy between thehistorical $420 K per kg cost of developinghuman-rated spacecraft and theapproximately $55 K per kg cost ofdeveloping commercial aircraft over thesame period. Because NASA’s traditionalprogram management methods are largely afunction of organizational culture, it may beinstructive to ask what might be the value ofreducing the cost of developing human-ratedspacecraft to two-thirds of previous levels.In that case, we can estimate the present


value of the future savings at about $1.2 Btoday. Thus, it is economically worthwhileto invest up to $1.2 B in changing themanner in which NASA manages itsprograms, if that change will result inlowering future development costs to two-thirds of historical levels.

Cost SummaryA Mars Exploration Program starting in2014, launching a first mission in 2024 anda mission every 26 months thereafterthrough 2044, is estimated to have a totalcost of no more than $129 B over thatperiod, or about $4.3 B per year.

Development of a nuclear thermal rockethas the potential to save $7.9-10 B if spacelaunch costs remain at current levels.Lowering space launch costs to 50% ofcurrent levels would save $20-25 B.Reducing the cost of NASA human-ratedspacecraft development to two-thirds ofhistorical levels could save an additional$19.3 B.

Policy Implications and

Recommendations for Shuttle

Retirement

We assume that the Shuttle Orbiter willreturn to flight in 2005, as NASA hasindicated. However, regardless of the safetymeasures incorporated, it will remaininherently deficient in its capability toprovide the crew with an escape option inthe event of a catastrophic failure at somepoint in the mission. Such a failure isinevitable if the Shuttle continues to flyindefinitely; we therefore agree with theAdministration’s decision to retire thevehicle in the 2010-2011 time frame.

Indeed, some have advocated that theShuttle be retired now. We do not advocate

this view; we believe that the Orbiter will beadequately safe to fly more missions intospace, although its retirement should beaccomplished as soon as practicable, giventhat it will have seen 25 years of use. Webelieve it is reasonable to fly again tocomplete construction of the ISS, at least tothe “U.S. Core Complete” stage, whichshould be reached after only six to eightadditional Orbiter missions and about twoyears. It appears, however, that to reach“Assembly Complete,” with theinternational modules (JEM and Columbus)and perhaps the U.S. Habitation module inplace, will take more than 20 additionalflights and an additional four or five years.This appears to be stretching the programtoo far and, perhaps equally important,extending funding for the Shuttle Orbiter toofar into the next decade, limiting fundsavailable to move into succeeding Stages ofthe Exploration Initiative. There seems to besome official ambiguity on this point.NASA has indicated that the Orbiter will beretired in 2010, though this is likely to bewell before “Assembly Complete” andwould once again place the United Stateswith no capability to reach LEO or the ISS,presumably necessitating reliance on theRussian Soyuz once again.

We propose instead that NASA plan touse the Orbiter only to “U.S. CoreComplete” and plan to deliver the otherheavy modules to the ISS with other launchvehicles, provided that agreement on thispoint can be reached with the internationalpartners. Launch options are described insection 4, above. In this way, larger andinternational crews can begin to utilize theISS even earlier than in the current planning.Also essential to this plan is the earlydevelopment of a simple and robust CEV,designed to transport four to sixcrewmembers to and from LEO , and to


remain with the crews during their stays atthe ISS. When this is done, which should bebefore 2010, then the Orbiter can be retiredand human access to space for the UnitedStates will not suffer another painful hiatus.The CEV design for this first stage might beidentical externally to the more capableversions adapted for interplanetary travel ina later Stage of Exploration. It can besimpler, less massive, and cheaper todevelop at this early phase because it willnot be required to spend long mannedperiods in space, travel to distantdestinations, or survive reentry at hyperbolicspeeds.

Overview, Significant Issues,

and Recommended Studies

We have described a three-stage plan forMoon/Mars Exploration. Stage 1 firmlyestablishes our Earth orbital capabilitieswith a new CEV capable of carrying four tosix persons to and from LEO, including theISS, before 2010. It requires the concurrentqualification of an appropriate launchvehicle, which we have suggested could bebased on a single SRM augmented with anew lox-hydrogen upper stage (Fig. 1). Withthis system in place, and with theconcurrence of the international partners, theShuttle Orbiter can be retired at any stage ofISS assembly following “U.S. CoreComplete.”

In Stage 2 of the Exploration program,destinations at the Moon, NEOs, theLagrange points, and the vicinity of Mars,including the Martian moons Deimos andPhobos, become possible. Each of theselocations offers the potential of fascinatingscientific return and broad public interest,while remaining within reasonable fiscalbounds. Finally, in Stage 3, human landingson the Moon and Mars are achieved.

Before the internal steps along the routethrough these three Stages can be fullydefined, a number of confirming or definingstudies must be completed.

• We have suggested that the earliestversion (e.g., “Block 1”) of the CEV bea simple design capable of carrying acrew of four to six to and from LEO. Itwould not be intended for long periodsof independent free-flight or tripsbeyond LEO but would provide U.Saccess to LEO and the ISS and allowthe Shuttle Orbiter to be retired. TheCEV Block 1 design would allow theISS be used by the United States both toqualify more and larger crews for laterinterplanetary travel and to assuremission planners that the internaldynamics of crew selection and skillprovision were appropriate for longduration missions. It would also allowour international partners to begin theirlong-delayed research in the U.S. Laband other existing facilities at theearliest possible time. It is expected thatthis Block 1 design could be availablefor testing by 2008 and manned by2010. The question of the proper designconfiguration remains, and is important,because successive versions will beunlikely to (and should not) alter thevehicle’s basic mould lines.

• The mass of the Block 1 CEV should bein the 13-15 metric ton range, includingthe abort system. We have suggestedthat the most suitable launch vehicle forthe LEO CEV could consist of a singleSRM, with a new lox-hydrogen upperstage. Candidate upper stage enginescould include the Apollo-era J-2S or theSSME (likely modified for cheaperproduction if it is to be expended upon


each use). Heavy-lift versions of DeltaIV and Atlas V have been mentionedfrequently as candidates for a CEVlauncher and have ample payloadcapacity for the task, thoughconsiderable work may be required tohuman-rate them. Some study should bedevoted to determining which option isbest suited to early use; what choiceswould be the most cost-effective, safest,and most reliable; and what additionalinfrastructure would be required foreach option.

• It would seem sensible to considerusing various international launchvehicles at sites other thanKSC—particularly Kourou, befitting itsadvantageous location—for launchingboth the CEV and other explorationhardware. Whether this is politicallyviable or practically implementableremains to be determined, but it shouldbe studied.

• It will be necessary to upgrade the CEVto a “Block 2” version for missionsbeyond LEO. The Block 2 version willhave requirements yet to be determined,but at a minimum it must be capable oflong-duration interplanetary cruisemissions to any of the destinationslisted for Stage 2 or 3 exploration,presumably in combination with othermodules (e.g., Hab, Lab, Consumables,Propulsion), which must be attached to2the CEV before departure oninterplanetary trips. We are confidentthat a CEV “growth strategy” alongthese lines will allow an explorationversion of the vehicle to be developed atthe least possible incremental cost.

• NASA must define the fleet of U.S.launch vehicles desired to supporthuman exploration beyond LEO.

• We have assumed that, at least initially,the assembly node for the collectedmodules will be in low-inclinationLEO, and we have pointed out some ofthe penalties associated with the use ofhigher-inclination orbits. We haveaddressed some of the advantages ofhigh-altitude assembly nodes but havenot considered them likely candidatesfor early use. These issues should beaddressed in more detail.


References

1. Morring, Jr., Frank, “Off the Ground,”Aviation Week and Space Technology, 28June, 2004, p. 26.

2. "The Next Steps in Exploring DeepSpace," a cosmic study by the InternationalAcademy of Astronautics, Review Draft 23March 2004http://www.iaanet.org/commissions/presentation.pdf

3. http://www.esa.int/SPECIALS/Aurora/ESA9LZPV16D_0.html

4. “Putin Acknowledges Budget Problems inSpace Exploration,” Moscow, ITAR_TASSin English, 1730 GMT, 12 April 2004.

5. Murray, Charles, and Cox, Catherine Bly,“Apollo: The Race to the Moon.” (1989),pp. 100-102.

6. Setlow, R.B., “Mitigating Hazards ofSpace Travel,” Space News, 15 Mar 04, p.13.

7. President’s FY 2005 NASA BudgetRequest:http://www.nasa.gov/pdf/55385main_01%20Front%20page%20Total%20Summary%20Table.pdf

8. Planetary Society Workshop: Steppinginto the Future, a Workshop in Memory ofthe Columbia 7, April 29-30, 2003.http://planetary.org/workshop/index.html

9. Morring, Jr., Frank, “Space NuclearPower,” Aviation Week & SpaceTechnology, Vol. 60, No. 22, May 31, 2004,p. 66.

10. http://www.alfanaes.freeserve.co.uk/Session981.htm

11. Dewar, James A., To the End of theSolar System—The Story of the NuclearRocket, The University Press of Kentucky,Lexington, KY, 2004.

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