Interstellar Travel

ASTR 1420

Lecture 24

Sections 13.1 & 13.2

Four spacecrafts flying away from the Sun

Pioneer 10• Launched on March 1972 (moving away from the Sun at 12.2 km/sec)

now at ~28 light minutes away!

will take ~120,000 years to the nearest star (if it were aimed directly at it).

Scenes from Earth

Rockets = Newton’s 3rd Law• For every action, there is an equal and opposite re-action!

Exhaust Flow pushed backwardEngine (and rocket) being pushed forward

Tyranny of Distance

the largest rocket ever built (Saturn V; used in the Apollo mission)

?? If we build a larger version of this rocket, can our descendants travel among the stars?

No!

Chemical Rockets are limited by Mass ratio• Need to accelerate fuel also! Mass ratioo mass ratio = weight of a rocket with fuel / without Current technologyo To escape from Earth: mass ratio = 39o Best single-stage rocket: mass ratio < 15

• Multi-staged rocketso Necessary, and used, to leave Earth, or even for

intercontinental ballistic missileso Interstellar travel: impractical, hundreds to

thousands of stages required

Even with the most powerful hypothetical rocket engine with 100 stages

achieve 0.001ctakes 4,000 yrs to the

nearest star!

Space Elevator

• SpaceElevator Competition ($1M USD)

Can reduce (by x100) the cost of sending material to space!

How about different types of rockets?

Nuclear Rockets

o Advantage: higher energy/mass ratio of nuclear reactions

o Disadvantage: difficult controlled use, especially fusion

o Maximum speed: ~ 0.1c,

• Project Rover (1955-1972)o Fission rocketo Achieved speeds 23 times those of

chemical rocketso Application: manned mission to

Mars, since abandoned

Nuclear Thermal Rocket Engine

1st ground experimental nuclear rocket engine

Dec 1, 1967

Project Orion• Explode H bombs behind the spaceship and

let the shock waves propel the spaceship

• Too expensive, also violates “ban on nuclear explosions in space”

Project Daedalus

Use pellets of 2H and 3He, ignited by an electron beam from the spacecraft

UK plan to reach Barnard’s star (5.9 Lyrs away)

Construction in theEarth orbit

Ion Engine• Using heavy noble gas (e.g., Xe, Ar, etc.) as fuel gas.• Ionize them by shooting electrons on them• Ions are accelerated by specially designed electric acceleration grid.• accumulated electrons are dumped outside.

Ion Engine• Continuous firing…but weak thrust!• Need to be free from other ions…• In vacuum. Much more efficient!• Already used by NASA (1998, Deep Space 1)

and ESA (2004, SMART-1, lunar orbiter).

Picture of the Deep Space 1’s ion engine.

weak thrust but continuous firing for long, long time!

Solar wind only reaches 0.003c, need to use sunlight

decreasing sunlight with distance causes a challenge.

Solar Sailing

Planetary Society - Cosmos 1

June 21, 2005, launched on Volna rocket from a Russian submarine. Failed to reach orbit.

privately funded project.

Solar Sail feasible?• 10-ton payload, sail 1000 km x 1000 km in size maximum speed is then

only 0.04 c.• It would take roughly 75 years to get the nearest star (3 Ly away; ignoring

deceleration & stopping)

• Oops! The SAIL ALSO has mass!

• A 1000 km x 1000 km. A gold leaf sail 1 atom thick (a real sail would have to be much thicker) would have a mass of 170 tons (it effectively becomes the payload), and so the top speed is 0.009 c. Now it takes over 300 years to get anywhere!

• Continuous powering up the sail from Earth by using a focused laser is too expensive. Return trip? How to stop?

Interstellar Ramjets• Accelerating fuel is a problem. Then, what about collecting fuel on the way?

• Ramjetso Collect Hydrogen from the interstellar medium and fuse it (magnetic funnel for

fuel collector).o Need a scoop that is hundreds of kilometer wide Spaceship the size of worlds

Antimatter rocket (hypothetical)• Antimattero Exists: all matter has antimattero Matterantimatter annihilation: all mass released as rays (100% efficiency!)

(E=mc2 , only <0.7% of mass change to E in nuclear reactions)o Problem: controlled storage

• Edward Purcell : Imaginary antimatter rocketship with 100% engine efficiency…o maximum speed of 0.99c but still requires 14 time more mass in fuel than the

payload.o if we want to stop at the designation 14 times more fuel to stop 14×14 = 196 times more fuel

o for a round trip at 0.99c speed 196x196 more fuel ~40,000 than the mass of the payload!!

Even with a hypothetical antimatter rocket, interstellar travel is very difficult!

Speed Limit• Einstein’s special theory of relativity:

For anything with a mass impossible to travel faster than the speed of light

• Even at the speed of light, the nearest star is α Centauri at 4.4 light-years away.fastest round trip takes still 8.8 years!trip across the Galaxy takes 100,000 years!

• Could it be that Einstein’s theory is wrong and that we will someday find a way to break the cosmic speed limit?

Highly Unlikely! possible that a more comprehensive theory in the future may replace Einstein’s

relativity theory, but such one will be inclusive of many verified results including the speed of light barrier.

Travel Time at high speed

• Time dilationtime is different for high-speed travelers than for people stay at home

time runs slower at high speed!

€

Tship = TEarth 1− vc ⎛ ⎝ ⎜

⎞ ⎠ ⎟2

• A spacecraft traveling at 0.1c hit by an 1-mm grain (mass of 0.012 grams) Same energy as a 1-ton object hitting at Mach9.5 (7,000 mi/hr)!!

HAZARD of interstellar flight

Unless there is a way to screen out all interstellar dust, the spacecraft will

be easily destroyed!

Additional Energy requirement!! imaginary spaceship with a plasma shield

Interstellar Arks

OK, fast traveling is challenging. How about slow traveling over long time?

• Hibernationo How do we put people to sleep? hibernation gene?o How do we wake them up?

• Long life:o Pure speculationo Robotic mission would be simpler

• Multi-generational:o Perseverance in the mission and/or infightingo Loss of expertise

Human hibernation??• A Swedish man, Peter Skyllberg age 44, survived in a snow-trapped car for

2 months!• Experts believe that he might have been in a kind of human hibernation.

Feb, 2012

2 monthsat -30°C

Simulated long-term space journey• Nov 11, 2011 -- Six members of

Russian Mars500 simulator stepping out of the windowless capsule after 520 days.

• simulated the confinement, stress, weightlessness, and fatigue of interplanetary travel.

• long confinement put the team members under stress as they grew increasingly tired of each other's company.

Energy use of an Interstellar Ark• Velocity for interstellar travelo Escape velocity from Earth: 11 km/so Travel velocity, say 0.001c = 300 km/s, dominates energy requirement

• Kinetic energyo Mass: say 108 kg (105 ton) for 5,000 peopleo = 9 x 1018 Joules = 2.5 x 1012 kW hro 1% of the world annual energy consumptiono 250 billion dollars (@ $0.10/kW hr)o 1/5 of US GDP

o Add cost of provisions, energy efficiency, deceleration!

Hyperspace and Wormholes• Hyperspaceo General theory of relativity (1916): space is warped by gravity

• Wormholeso Rotating black holes connect to another flat spaceo Other flat space may connect to ours somewhere, but may be noto We will know only after we go through the wormholeo Stellar black holes: have too strong a tidal force, which would rip us aparto Massive black holes: only known in galactic nuclei, have to get there

In summary…

Important Concepts• Need to accelerate fuel also• Advantage of in-orbit launch• Various rocket types and their pros

and cons• Speed limit and time dilation• Difficulties of high speed space

travel• Difficulties of low speed space

travel

Important Terms• Mass ratio

Chapter/sections covered in this lecture : 13.1 & 13.2Terra-forming and Colonization : next class!