NeTE - THE NEPTUNE TRITON EXPLORER

Alpbach Summer School 2012Team Red

SOLMAZ ADELI, MARKUS ARZT, TATIANA BOCANEGRA BAHAMON, VINCENT BONNIN, COLM BRACKEN,

ALEXANDER BROBERG SKELTVED, CHRISTOPH BURGER, ALEJANDRO CARDESIN MOINELO, ELIE

DAWIDOWICZ, ROMAN FERSTL, MADELEINE HOLMBERG, ALEXANDER HYGATE, CHRISTOS LABRIANIDIS,

KATALIN LUKÁCS, ALBERTO NARDIN

Introduction

The Neptunian system provides the opportunity to investigate two intriguing Solar System objects: Neptune,as an archetype of an icy giant, and its moon Triton, as a possible Kuiper Belt Object (KBO). Our present dayunderstanding of Neptune and its planetary environment is based on the measurements performed during theflyby of the Voyager 2 spacecraft in 1989. To this day it is the only spacecraft that has visited this enigmaticsystem. Amongst many other findings, Voyager 2 discovered auroras, four rings and six new moons. Italso showed Neptune’s atmosphere to be very dynamic with wind speeds of up to 2,000 km/h and withanticyclone formations like the Great Red Spot on Jupiter (Suomi1991). The Voyager 2 flyby proved to bevery successful, but left us with many questions yet to be answered. NeTE will be dedicated to explore theNeptunian system and answer many of these questions.

1 Science objectives

NeTE will characterize Neptune as an archetype of an icy giant. NeTE will be the first mission solelydedicated to study and orbit an icy giant. The orbit around Neptune will take NeTE to less than 1.2 Neptuneradii (RN) from the center of the planet. It will primarily focus on measurements which will help mapping thestructure of the planet as well as the surrounding environment. The orbiter will do remote observations of theatmosphere using the wide and narrow angle cameras to monitor the global climate and meteorology (cloudmorphology and wind velocities), perform detailed measurements with the Visible, near infrared (NIR) andultraviolet (UV) spectrometers (in order to investigate the composition of the atmosphere), radio occultationsin the X band (to obtain the vertical density and temperature profiles), ion and neutral mass spectrometer(to investigate the ionosphere interaction with the magnetosphere) and gravity experiment (to measure thegravity field and constraint the models of internal structure). NeTE will also carry the entry probe Hypate,further described in Section 3, which will carry a mass spectrometer, gas chromatograph, photometer and anatmospheric instrument package in order to study the Neptunian atmosphere in detail. Furthermore, NeTEwill carry a magnetometer in order to measure the magnetic field of Neptune, which is close to a shifted andtilted dipole.

NeTE will shine new light on solar system formation models and will provide detailed con-straints on the formation of icy giants Investigating the abundancies of nobel gases, in particular theNe/H2-ratio, will test and improve the current planet formation theories for icy giants. NeTE will performgravity experiments to put constraints on the internal structure which is related to the formation of solarsystems. The discoveries of NeTE will in the end give us the opportunity to better understand the formationof our Solar System. Studying the Neptunian environment will also help understand the formation of distantsolar systems. Exoplanets of similar mass have been observed and may be Neptune like planets at an earlierformation stage. The distance from Neptune to the Sun make the conditions surrounding the Neptunianplanetary system very similar to what is experienced by many known exoplanets. In addition NeTE will pro-vide important new information which will increase our understanding about the formation and migration ofexoplanets.

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NeTE will improve the understanding of the planetary environment of Neptune The Neptu-nian magnetosphere is unlike any of the other magnetospheres investigated to this day. The large axial tiltbetween the rotational axis and the magnetic dipole axis creates a very dynamic system. The polar cusp altersbetween an open cusp in the sunward direction and Earth-like magnetospheric configuration in just ∼8 hours.Investigating this variable system in situ will improve the understanding of the Neptunian magnetosphereas well as magnetosphere structures in general. The magnetosphere will be investigated with a fluxgate andsearch coil magnetometer (to map the magnetic field and test present day dynamo models), a plasma andradio wave package (to obtain plasma densities, temperatures, ion velocities, Neptunian Kilometric Radiationwhich will give better accuracy on the internal rotation rate of the planet), dust detector (for size-distributionand the distribution of cosmic dust) and an ion and neutral mass spectrometer. NeTE will also investigatethe Neptunian rings, moons and the interaction of Neptune and its planetary surrounding. Especially themoon Triton is of great interest.

Triton, potentially a Kuiper-belt object Triton is the largest moon of Neptune with a diameter of2,705.2 ± 4.8 km (Stone1989). It has a thin atmosphere dominated by N ice particles (Cruikshank1995) andspectroscopy from Earth has shown condensed N and CH4 on the surface (Cruikshank,1984). The othermolecular compositions observed on the atmosphere of Triton are CO22, H2O and H2S. Triton and Plutohave similar compositions, dimensions and mean densities, suggesting a similar or common origin. In additionthe retrograde orbit of Triton suggests it is of external origin, possibly from the Kuiper belt.

The origin of Triton is still an open question which could be answered by more precise and frequent remotesensing studies. The NETE mission will provide an investigation of the atmospheric and surface compositionof Triton. The IR, Visible, UV and mass spectrometers on-board NETE offer a unique opportunity to studythe presence of N , NH3, C, H and H2O. Furthermore this body can be compared with Pluto and possiblythe other Kuiper-belt objects.

Cryovolcanism, the geological activity on the southern pole of Triton Photogeological analysisof images taken by Voyager 2 revealed Triton as a frozen world with a geologically fresh surface (Cruik-shank,1995). The evident traces of deformations and melting/flooding episodes seen upon the icy surfaceof Triton may refer to volcanism and tectonic activities (Bazilevskii1992). In the south polar cap a set of”cryovolcanic” landscapes, such as dark spots assumed as plumes, apparently produced by icy-cold liquidserupted from Triton’s interior.

Composition of ejected material from the plumes, particle size, seasonal variation of cryovolcanism activ-ity, interaction between surface and atmosphere, evolution of Triton’s surface morphology and composition,its topography and relative age, planetary interior activity, the surface morphology of northern hemisphere,existence of a potential subsurface ocean, habitability condition are the questions that will be addressed inthe NeTE mission. The NeTE orbiter will carry narrow and wide angle cameras; IR, Visible, UV and massspectrometers; and a subsurface radar, that would be used to investigate the geology and interior structure ofTriton. Currently the surface of Triton is completely or partly covered by materials ejected from the plumes.Remote sensing would therefore not be adequate for measuring the composition of the surface before thedeposition of cryovolcanic materials. This goal will be reached by using an impact probe as explained below.

Triton Impact Probe (TIP) The Triton Impact Probe is planned to hit the southern pole, close to aplume, based on the investigation of the geological context of this area by at least 5 flybys. The releasedenergy by impact of a 50 kg-weight probe with the Triton’s surface is around 1.28 ∗ 109J (equal to an ex-plosion of 305 kg of TNT). This impact energy on a pure nitrogen surface could release 5.6 ton of volatileswhich would be detectable by spectrometers and cameras on-board the NETE orbiter. This value for a pureammonia surface is 0.7 ton. This huge amount of released volatiles will allow a characterization of thecomposition and stratigraphy of subsurface materials. The high-resolution camera on-board the impactorwill acquire images of the surface features from orbit to impact.

Planetary protection plan In the report of ”COSPAR workshop on Planetary Protection for Outer PlanetSatellites and Small Solar System Bodies 2009”, Triton is classified in the same category as Titan, which iscategory II+. As the cryovolcanic activity in the southern pole of Triton may present an environment forresistant microrganisms to contaminate the potential subsurface ocean, further precautions over and abovethose required for category II+ would be taken. Therefore, the Triton Impact Probe will follow the require-ments for Europa lander (category IV) which requires to reduce the probability of inadvertent contaminationto less than 1 ∗ 10−4 per mission (see COSPAR report 2009).

Jupiter gravity-assist Since the spacecraft passes Jupiter in 2032 for gravity-assist various observations,

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measurements and a system calibration will be performed in the Jovian system. A fly-by distance of ap-proximately 8 RJup will, depending on the definite constellation and available power, allow for investigatingJupiter itself (atmosphere, magnetosphere) and the 8 inner moons. Imaging in the optical and infrared willprovide data produced by the latest generation of instruments. Current plans give the possibility of a fly-byat Europa, but definite parameters might be changed slightly. With planetary protection level 4 those issuesare treated as well.

Payload Improvement in payload technology would improve the accuracy of the measurements but is notnecessary for the scientific objectiv. The majority of instruments are of Juice heritage. Therefore the massmaybe a bit lower due to less needed shielding but the exposure times will be rising due the less illuminationif the instruments would be exactly the same.

Structure Instrument Measurments Ranges Known values Mass, Power, TM

Orbiter

2 Langmuir probes

Electron density 10−4 - 105 cm−3 ∼ 1 cm−3 1kgIon density 0.1 - 105 cm−3 ∼ 1cm−3 1kbpsElectron tem-perature

0.01 - 20 eV

Ion temperature 0.01 - 20 eVSpacecraftpotential

± 100V

Ion drift speed 0.1 - 200km s−1

Electric field DC 100V

Radiowave

Voyager (< 1362kHz)

100kHz - 45MHz ∼ 2MHz 1.5kg

Voyager 2 (3 - 60GHz)

100kbps

Plasma densityρ

0.001 - 106 cm−3

Temperature 0.01 - 20 eVLow frequencyfluctuations

0.1Hz - 4kHz

Search coil magnetometer Low frequencyfluctuations ofthe magneticfield

TPD 1kg, 1kbps

Narrow angle camera High resolutionimager auroras

∼ 1 mdeg 10kg, 15W, 75kbps

Wide angle camera Field strength ∼ 10 mdeg, Orbit< 10,000km

4.5kg, 3W, 5000kbps

Flux gate magnetometer Field strength 0 - 100,000nT, res-olution 2nT, Widevariety of orbits

Voyager:100,000nTat surface

1.8kg, 2W, 5000kbps

Radar antenna Subsurfacestructure at lowresolution ∼1km

1kHz - 25MHz 10kg, 20W, 300kbps

Visible-IR spectrometer Composition ofatmosphere

λ = 400 - 5200nm,δλ = 2.3nm @< 1.7µm, δλ =5.8nm @ > 1.7µm, IFOV: 0.125- 0.25m rad, FOC:3-4◦

17kg, 20W

UV-Spectrometer Compositionand dynamics ofthe atmosphere

λ = 50 - 230nm,IFOV: 0.01m rad,FOC: 2◦

4.5kg, 4.4W

Sub millimeter wave instrumentSpectral range 200 - 500 µm < 580m s−1 9.7kgWind velocity FOV < 0.2 deg 39W

Doppler Wind 1 m s−1 10kbps

Ion neutral mass spectrometerElectron en-errgy

15keV - 1MeV 6kg

Ion energy 3keV - 5MeV 27WENA 10eV - 10keV 1.5kbps

Radio science transponder2 way Dopplerwith Ka-bandtransponder

TPD 2.5kg

Ultra stable os-cillator

0.1Hz - 4kHz 26W

ENA 0.1Hz - 4kHz

Probe

Gas chromatograph mass spect. Composition,abundances ofnoble gases

TPD 5kg, 10W

Doppler wind experiment Atmosphericwind velocities

TPD 1.5kg, 18W

T and p profile instrument Temperatureand pressurestructure

TPD 0.5kg, 5W

Photometer Absorption TPD 0.3kg, 2WAccelerometer Density profile TPD 0.4kg, 5W

Impactor Low resolution camera Surface images TPD 6kg, 30W

Table 1: Payload of Orbiter, Probe and Impactor for the NeTE mission.

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2 Mission Scenario

Our orbit design started from the specification of numerous scientific requirements. In order to comply withthose, certain trade-off had to be made regarding time of flight (ToF), and spacecraft dry mass.

Mission Profile Requirements

• The Earth-Neptune trajectory propulsion system shall be chemical.

• The selected launcher shall be selected among ESA’s portfolio.

• Gravity assists swing-bys should comply with required clearances (i.e. atmosphere, radiation).

• The orbiter shall accommodate the instruments for their intended science purpose.

• The spacecraft shall accommodate a Neptune atmospheric probe.

• The spacecraft shall accommodate a Triton impacter.

Earth to Neptune Trajectory

The transfer orbit scenarios were firstly assessed using existing literature studies and by Tisserand graphicalapproach. From these, different configurations were further investigated by performing trajectory optimisa-tion with the software tools Pagmo/STK. Tradeoffs between launch mass versus time of flight were computed.Out of those results, a VEEJ sequence of gravity assists was selected.

Sequence of events

The mission timeline is listed in Table 2.

(a) Table 2: Mission timeline. For the time column: ‘L’= launch,‘I’= insertion at pericenter.

(b) Earth to Neptune transfer orbit with gravity assist in VEEJ

Neptunian system orbit selection

One approach is to insert a high energy orbit which will require a lower impulse, hence saving propellantmass. This influences also the orbit period. Trade-offs were computed between period and burnt propellantmass for the orbit insertion. In order to fly by Triton, a 1:5 resonant orbit to Triton was chosen, with a periodof approximately one month (Triton period ∼ 5.8days). This orbit allows to take the best out of the save

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Orbit parameters Valuerp 28500 kmra 2.0448 × 106 kmT 29.5 daysInclination 60◦

Table 3: Relevant values of orbit transfer design.

in mass, while complying with the desired science Neptune altitude requirement (∼ 4000km) and enabling abigger window for data transmission. The final orbit parameters are given in Table 3.

The probe separation with the spacecraft occurs 101 days before atmospheric entry. The atmospheric entryis defined 600 km above the 1 bar pressure level. The probe enters atmosphere at hypersonic speeds in astable and aeroshell-face-down-position and gradually slows down until reaching Mach 1, 60 seconds afterentry. The parachute and front shield release procedure takes 30 s, the altitude level is ∼ 1 bar. Then anhour of descent gets the probe from 1 to 10 bars where the back cover and drogue parachute are released andthe probe continues it free fall descent until levels up to 100 bars it would reach 30 minutes after back coverrelease. During entry and descent, the spacecraft probe range does not exceed 80000 km and the apparentspacecraft elevation from the probes is at least 60 degrees.

As for the impactor, different Triton approach scenarios were considered. A definite scenario would be selectedout by mapping Triton using a couple of flybys before releasing the impactor. An optimisation of the orbitfor Triton fly-by should be performed (current phase angle is not favorable for measurements).

(a) Table 4: Relevant values of orbit transfer design. (b) Earth to Neptune transfer orbit with gravity assist in VEEJ

Figure 1: The solid line shows the provided thrust by the main engine we will use and the dashed line shows the old main engine thrust.Therefore the additional propellant mass due to gravity losses drops from originally 785kg to 260kg.

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3 Mission Structure

Structure The S&M subsystem has to fulfill various tasks: a) to provide a mission-oriented, force-adaptedprimary structure, b) to provide a secondary structure including attachments for subsystems, payload andinstruments, c) to provide adequate mechanisms for deployment and separation processes. The drivingparameters of the system are the environmental conditions during the launch (steady state accelerations,dynamic acceleration, resonance frequencies, acoustic noise as well as temperature etc.), mass-reduction andcompactness as well as the adequate mission-oriented positioning of the payload. A central tube configurationshall be used as primary structure. The central tube contains mainly the 2 main spherical propellant tanks;its outer diameter is defined by the launch adapter ( d=1660 mm. A ground plate provides additional shearstiffness against bending forces during the launch and the firing of the main engine. A supporting aluminumalloy truss structure, which also holds the HGA, builds the upper shear stiffness-providing structure. Thecentral tube serves additionally as a secondary structure and provides attachments for subsystems (such asthe RTGs, reaction wheels, attitude control thrusters etc.). A main octagonal payload bay comprises all thesubsystems and provides appropriate attachments possibilities for the payloads. The technology used forthe structural configuration is flight proven and has a TRL=9. The spacecraft has an overall dimension of4.4m x 4.5m (diameter X height in undeployed configuration, height measured from launch adapter). Mostof the subsystems are structurally designed in frame configuration with an adequate thickness in order toavoid radiation harms on electronics. Various mechanisms are required for the fulfillment of the mission.Booms are used for the Langmuir probes, antennas (radar, E-field) as well as for the magnetometers; eachof them is driven by a spring mechanism, which is initiated by a pyrotechnical device (high reliability).Special separation mechanisms are used for the separation of the attached atmospheric and impact probes(pyrotechnical bolt cutters, TRL=9).

HYPATE entry probe has a structural configuration as defined in the PEP-study of ESA. Figure illus-trates the configuration.

MESE Triton impactor is made of a simple rectangular frame structure made of EN-AW7075-T6 with anadequate thickness In order to avoid radiation harms. It comprises a camera system, a tetrahedral reactionwheel configuration with g-sensors, a secondary battery, an OBDH and an S-band antenna.

Thermal Control System The TCS provides a stable thermal environment around 280 K inside the space-craft. The High Gain Antenna, painted with white epoxy, shields the spacecraft sufficiently in the innersolar system, while a cover with 15-layers MLI reduces thermal absorption and emission. If required internalheaters (460 W) provide power in addition to thermal dissipation and radiators on the surface (420 W) givethe possibility of radiating spare RTG power (including double redundancy each). The thermal circuit willconsist of two toroidal heat pipes for the payload-section and spiral-shaped ones for the cylinder sectionto distribute the generated heat. Louvers have been avoided in order to increase reliability of the TCS.Rough thermal calculations resulted in a constraint of non-MLI-insulated spacecraft area of 1.73m2, used forthrusters and instruments. The driving case occurs during communication at Neptune according to the poweroutput of the antenna. For scenarios of too high internal power output (Triton/Jupiter flyby) the spacecrafttemperature will be increased by 15 K at most. Furthermore, using the thermal losses of the RTG’s canprovide additional energy if the final design makes it necessary. The TCS has been designed for a 24 yearmission but allows for 3 to 4 additional years due to temperature margins.

Propulsion The propulsion system has to fulfill the following tasks: a) provide adequate ∆v to ensure thatthe required change of orbit can be achieved by the spacecraft, b) facilitate the correction of orbit (DSMs)during the interplanetary trajectory transfer, c) insert the vehicle into Neptune’s orbit and change its altitudeand inclination, d) facilitate the separation of the atmospheric probe and the impact probes, e) desaturatethe reaction wheels. The driving parameters of the system are the required ∆v for the orbital maneuvers andattitude control sequences ∆vreq=1700 m/s as well as the required thrust F. A bipropellant propulsion system(MON/MMH) shall be used for the NETE-mission in a blow-down configuration. A 1.1 kN main engine shallbe used in order to minimize the gravitational losses during the maneuvers (flybys, orbit insertion); it shallbe integrated on a gimbal mount, in order to compensate off-axis movements driven by COG drifts duringthe flight. The AOCS thrusters are also fed by the system system. Spherical tanks made of TiAl6V4 shallbe used for both fuel/oxidizer (required tank pressure p=22 bar in bladder tank configuration) and helium(required tank pressure p = 250 bar), in order to minimize weight and optimize the propellant feed. Fullsingle redundancy shall be provided for the entire feed system (valves, filters, mass flow controllers, pressureregulators, pressure transducer etc.). The technology used for the propulsion configuration is flight proven

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and has a TRL=9 (exception: main engine with TRL1/2).

Attitude and Orbit Control System NETE’s AOCS has to fulfill three major tasks: a) attitude deter-mination, b) computation of attitude deviations and required rotation angles depending on preprogrammedoperational modes, c) active rotation of the spacecraft. The driving parameters of the system are the mini-mum required rotational angular rate of the spacecraft ∆φ̇req = 0, 054deg/s (in 3 axes) as well as the pointingaccuracy of ∆φreq=23 arcsec (spacecraft-Earth optimum telecommunication connection). Two star trackers(single redundancy in each axis), six fine sun sensors (single redundancy in each axis, special design forNeptune orbit distance has to be taken into account) and one inertial measurement unit shall provide therequired attitude determination (relative sun position, rotation angles and angular velocity) with an accu-racy of ∆φ=0.1∆φreq resp. φ̇=0.1∆ ˙φreq. The AOCS controller calculates the required output signals andtransmits them directly to the four reaction wheels (single redundancy in each axis in tetrahedral configu-ration, Figure), which are compensating the computed attitude differences. The OBDH defines the actualoperational AOCS mode (7 different AOCS modes have been accurately defined) and therefore the requiredangular position. It also determines the actual momentum of the wheels and determines if a desaturation ofthe reaction wheels is required, which has to be done by 12 monopropellant thrusters (single redundancy ineach axis). The technology used for the AOCS configuration is flight proven and has a TRL=9.

Electronic Power System The amount of energy we needed to provide for the space craft was constrainedprimarily by the extreme case of an fly-by near Triton. There we only have around one hour of possibleobservation and therefore need all the instruments working at the same time, which leads to a peak powerEOL of 332 We.To provide this energy we were considering different energy sources and primarily due to the long life timethe system should be designed for we decided to use 4 MMRTGs. The energy production dropped in Voy-ager II after 21 years to 68%. Suppose linearity the energy loss per year is roughly 1.5% mainly due to thephosphorus doping of the TECs. By development we hope that we can lower this number to 1.3% and endup with 300We after 2 year mission in the Neptune system. This solution is more massive than ASRGs butcheaper and more reliable. Another advantage of the MMRTGs is the already controlled Voltage output of28V but due to the long mission time we still decided for a PCDU unit instead of a PDU.The primary energy source is supported by 1kg of Li-Ion secondary batteries to cope with Voltage maximums.We also need 27kg of Li-Ion primary batteries for MESE (1kWh) and HYPATE (0.3kWh).

On Board Data Handling The OBDH sub-system is a crucial element of the spacecraft, since it performsall data processing, routing, and storage. It consists of a central computing unit and a spacecraft-wide databus system, providing data interfaces to the other subsystems. The OBDH has to fulfill the following tasks:a) provide a data bus system with prioritized data routing for all sub-systems, b) provide different operationalmodes in order to accommodate varying requirements to the OBDH in different mission phases, c) provide areference system time for scheduling events along the mission timeline, d) provide a standard data protocolfor data transfers among sub-systems, e)provide storage for all science, telemetry, control, and system data,f) handle all TT&C and payload data, route it and store it. The OBDH contains a solid state mass storageunit with a memory size of 8 Gbit. All three computers have a direct data connection to this mass storage.The write access is controlled by the memory controller which is connected upstream of the mass storage.Highly ruggedized mass storages for deep space environment will be used. The PDH has a mass storage unitwith a maximum memory size of 256 Gbit. The two computers have a direct data connection to this massstorage. The MRC monitors the OBDH and PDH processes and controls the system redundancy; controllerresets are performed if a controller is consistently producing differing data. A CAN bus system providesthe interconnection between the different subsystems and OBDH/PDH components, a parallel bus transfersdata to the mass memories, a MIL bus is used for the payload data transfer because of the high data rateproduced by the camera systems. The technology used for the OBDH/PDH configuration is flight provenand has a TRL=9. Due to the time-critical mission profile, it is essential to use a real-time operating systemon our computers. We have chosen VxWorks by Wind River, which is commercially available, and proven inseveral deep space missions. It has been used by NASA and ESA. For minimizing the BER, error detectingand correcting codes will be used. To achieve a higher compatibility a communication protocol, conform toCCSDS, will be used.

Telecommunication

Orbiter - Earth The NeTE telecom system includes S, X, and Ka Band components. The S Bandfrequencies will be used with two LGA for emergency mode and are also required for relay between theorbiter and planetary entry probe (PEP); S-Band should be capable of penetrating Neptune’s atmosphere at

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the desired science altitudes (at least 20 Bar). The Ka band will be used for the transmission of Science datato Earth (downlink) and housekeeping while the X band will be required for safe mode (all phases). The Kaand X band links employ 60W traveling wave tube antennae (TWTA) in combination with a fixed 3.5m highgain antenna (HGA) (58.8 dB). This arrangement will be capable of returning 7 Gbits of data per orbitalperiod of 1 month to the 35m ESA station located at Cerebros, Spain. This was determined by calculating alink budget based on 60W of RF transmitted power (120W input), using realistic efficiencies and with a 6dBsignal to noise ratio (SNR); this includes a 3dB margin. The above data volume was also based on 8 hoursof downlink per day to the Cerebros station. The Ka and X-Band links will also be used with a mediumgain antenna (MGA) to provide Earth communication up to LEOP stage and in place of the HGA duringphases that bring the spacecraft close to the sun; the HGA will be used as a sun shield for these phases.Redundancy is provided on all elements except the MGA and HGA as both the MGA and HGA have verylow failure risk.

HYPATE - Orbiter - Earth The data relay subsystem between the probe and orbiter will provide aone-way communication link from the probe to the orbiter. There is full redundancy on the transmitters inthe probe and the receivers in the orbiter. During descent each transmitter sends data, via its own antenna,from the probe instruments to the orbiter over the S band. The orbiter then stores the data in its onboardmemory for later transmission to Earth during downlink time described above.

Mission Development Plan

The NeTE Definition Phase A system study is expected to start in 2016 for a period of about 24 months,with the objective to start implementation phase in 2018. The planned launch would be carried out in 2026,resulting in an arrival at Neptune in 2048. Technology activities will be initiated in parallel to the definitionphase, and will provide input to the system study.

The implementation phase would last slightly less than 6 years, with a contingency margin of more than oneyear, considering the importance of the launch opportunity window. The prime contractor for the missionimplementation will be chosen for phase B/C/D through open competition. The instrument developmentwould follow the spacecraft development and implementation schedule, with an AO being issued in parallelwith the ITT for the Definition Phase.

Item Detail Costs [Me]

Launcher Ariane 5 175

Spacecraftdry platform 1100kg 300propellants 2300kg

Payload 83kg 200Probe 300kg 200

Impactor 50kg 100Total power 4 MMRTGs 220

Specific needsMMRTG development 10

probe development 20

Time dependent costs22a cruise duration 120

2a operational phase duration8h/d ground based antenna

Science operation 70Project management 100

Sum including 10% margin 1660

Table 2: Cost estimation of the NeTE mission.

Mass budget

The characterization of the Neptunian system demands a payload mass comparable to the Juice mission. Theinvestigation of the atmospheric composition of Neptune longs for a probe and Triton surface analysis requiresan impactor to fulfill scientific measurements. From the technical point of view the needed redundancy andthe impossibility of solar arrays in the outer solar system drove the dry mass budget.Including harness and margins NeTE will have a dry mass of 2.3 t and a propellant mass of 2.3 t. With Ariane5 ME we can launch 4.8 t to the earth orbit. Therefore NeTE is roughly 0.2 t lighter than the maximumpossible mass.

Risk Management

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There are three hazardous single point failures in this mission design: (1) High Gain Antenna malfunction,(2) main engine failure and (3) launch window slips. On balance (1) is highly mitigated due to TRL 9classification and very low probability of occurrence. To minimize the risk of (2) the engine can be studied,designed and tested in order to maximize reliability. Moreover, in case of failure, a flyby of Neptune is stillsufficient to release the probe and achieve main scientific goals. Eventually, if case of (3), due to delays, themission can be easily converted into a Uranian System exploration mission with very similar primary scienceobjectives regarding icy giants.

Public Outreach

NeTE is expected to attract broad public interest. Hence, the mission will be given adequate exposure withinthe communication activities of the Science Program. ESA would have the overall responsibility for planningand would be coordinating with national agencies activities around key milestones and major achievementsof the mission. Such outreach activities will be supported by the members of the SWT with the publicationof scientific results and high level data products. Moreover, this long term mission would federates an entirescientific and engineering community throughout a generation. As for ESA education mission, NeTE wouldinspire younger generation and rise interest in space activities.

Scientific Conclusions

The proposed NETE mission will return invaluable scientific data for Neptune and its moons and rings. By themethods described in this paper we will acquire detailed information pertaining to the chemical composition ofNeptune’s atmosphere, its internal structure and the planets magnetic and gravitational fields. These data areessential in order to constrain current formation models of ice giant planets and will be of great importance forunderstanding Neptune-type exoplanets. Neptune’s ring system will be accurately characterized in terms ofparticle size, number density and composition; these parameters are critical for understanding the origin andevolution the rings and nearby moons. Also, by observing Triton over unprecedented timescales and spatialresolution, and by comparing Triton to the other moons of Neptune in terms of ice to rock ratios, we willcollect the necessary data for constraining formation theories for Triton and its possible origin in the KuiperBelt. Furthermore, valuable information on the internal structure of Triton will be gained by analyzing themoons cryovulcanism and geology. The design of this mission has been driven by these longstanding questionsregarding the Neptunian system. Eventually, NeTE could be a major milestone of ESA Cosmic Vision byachieving challenging scientific goals and giving a single long term goal to European space industries R&Dinvestments. NeTE would consolidates ESA as a world space leader for space exploration and science.

Bibliography

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NeTE : Neptune Triton Explorer Key mission goals Characterize Neptune system as an archetype icy-giant

Provide key science information on the formation of icy-giants and derive constraints on the evolution of the solar system and Neptune-like exoplanets

Characterize Triton as a potential Kuiper Belt Object and investigate its origin

Develop European capabilities for long duration missions to the outer parts of the solar system

Payload : Orbiter, Probe, Impacter

(O) : Narrow angle/wide angle camera, sub-surface radar, VIS-IR Spectral imager, UV spectrometer, sub-millimeter wave sounder, plasma package (Langmuir probe, radio wave instrument, Ion and neutral mass spectrometer), cosmic dust detector, flux gate/ search coil magnetometer, radio science transponder with ultra stable oscillator (P) : Gas Chromatograph Mass spectrometer, Doppler wind experiment, temperature and pressure profile instrument, photometer, accelerometer (I) : low resolution camera

Mission profile - 27/08/2026 Single Ariane-5 launch from Kourou - VEEJ Gravity Assist sequence - Probe release and separation (20 days before orbit insertion) - 60° inclination orbit insertion - 19/4/2048 Neptune 1 month orbit and Triton flybys (5/month) for2 years - Impactor release when sufficient Triton topographic data (in plumes) - Total v = 1.2 km/s

NeTE Orbiter Hypate Probe Mese Triton Impactor

Stabilisation 3-axis Spin Spin - Earth Escape

- VEEJ Gravity Assists - Neptune Orbit insertion - Neptune observations - Triton flyby observations - Relay link during probe entry and impactor descent

- Release before insertion - Atmospheric entry (up to 100 bar) - In-situ measurements during descent - Remote observations from NeTE before entry

- Release before fly-by - High velocity descent and impact on Triton surface (7 km/s) - Observations during descent - Remote observations from NeTE before and after impact

Power 440 W (MMRTG x4) 416 W (batteries) 120 W batteries Communications Ka-, X- and S- band S-band S- band

Data volume (comp) 7 Gbit/orbit (month) 20 Mbits 10~50 Mbits

Payload 83 kg 10.25 kg 6 kg

Dry mass 2047 kg 321 kg 55 kg

Wet mass 4348 kg

Launch mass 4725 Kg (incl. launcher adapter)

Ground station 35m ESTRACK Station 8 hours/day ESA Cost at Completion estimate

1500 – 1700* M€ 200 M€ 100 M€

Complete mission : 1.80 – 2.0 B€

Key Capabilities: - Long lasting electronics - Power budget at arrival - Thermal control in cold Neptune environment - Long distance TT/C and data downlink - Technology for entry probe and impactor

* According to R&D progress in key capabilities.

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