Triton Hopper: Exploring Neptune's Captured Kuiper Belt Object

The Triton Hopper is a mission concept study funded as a phase 1 effort by the NASA NIAC program. This work serves as the final report for this effort. Neptune's moon Triton is a fascinating object, a dynamic moon with an atmosphere, and geysers.1 Triton is unique in the outer solar system in that it is most likely a captured Kuiper belt object (KBO) - a leftover building block of the solar system.2 When Voyager flew by, it was the coldest body yet found in our solar system (33 K), yet had volcanic activity, geysers, and a thin atmosphere (see figures Figure 1-1 and Figure 1-2.) It is covered in ices made from nitrogen, water, and carbon dioxide, and shows surface deposits of tholins, organic compounds that may be precursor chemicals to the origin of life. At a distance of over 30 astronomical units, it would be by far the most distant object ever landed on by a spacecraft. The Triton Hopper effort set out to design a mission to not merely land, but repeatedly fly across the surface of Triton, utilizing the volatile surface ices (primarily nitrogen) as propellant to launch across the surface and explore all the moon's varied terrain. We have determined that such a hopper can be developed using simple methods of collection and propulsion. Gathering 100 kilograms of surface nitrogen ice with either a robotic scoop, or atmospheric gas using a cryopump provides sufficient propellant to allow this ~300 kg, highly instrumented lander to hop 5 km across the surface once a month, using a low-temperature radioisotope-thermal engine. More sophisticated propulsion could increase the flight range. Two years of hopping would allow this lander to hop 150 km and visit 30 sites! The final hopper design is shown in Figure 1-3. The concept of operations for the hopper is shown in Figure 1-4. While the phase I effort focused primarily on design of the hopper vehicle, not the delivery to Triton, a top-level analysis was performed to show that it is possible to deliver the vehicle from Earth to Triton and land it on the surface. Figure 1-5 summarizes the delivery trades. While use of nuclear electric propulsion would allow getting into low Triton orbit (and thereby minimizing the Triton descent propulsion system size), the baseline design choice was use of a solar electric propulsion (SEP) stage and aerocapture system, leveraging the investment in SEP by other NASA technology efforts, including the piloted Mars mission's use of SEP and aerocapture. Using the SEP and aerocapture system produced a launch and delivery concept similar to that explored in Mission Trades for Aerocapture at Neptune (R. Bailey, M. Noca, Mission Trades for Aerocapture at Neptune, AIAA-2004-3843, 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Fort Lauderdale, FL, July 2004). As such, the concept developed in the reference was assumed to be the delivery concept to Neptune. New trajectories to Neptune using SEP and aerocapture for a 2029 launch date were developed as was a notional mission and combined solid/biprop landing stage to get the hopper nearly to the surface. The baseline conops for the delivery to Triton is shown in Figure 1-6. The main focus of this NIAC effort was to determine feasible methods of gathering, processing and using in-situ propellant. According to Voyager the surface is predominantly nitrogen, in the form of both ice and snow form on the surface (Figure 1-7). Triton has a very thin atmosphere (~1 Pa), again mostly of nitrogen. Both surface and atmosphere sources were analyzed for propellant collection. Since nitrogen was accessible both frozen on the surface and in the atmosphere, it was selected as the easiest propellant to utilize and process. Four main approaches to gathering propellant were considered, each having various options (e.g., shovels or drills or scrapers). These methods are shown in Figure 1-8. The obvious choice is to gather the propellant in its frozen form on the surface. Since the science system requires a robotic arm with a scoop to gather samples for analyzing, a simple approach is to reuse this scoop to gather frozen nitrogen. Even at low temperatures, nitrogen ice is no harder than water ice (at 0 °C), and could be even easier to gather in the form of snow. However, the low gravity of Triton (~1/2 lunar gravity) and icy terrain might make it difficult to gather nitrogen at all locations. Additionally, the issue of other contaminants in surface ices needs to be addressed. An alternate source, the atmosphere, is available regardless of surface conditions, and could be compressed using a cryopump. In order to reduce power requirements for the cryopump it was determined that the cyropump should operate at very low temperatures and reject its waste heat (~90% of the 50 W of power) through a deployable arm to the surface thermal sink. Good conduction through the arm and a large opening (~10 cm diameter) are key to making the cryopumping option work. In the baseline design, both propellant gathering techniques were incorporated. The cryopump atmospheric inlet doubled as the funnel for frozen nitrogen from the shovel. Either system can gather the amount of nitrogen in roughly a week with 50 W or less of power. Once filled, a unique interior door system will seal the tank and the pressure will keep it sealed (Figure 1-9). Once the tank is filled, the propellant is heated to gas. Nitrogen gas has been used as a very simple, although low performance, propellant for reaction control systems. Besides the low Isp (~60 s Isp) the high pressure required makes relatively heavy tanks necessary, but such high pressure simplifies the propellant feed system. Temperature and pressure of the cold gas was optimized, trading off the tank mass against the engine performance. Approximately 100 kg of Nitrogen is vaporized and heated to 300 °K and 2000 psi in about 11 days using 60 W of heaters. Use of waste heat from the radioisotope generator was complex and was detrimental to power production, while use of RTGs would provide four times the waste heat but would also be more challenging to integrate to the low temperature platform during propellant collection. Heating the tank will also heat the propulsion system so it will not need to operate at extremely low temperatures. Use of additional propellant heating by a resistojet was also analyzed. This would improve the specific impulse (>100 sec Isp), but requires large amounts of electrical power, and the trade analysis indicated little or no net performance gain from this approach. Increasing the gas temperature from heat stored in a thermal mass could also provide performance. The thermal mass would be heated slowly over the week spent gathering propellant, and the nitrogen propellant is then passed over it during flight. While analysis of this approach showed that net improvement in performance could be achieved, with over 100 sec of specific impulse possible, detailed results were not ready until near the end of the project, and the simpler pressurized gas system was used for the Phase I design effort. The more advanced engine with improved Isp performance could quadruple the distance achieved per hop, so this approach will be further investigated in a Phase II. Given the cold gas nitrogen thruster approach the hopper can still hop surprising distances due to Triton's low gravity. Figure 1-10 shows a nominal hop. A thrust to weight of 4:1 was required by the trajectory. Six 220N cold gas thrusters were used, along with six 4.5N thrusters for reaction control. The landing sites will be mapped by the orbiter during flybys to allow mission controllers to program the hopper (with use of a IMU and startrackers) to precisely hop to a 100-m diameter landing zone. A combination of lidar and radar systems will provide soft landing and allow the hopper to autonomously avoid landing on debris > 10 cm in size or extreme slopes. The orbiter will also relay the estimated 1 Gb of data that the science suite gathered on the hopper and sent using a 10W x-Band communications system. Science \uf0a7 Narrow-field and wide-field stereo cameras. \uf0a7 Visible/infrared spectroscopy \uf0a7 Surface sample collection, with associated sample analysis by: \u2212 Microscopic imager \u2212 Thermal Evolved Gas Analysis \u2212 Gas chromatography \u2212 Mass spectrometry \u2212 x-ray crystallography \u2212 APxS \u2212 Organic sample analysis \u2212 Meteorology \uf0a7 In-flight atmospheric sampling and analysis \uf0a7 Ground-penetrating radar \u2212 Look under surface for geyser sources \uf0a7 Seismometer The huge of the impact of such a hopper is clear when considering that it replaces 30 single-site landers on Triton's surface! Even a Triton rover would be hard pressed to cover 150 km in two years (for comparison, the MER rover Opportunity covered 42 km in 11 years.) This report will go into detail on the trades and point design created for the Triton Hopper. A summary of the requirements, assumptions, design, and trades to be discussed is shown in Table 1.1. The conclusion of the design study was that it is possible to design an innovative hopper vehicle propelled by in-situ resources that could accomplish a roving mission on Triton, a captured Kuiper belt object of high scientific interest. The approach developed for the Triton hopper concept can be adapted utilized on other bodies, whether icy or not. By the use of cyropumps grounded to the cold surface even thin atmospheres can be pumped to create propellants. Ices are even easier to collect and process given the right tools. Phase II efforts plan to explore the higher performing high temperature thrusters, lightening the lander, and/or speeding up propellant collection/processing in order to extend hop distance and frequency. A complete launch and delivery analysis to Triton is also planned. Finally, use of a Triton-type Hopper on Pluto should be analyzed.