A new epoch in the robotic exploration of the Solar System has begun and it promises new and unexpected discoveries. The Mars Exploration Rovers (MER) and Mars Science Laboratory (MSL) missions present novel and exciting findings and observations, yet these missions are only the beginning a new era of detailed, in-situ exploration of Earth’s planetary neighbors.
The next generation of scientific missions to Mars and other bodies in the Solar System requires technology advances in six key areas:
- Entry, descent, and landing (EDL): JPL is looking to increase its current capabilities in EDL to larger scales, higher speeds, greater precision, and higher unit loads for planetary entry, as well as to develop capabilities for higher-density atmospheres like Venus and Titan while providing greatly improved landing precision on such bodies.
- Close Proximity Operations (Prox Ops): Exploring comets, asteroids and other small bodies requires technologies and capabilities different from those used for planetary operations. Formation and constellation flying, differences in gravity, the possibility of ejecta and uncertainty regarding the behavior of these small objects poses a new set of challenges to close proximity operations.
- Mobility: Advances in this area could extend existing capabilities to provide rovers, helicopters, submarines and boats, with greater levels of autonomous navigation. It could also help provide greater range and speed, and help develop capabilities to explore the atmosphere, oceans and hazardous terrain.
- Sample acquisition and handling: Technological developments are necessary to improve and extend existing capabilities to capture and dexterously manipulate subsurface and surface samples.
- Autonomous orbiting sample retrieval and capture: The capabilities needed to achieve the long term goal of sample return to Earth from an extraterrestrial body requires technological innovation. Researchers are reviewing these capabilities to identify and meet the technological challenges in this field.
- Planetary protection: This area is essential to enabling uncompromised and safe exploration of planetary bodies in our Solar System that may harbor life.
Gathering in-situ scientific observations and data will require increasingly capable spacecraft, planetary landers and rovers. These spacecraft will be more complex than their predecessors. Earth’s atmosphere and atmospheric drag limit the diameter of launch vehicles, which in turn will limit the diameter of entry heat shields for planetary spacecraft designed to land. The continually increasing mass of planetary landers increases the per-unit-area heat load and force borne by heat shields and parachutes. The dense atmospheres and higher entry velocities at bodies like Titan and Venus will exacerbate these effects and heighten the need for technological advances to provide mission-critical capabilities.
The most compelling scientific questions often require investigations of sites on other planets that are inhospitable to a safe landing. As a result, rovers that are more capable in every way are needed to go farther more rapidly, into more hazardous terrain, autonomously, using less power. On some bodies, these requirements may result in the use of lighter-than-air (buoyant) vehicles, penetrators, or submersibles instead of rovers. Once a rover reaches a scientifically interesting site, it must obtain a specified sample and prepare and present that sample for scientific analysis.
In addition to substantial technology advances needed in the areas of EDL, mobility, and sample acquisition and handling, a major goal is to develop the capabilities that would enable a rover to launch a sample-bearing canister into space, enable a spacecraft system to detect the canister, rendezvous with it, transfer that sample to an Earth-return vehicle, and finally, return to Earth. This will require significant technology advances in a wide spectrum of disciplines. Any mission that would enter and operate within the atmosphere or on the surface of the Solar System’s extraterrestrial bodies must protect that body from biological contamination by the visitor from Earth. Therefore, spacecraft must be designed with planetary protection-related requirements in mind. To do so, improved sterilization capabilities drawn from the planetary-protection discipline are necessary to meet these requirements affordably and practically.
Current Research Areas
In-situ planetary exploration will be enabled by a set of capabilities that are key to future planetary and small-body exploration, whether of the inner planets or the outer planets. Validating these capabilities and making them available to these missions will be both technically difficult and resource intensive, and will require a thoughtful, long-term, sustained, and focused effort.
Entry, Descent, and Landing (EDL)
EDL is made possible by a broad spectrum of related technologies, including:
- the design and fabrication of a mass-efficient heat shield
- the design and deployment of a supersonic parachute
- the use of advanced navigation and guidance to reduce the size of the landing error ellipse
- the use of sensors during planetary descent to identify and avoid hazards in the landing area
- the design and implementation of a mass-efficient propulsion system to control the spacecraft attitude and rates during all phases of the descent
To date, heat shield and parachute designs for robotic missions derive from designs qualified for NASA’s Viking and Apollo programs. Any future missions to Titan or Venus would impose heat-transfer rates and pressures well beyond the qualification ranges of those prior missions. Technological advances in these two disciplines are essential for future cost-effective in-situ exploration.
Future in-situ missions would also require a significant reduction in the size of the landing error ellipse, as well as techniques to identify and avoid landing-site hazards. Landing error ellipses on Mars typically have a major axis on the order of 100 km. To make many of the scientific measurements desired in the future, this major axis must be reduced to less than 10 km, and preferably, a few hundred meters. The Mars Science Laboratory (MSL) demonstrated a reduction in the major-axis landing error to 20 km. To reduce the mass of the entry system and allow larger spacecraft to be delivered to the surface, ongoing work could make available a lightweight, inflatable supersonic decelerator. When used with the supersonic parachute (also being developed), the fraction of a lander’s mass devoted to the entry systems needed for more precise landings can be substantially reduced. Additional reduction of the landing error ellipse requires the ability to determine the vehicle’s precise location during descent and actively guiding it to a predetermined landing site. During the terminal phase of the descent, the system must be able to identify hazards and control the final descent to avoid them. Developments in these areas are led by newer, smarter and more precise sensors.
Proximity Operations (Prox Ops)
The commonly held view of in-situ space exploration brings to mind pictures of rovers and landers, drilling and exploring new planets. However, in-situ exploration of comets, asteroids and other small bodies presents a host of different challenges. When a spacecraft has to operate in very close proximity to a small body (comet/ asteroid or other spacecraft), it is tasked with gathering data, maintaining small body relative attitude and pose, and possibly collecting samples. This requires technological advancements in hazard detection, an intelligent landing system that can identify the safest areas and those that provide the richest science yields, advances in microthruster technology, terrain relative navigation (TRN), improved software and algorithms to control the spacecraft, and developments in the ability to deal with possible ejecta.
In addition, small bodies are often surrounded by dust clouds that pose a significant danger to any spacecraft in its vicinity. Since the composition of many small bodies is unknown or poorly characterized, the possible risks to a lander or proximally orbiting spacecraft due to dust are high. Other challenges to an orbiter or lander include the possibility of unanticipated ejecta causing a possible change in rotation of the comet or asteroid, and disturbances that can alter the spacecraft trajectory.
Multi-platform mission configurations is another area of proximity operations that involves a constellation of distributed spacecraft flying in formation, or mother – daughter concepts. Coordination between multiple spacecraft requires new, very accurate sensor technologies. The development of innovative new sensors that are smarter, more precise and accurate is a new, critical advancement in this field. A significant feature of constellation and formation flying is the ability of each spacecraft to autonomously navigate in space so as to maintain the correct attitude and orientation with respect to the other spacecraft in the formation, towards Earth and towards the destination or observation space. This calls for very high fidelity system-level simulations, advanced sensors, advanced algorithms for guidance and control, developments in microthruster technology, and hardware testbeds where hardware can be built, tested, and studied.
Mobility choices are constrained by the environment in which they have to operate. Exploring cliff faces and lava tubes on Mars requires capabilities that are different from those needed to conduct experiments in the microgravity environment of a small body, like an asteroid or a comet, or under the surface of a liquid ocean. To this end, researchers are experimenting with robots and rovers of different shapes and sizes and with widely varying features, while recognizing that energy sources could be extremely limited. A robotic rover like the Hedgehog pictured above has multiple modalities for mobility; it can hop and tumble in microgravity, leading to a new kind of rover that can function in sandy and rocky microgravity environments.
Sample Acquisition and Handling
Sample acquisition and handling is an essential element of in-situ analysis and exploration. Earlier NASA planetary exploration systems developed to obtain, manipulate, and deliver samples were limited to scraping and scooping, placing samples into an instrument’s opening using gravity and examining abraded rock surfaces. MSL advanced the state of the art since its design added the capability to drill into rock and to transfer the resulting particulate to instruments for analysis. The ability to obtain an intact core sample, to drill and extract that sample from a desired depth, to prepare that sample in different ways and deliver it precisely are capabilities that are currently being developed. Research is underway to mature technologies that could make it possible to precisely extract a sample and send it to Earth for analysis.
For any sample-return missions, the ability to place a pristine sample into a sample container must be augmented with the capability to ensure that such a sample is both unique and scientifically interesting.
Autonomous Orbiting Sample Retrieval and Capture
At present, the notional architecture of a Mars sample-return effort would include a sample container into which a pristine sample would be placed and maintained in that condition, a Mars ascent vehicle that would launch the sample container into an orbit about Mars, and an Earth-return vehicle to detect and rendezvous with the orbiting sample container and transfer the sample to an Earth-entry vehicle (while precluding back contamination), after which the vehicle would return to Earth. To implement this architecture, technological capabilities which are not currently available at acceptable levels of risk, must be developed and validated:
- The capability to detect autonomously a small, unpowered target in a poorly defined orbit around Mars.
- The capability to rendezvous autonomously with that small target starting from a separation distance of about 50,000 km.
- The capability to transfer the sample container autonomously to an Earth-entry vehicle while maintaining the samples in pristine condition and precluding back contamination.
Experiments so far have illustrated the difficulty of the work still needed to make a Mars sample-return campaign possible.
Other targets for sample extraction, capture and retrieval include small bodies such as asteroids and comets. The capabilities required for such missions are very similar to those required for a planetary sample capture mission. However, there are also some key differences:
- Smaller bodies have a much lower gravity (the spacecraft, sample capture and transfer system must function efficiently in a microgravity environment)
- Small bodies present large orbital uncertainties (this presents challenges for successfully capturing and transferring samples)
For any spacecraft entering an extraterrestrial environment, it is imperative that it not introduce terrestrial life forms capable of surviving in that environment (forward contamination). Otherwise, the mission would compromise not only the extraterrestrial body but also its own and all subsequent scientific studies. Planetary protection is a discipline that maintains techniques applied during spacecraft design, fabrication, and testing, to ensure sterility and, for sample-return missions, the containment of any returned extraterrestrial samples so that they would not present a threat to Earth (backward contamination). Because planetary protection can be an issue at every stage of a spacecraft’s design and assembly, it must be considered from the very outset of the design phase. Planetary protection techniques include:
- Sterilization, cleaning, and aseptic processing: This ensures that a robotic planetary spacecraft is sufficiently sterile not to compromise its scientific studies or those of subsequent investigations. Validated techniques that sterilize individual parts as well as the entire spacecraft, and maintain its biological cleanliness at every level during the fabrication, assembly, and testing, are under development at JPL.
- Recontamination prevention: Parts and assemblies, once sterile, must be maintained sterile throughout assembly, test, and launch activities. Techniques and processes to accomplish this goal require additional development.
- Sample handling and processing: For samples returned to Earth, planetary protection would provide two critical capabilities: the ability to return a sample while preventing back contamination of Earth from an extraterrestrial source, and the ability to prevent contamination of the sample while manipulating it from its extraterrestrial source to the terrestrial bio-containment facility in which it would be analyzed.
- Cost and risk-reduction management: The ability to estimate both the costs of different approaches to spacecraft sterilization and the efficacy of those approaches is necessary to develop a cost-effective approach to effective planetary protection for a given mission. The planetary protection community has developed and continues to improve these analytical techniques.
Selected Research Projects
Research and development in the In-Situ Planetary Exploration Systems area spans many dimensions: hardware and software, manipulation and mobility, small and large. Below is a selected subset of ongoing research, which represents the spectrum of activities.
Vehicles to Explore Extreme Terrain
Extreme terrain mobility could enable the exploration of highly valued planetary sites for future science and human missions. Lunar cold traps with evidence of water ice, Martian craters with seasonal putative briny flows (recurring slope lineae or RSLs), exposed stratigraphy and caves are a few examples of extreme terrain.
Access to deep and steep craters and caves requires robust and versatile mobility. JPL is investigating both tethered and untethered mobility. Tethered rovers that access extreme terrains could host instrument payloads of a few to tens of kilograms for in-situ measurements and sample return. JPL has been investigating long-range tethered mobility (10s of meters to kilometers) over challenging terrains with largely unknown regolith properties for week-long durations. Technological and engineering advances required to retire the risk for such systems include deployment and retraction of tethered mobile payloads, strong and durable umbilical tethers, anchoring and de-anchoring, in-situ instrumentation and sampling tools, remote control and operation (semi-autonomous and autonomous), miniaturized avionics, power/communication through long umbilicals, risk mitigation and fault management. Related technologies for approaching extreme terrain sites through precise landing or fast traverse are equally important.
Mobility in Micro-Gravity
The very weak gravitational pull of asteroids makes operations on their surfaces challenging. JPL’s testbed uses computer-controlled off-loaders to provide a test environment in which actual hard-ware is developed and validated. This environment allows engineers to test mobility devices, surface samplers, and coring tools in an environment closely replicating that of a small body, validating innovative engineering solutions to the unique problems a microgravity environment poses. An inverted Stewart platform, where the vehicle under test is suspended by six computer-controlled cable winches that maneuver a work platform in x, y, and z, and roll, pitch and yaw, was used to simulate the microgravity environment.