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In-Situ Planetary Exploration Systems
In-situ planetary exploration systems enable planetary and small-body surface, subsurface, and atmosphere exploration leading to sample acquisition, retrieval, and return to Earth.
A new epoch in robotic exploration of the solar system has opened and its promise of new and unexpected findings beckons us forward. The Mars Exploration Rover and Mars Science Laboratory missions entice us with their exotic findings and observations. Yet these missions, novel and exciting as they are, mark only the beginning of this 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 five key areas:
Entry, descent, and landing (EDL): Developments in this area are necessary to extend current capabilities to larger scales, higher speeds, greater precision, and higher unit loads for Mars entry, and to develop capabilities for higher-density atmospheres like Venus and Titan while providing greatly improved landing precision.
Mobility: Advances in this area could extend existing capabilities to yield rovers with greater range and speed and develop capabilities to explore through the atmosphere and beneath oceans.
Sample acquisition and handling: Necessary to improve and extend existing capabilities to obtain and dexterously manipulate subsurface and surface samples.
Autonomous orbiting sample retrieval and capture: Advancements in technology would create the capabilities necessary to return a sample from Mars to Earth.
Planetary protection: This area of technology could enable uncompromised and safe exploration of planetary bodies in our solar system that may harbor life.
Gathering in-situ scientific observations and data would require increasingly capable spacecraft, planetary landers and rovers. These vehicles would be more massive than those of today. Earth’s atmosphere and atmospheric drag limit the diameter of launch vehicles, which in turn would 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 technology advances to provide mission-critical capabilities.
The most compelling scientific questions often require investigations of sites on a planet 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, to do so autonomously, and to require less power. On some bodies, these requirements might 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 be able to 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, we must develop the capabilities that would enable the spacecraft system to detect a sample-bearing canister in space, to rendezvous with that canister, to transfer that sample to an Earth-return vehicle, and to return to Earth. This would 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 sterilization-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.
In the future, more demanding missions would require more capable onboard planning and data analysis, not only to permit distant missions to be productive despite lengthy round-trip flight times but also to provide the ability to take advantage of unanticipated or variable events of scientific interest. The issues associated with advancing autonomy technology are critical to in-situ planetary exploration.
Current Research Areas
In-situ planetary exploration would be enabled by a set of capabilities that are key to future planetary exploration, whether at the outer planets, Mars, or Venus. Validating these capabilities and making them available to planetary missions will be both technically difficult and time consuming, 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, and the design and implementation of a mass-efficient propulsion system to control the spacecraft attitude and rates during all phases of the descent. Similarly, the reduction of propulsion requirements made possible through the use of aerocapture can be essential for an in-situ exploration mission. 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. Technology 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 had a major axis on the order of 100 km. To make the desired future scientific measurements, this landing error ellipse must be reduced to a major axis less than 10 km, and preferably a few hundred meters. The Mars Science Laboratory (MSL) demonstrated the capability to reduce 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 will 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 the vehicle to a predetermined landing site. During the terminal phase of the descent, we must be able to identify hazards and control the final descent to avoid those hazards.
Top Image: Rover coring testing for potential future sampling missions to Mars. Bottom Image: Honeybee coring and abrasion tool testing at JPL.
When we think about mobility today in the context of in-situ exploration, rovers on Mars come to mind. But mobility is not restricted to rovers. Although mission designers recognize that the energy available would be extremely limited, plans for missions to perform in-situ investigations of cloud-covered planets and moons like Venus and Titan call for the use of robotic balloons called “aerobots” that would carry instruments to analyze the atmosphere and cameras to photo-graph the surface while flying thousands of kilometers. To follow the step first taken by the 1985 Soviet VEGA balloon mission, Venus mission designers envision helium-filled balloons with a polymer skin (for use in the cooler upper atmosphere) or with a thin metal skin (for use in the very hot lower atmosphere) as the technological approaches of choice. The denser, cooler atmosphere at Titan might be explored with a Montgolfier balloon, one whose lift would be gained by containing and then heating the local atmosphere with the waste heat from a radioisotope power source that would supply electrical power. At present, early proof-of-concept activities have been accomplished, and for use at Venus, full-scale prototypes have been constructed and are being validated. Materials suitable for use at Titan’s cryogenic temperatures have been developed and prototype designs are being evaluated. Ongoing efforts are directed toward bringing these technology advances to the necessary level of maturity for flight-mission implementation and toward addressing their integration into vehicle design.
Sample Acquisition and Handling
Sample acquisition and handling is an essential element of in-situ analysis and exploration. The NASA planetary exploration systems developed prior to Curiosity’s development 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. Curiosity’s 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 remain to be realized and implemented.
For 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 architecture of a potential Mars sample-return mission includes a sample container into which a pristine sample would be placed and maintained in pristine 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 the Earth-entry vehicle (while precluding back contamination), after which the vehicle would return to Earth.
To implement this architecture, technological capabilities not now available at acceptable 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 approaching 50,000 km.
The capability to transfer the sample container autonomously to the Earth-entry vehicle while maintaining the samples in pristine condition and precluding back contamination.
Some work to develop the capability to rendezvous autonomously in space has been undertaken by the U. S. Department of Defense. The resulting experiments illustrated the difficulty of the work still needed to make a Mars sample-return mission possible.
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). Other-wise, the mission would compromise not only the extraterrestrial body but also its own and all subsequent scientific studies. Planetary protection is the collection of technological capabilities that are applied during spacecraft design, fabrication, and test to ensure its sterility and, for sample-return missions, the return containment of the extraterrestrial sample so that it would not present a threat to Earth (backward contamination).
Because sterilization is an issue at every scale of a spacecraft’s design and assembly, it is an issue that must be considered from the very outset of the design phase. Planetary protection is a discipline that depends on many technological capabilities:
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 to sterilize equipment and maintain its biological cleanliness at every level during the fabrication, assembly, and test, from piece part to whole spacecraft, are needed but not yet in hand.
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 to be perfected.
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 by 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.
In-Situ Exploration Impacts
The United States, NASA, and the Jet Propulsion Laboratory have had great success using in-situ, robotic exploration to learn about the solar system and relating that knowledge to Earth. That success has certainly not been easy to achieve. Yet the hard-fought knowledge we have gained has only served to raise questions about the Earth itself as well as about the solar system that can be answered only with future investigations, investigations more difficult and demanding that those that have gone before. Aided by the innovations made possible by technology advances in robotic, in-situ planetary exploration systems, these new questions will yield, eventually, to human inquiry and provide us with a better understanding of our world and its impact on our lives.
Selected Research Projects Overview
Research and development in the In-Situ Planetary Exploration Systems area spans many dimensions, among them hardware to software, manipulation to mobility, small to large. Below is selected subset of ongoing research, which represents the spectrum of activities.
Top: An example of a four-wheeled rover anchored near the rim of an RSL crater, deploying its front axle, which in itself is an instrumented tethered rover, termed Axel. A spectrometer is deployed from a turret-like instrument bay that is protected by the wheels. (Physical rover pictures were rendered on a Mars background that is not to scale.) Bottom: The full-scale metallized balloon, produced by ILC Dover, could be used today for operation in the Earth-like temperatures found at 55 km altitude in Venus’ atmosphere for mid-altitude exploration of the plan-et’s surface.
Vehicles to Explore Extreme Terrain
Extreme terrain mobility would 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 such examples.
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 re-turn. 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. Technology and engineering advances to retire risk for such systems include deployment and retraction of tethered mobile payloads, strong and durable umbilical tethers, anchoring and deanchoring, 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 also important.
Autonomous Aerobots for Exploration of Titan and Venus
An aerobot is a robotic aerial vehicle that uses buoyancy to provide the lift needed to fly. Such vehicles are essentially balloons with scientific payloads suspended underneath and, optionally, propulsion systems (e.g., propellers) mounted on either the balloon or payload compartment.
JPL is developing aerobot technology for potential use in future missions to Mars, Titan and Venus. The different environments at these three worlds dictate the use of different aerobot designs and components, which in turn would lead to different kinds of possible missions:
Titan has a very dense but very cold atmosphere comprised mostly of nitrogen gas. JPL is developing both wind-blown and self-propelled aerobot vehicles using cryogenic balloon materials. Payloads of up to a few hundred kilograms would be possible for mission durations of 6-12 months.
Venus has a very dense carbon dioxide atmosphere that is relatively cool at high altitudes but extremely hot near the surface. JPL is developing both wind-blown and self-propelled aerobot vehicles for a variety of mission concepts that could either stay high, stay low or traverse the entire atmosphere. Payloads could range from tens to hundreds of kilograms in missions lasting days or weeks.
Mars has only a tenuous carbon dioxide atmosphere, which means that very large, light-weight balloons would be required to float even small payloads of a few kilograms. JPL is focused on developing simple, wind-blown balloons that could fly for weeks or months.
Mobility in Micro-Gravity
The very low gravity present on asteroids makes operations on their surfaces difficult. 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 micro-gravity 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 micro-gravity.
The All-Terrain, Hex-Limbed, Extra Terrestrial Explorer (ATHLETE) Rover