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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 Pathfinder and Mars Exploration Rover missions enticed 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 here would 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 would 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 the 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 and must do so autonomously, while requiring 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 the rover reaches a scientifically interesting site, it must be able to obtain a specified sample and to 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.
The full-scale metallized balloon, produced by ILC Dover, could be used today for operation in the Earthlike temperatures found at 55 km altitude in Venus’ atmosphere for mid-altitude exploration of the planet’s surface.
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, Europa, 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. Present landing error ellipses on Mars have 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 perhaps as small as a few hundred meters. (The Mars Science Laboratory [MSL] has developed the capability to reduce the major-axis landing error to 20 km, but this technology has not yet been accomplished on Mars. MSL is planned for launch in 2011.) To do so would require both improvements in entry systems, the ability to determine the vehicle’s precise location during descent, and to actively guide 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.
When we think about mobility today in the context of in-situ exploration, the 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 photograph 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 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.
Top Image: Rover coring testing for potential future sampling missions to Mars. Bottom Image: Honeybee coring and abrasion tool testing at JPL.
Sample Acquisition and Handling
Sample acquisition and handling is the most essential element of in-situ analysis and exploration. The NASA planetary exploration systems developed to date to obtain, manipulate, and deliver samples have been limited to scraping and scooping, placing samples into an instrument’s opening using gravity and examining abraded rock surfaces. 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 need 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 its current 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 autonomously detect a small, unpowered target in an orbit around Mars that is not well-defined.
The capability to apportion requirements between the planetary surface ascent vehicle and the Earth-return vehicle to optimize the mission/flight system design.
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 have illustrated the difficulty of the work remaining to make a Mars sample-return mission possible.
For any spacecraft that enters an extraterrestrial environment, it is imperative that it not introduce terrestrial life forms capable of thriving in that environment (forward contamination). Otherwise, the mission would compromise not only the extraterrestrial body but also 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 level 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 is to ensure that a robotic planetary spacecraft is sterile to the levels required by international agreement, techniques are developed to sterilize equipment and maintain their biological cleanliness at every level during the fabrication, assembly, and test processes, from piece part to whole spacecraft.
Recontamination prevention: This research area is comprised of design and implementation of biobarriers to maintain a sterile environment and prevent the recontamination of sterile parts and assemblies as they are stored or incorporated into larger assemblies is a planetary protection capability needed throughout the project’s fabrication, from deployment to the mission target.
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 biocontainment facility in which it would be analyzed.
Cost and risk-reduction management: This is necessary to provide the ability to estimate both the costs of different approaches to spacecraft sterilization and the efficacy of those approaches to management of different designs, planetary protection seeks to provide the analytical tools that would allow spacecraft managers to incorporate the most cost-effective approach to effective planetary protection for their mission design and architecture.
Selected Research Projects Overview
Research and development in the In-Situ Planetary Exploration Systems area spans from hardware to software, manipulation to mobility, small to large, and many other dimensions. Below is selected subset of ongoing research, which represents the spectrum of activities.
ATHLETE rover during NASA's Desert RATS field campaign.
All-Terrain Hex-Limbed Extraterrestrial Explorer (ATHLETE)
In September 2010, The All-Terrain Hex-Limbed Extraterrestrial Explorer (ATHLETE) rover, currently under development at NASA's Jet Propulsion Laboratory, Pasadena, Calif., was in the Arizona desert this month to participate in NASA's Research and Technology Studies, also known as Desert RATS. The desert tests offer a chance for a NASA-led team of engineers, astronauts and scientists from across the country to test concepts for future missions. ATHLETE demonstrates mobility advances on different trains.
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, lightweight 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.