This article focuses on the exploration of our solar system that has, in a very literal sense, extended the reach of mankind. Developing the technology of that exploration has extended immensely the capacity of engineering. The new technologies and key capabilities being developed include intelligent robotics, advanced propulsion systems, power generation, avionics, telecommunications, and instruments. Technology for sample acquisition and return encompasses power and propulsion, robust landing, sensors, handling and packaging systems, ascent vehicles, and autonomous rendezvous and capture systems. Measures are needed to ensure that the samples are not contaminated during collection or the return to Earth, and that samples cause no harm to the Earth's environment. Some of the future solar system missions will experience extreme environments. The extreme cold and intense radiation around Europa, or the searing heat and crushing pressure of Venus, would limit the lifetime of systems built with present technology to just minutes. Improved pressure vessels, thermal control, environmentally tolerant electronics, and low-power systems are needed to prolong the lives of vehicles and instruments for these missions.
Spacecraft have traveled to almost every planet in our solar system. The first planetary mission, a flyby of Venus, dates to 1962. Since then, people have walked on Earth’s moon, and robotic probes have gone much farther. Missions have included visits to every confirmed planet except Pluto, as well as to some planetary satellites along the way. The exploration of our solar system has, in a very literal sense, extended the reach of mankind. Developing the technology of that exploration has extended immensely the capacity of engineering.
Today, we take for granted many engineering tools and practices whose development was initiated or accelerated by the U.S. space program. They include life-cycle modeling and simulation, multidisciplinary integration and optimization, virtual reality, database management, concurrent engineering, and collaboration among geographically dispersed diverse teams. In fact, finite element analysis, which is the backbone of engineering simulation systems, grew out of the aerospace industry. The developments of hand-held and embedded computers, and information appliances, all widely used today, were accelerated by the needs of the space program.
Since the beginning of space exploration, more than 1,300 NASA and other U.S. space technologies have been used on Earth in fields as diverse as mechanical engineering, transportation, textile manufacturing, and medicine. Commercial applications range from satellite communications to image processing used in computerized tomography and magnetic resonance imaging. Space exploration technology gave rise to surgical probes used to treat brain tumors and to hand-held infrared cameras.
Now there is a renewed vision for space exploration. The United States is looking once more at the moon as a destination for humans, and we’ll try to take people to Mars as well. Just as important will be numerous robotic missions in the next two decades that will carry sophisticated payloads to our moon and to the other destinations in our solar system and beyond.
The technology needed to take us and our mechanical surrogates across interplanetary space will also take us to new heights of achievement. The new technologies and key capabilities being developed include intelligent robotics, advanced propulsion systems, power generation, avionics, telecommunications, and instruments. All are likely to have many Earth-bound applications that significantly improve the quality of life.
The opportunities for engineers are indeed great. And NASA needs the contributions of engineers. It has gone through a major restructuring of its workforce and is forming partnerships with industry and universities to develop the new technologies in support of the new vision for space exploration.
Future missions may include the study of Mars by a series of robotic missions, surface exploration of Venus and Titan, orbiters around Jupiter’s icy moons, and the founding of a lunar outpost for extended human occupation. Plans also call for vehicles to carry samples back from distant bodies, from as far away as Venus or a comet’s surface, and for an investigation of the moon as a source of minerals for future space manufacturing, and maybe for earthly use.
Developing the technology for this kind of exploration, under the current budget constraints, will require much higher levels of collaboration between diverse engineering and science teams, as well as more creativity and innovation than before.
There are four primary modalities for robotic exploration of the solar system.
Flyby spacecraft pass the target object at high velocity and observe it with remote sensing instruments over the brief period. Flyby spacecraft have now visited all the planets in the solar system except Pluto, which is the target of NASA’s New Horizons mission.
Orbiter spacecraft are often equipped with instruments similar to those aboard flyby spacecraft, but in orbit around the target object, they can then acquire more information over an extended period. The Cassini spacecraft, in an interesting hybrid of flyby and orbital observation, recently entered orbit around the planet Saturn, where it can also conduct numerous close flybys of Saturn’s moon Titan.
In situ exploration sends vehicles into the atmosphere and in some cases onto the surface of a body. The Mars Exploration Rovers—Spirit and Opportunity—are current examples of in situ systems. Future examples will include deep probes into the Jupiter atmosphere, long duration aerial platforms to Mars, Venus, and Titan, and subsurface probes of Jupiter’s moon Europa.
Sample return systems are designed to bring specimens from distant atmospheres or surfaces back to Earth. A sample return system must include provision for return to Earth, a frequently hazardous final stage.
The Drive to Get There
To open the solar system to vigorous robotic and human exploration, purely chemical and other modes of propulsion that have reached a plateau in terms of technology and capabilities are not sufficient. New forms of propulsion are being sought to make faster trip times and to allow long-term surveys of planets and other bodies in the solar system. Among the concepts being considered are solar electric propulsion, nuclear electric propulsion, aerocapture, and solar sails. A number of technology programs, including Project Prometheus, are currently focused on the development of nuclear electric and other advanced in-space propulsion concepts. These initiatives will require the skills of engineers and scientists with systems engineering and nuclear physics backgrounds.
Solar electric propulsion uses electricity derived from solar power to accelerate ions of a propellant (xenon, for example) to very high velocities. Although the thrust produced is low, a small mass of propellant can go a great distance. The technology can reduce the propellant load by a factor of 10 or more, compared with systems that use chemical fuel as a propellant. The capability of the system is limited by available solar power, so it is expected to provide substantial benefits for missions to inner planets and to the asteroids and comets.
Nuclear electric propulsion is based on the same principle as solar electric propulsion, but uses fission-derived power for electric propulsion instead of solar power. It requires the development of a compact and efficient power source coupled to advanced electric propulsion systems. Nuclear electric propulsion will enable missions that can visit several planets and satellites, deliver large payloads, and return samples from virtually any destination in the solar system, including locations that cannot be reached by other methods of propulsion.
Aerocapture uses a planet’s atmosphere and aerodynamic forces to alter the orbit of the spacecraft or its travel velocity. In the most conventional approach, aerocapture refers to using a planet’s atmosphere for both lift and drag to send a spacecraft into the correct orbit. An aerocapture vehicle approaching a planet is captured into orbit as it passes through the atmosphere without the use of onboard propulsion. This fuel-free method could reduce the mass of an interplanetary spacecraft by more than half, allowing for smaller and less expensive launch vehicles and/or increasing the science payload capacity. This can be particularly efficient for outer solar system missions.
To conduct an aerocapture, maneuver, a spacecraft requires adequate drag to slow its speed and also requires adequate protection from the heating environment. Among the concepts being studied to accomplish these objectives is the use of an inflatable drag device, the ballute, a hybrid of balloon and parachute made of durable, thin material and stowed behind the spacecraft for deployment.
When solar electric propulsion is used to accelerate the spacecraft near the sun and is coupled with aerocapture, it may enable rapid and cost-effective delivery of orbital payloads to outer solar system missions, as well as much larger payloads into orbit around Mars and Venus.
Solar sails, consisting of very lightweight films deployed over large areas, can develop thrust from the constant impingement of solar radiation. They represent the height of propulsion efficiency because they require no propellants—just sails, booms, and deployment systems. They would need sophisticated flight control techniques.
Solar sails were considered for a U.S. mission to rendezvous with Comet Halley during its last pass through the inner solar system in 1984. Their principal application is likely to be station keeping in non-Keplerian orbits, which force the sailcraft to stay at a fixed angle above the plane of the Earth’s orbit around the sun by constantly compensating for solar gravity, essentially creating an orbit parallel to that plane—to monitor the sun, for example. They are unlikely to be useful for exploration missions to nearby destinations or for human exploration missions, for which very large sails are needed (on the order of many square kilometers). Deployment and control, particularly in operations near planets and satellites, present major challenges.
Solar power is generally insufficient for missions beyond the main-belt asteroids. Future outer solar systems, as well as other missions for which solar power is not sufficient, will require reliable, long-lived rugged power sources, ranging from milliwatts to kilowatts, to support instruments and communications. Advanced radioisotopic power systems under study would operate not only in the vacuum of space, but also on planetary surfaces with atmospheres. Other technologies under development could provide up to five times the electrical conversion efficiency of current thermoelectric generators. Specialists with systems engineering skills are needed for that effort.
Project Prometheus aims to adapt nuclear fission for possible applications in support of a range of missions. The power available from small fission reactors can exceed 100 kW. Such a power source would not only enable advanced nuclear propulsion systems, it would also increase the operational lifetime of spacecraft and instruments, improve scientific measurements and mission options, and eventually support human explorers.
The propulsion capability that fission can enable has great potential for robotic and, eventually, human exploration of the solar system. A mission now being planned may use an electric propulsion system powered by a nuclear fission reactor to maneuver among the icy satellites of Jupiter—Callisto, Ganymede, and Europa. The power available could enable the spacecraft’s radar to penetrate deep into icy surfaces and allow extensive surface mapping in enough detail to see features on the order of tens of meters.
Major improvements have been made in deep space telecommunications, including the implementation of high-bandwidth communication systems operating at Ka-band (Kurtz above radio frequency of 32 GHz, which is off-limits to commercial users); it will be demonstrated on the 2005 Mars Reconnaissance Orbiter mission. Ka-band is a factor of four higher in frequency than the current X band technology used in deep space communications. It will provide a data rate from Mars of more than 2 megabits per second. Today, the maximum data rate transmitted to Earth by spacecraft at Mars is about 128 kilobits per second.
Current technology developments are directed toward improving radio communications performance through the use of large deployable spacecraft antennas and ground-based antenna arrays. In addition, optical transmission, which relies on laser light instead of radio waves, is being explored to enable video-rate communications from Mars, and large gains in data rate for outer solar system exploration. Laser communications from deep space will be received by optical telescopes, operating either on the ground or on platforms above Earth’s atmosphere. Real-time access by the science community can be anticipated in the future using a “trunk line” from the Earth to a relay spacecraft in orbit around a distant planet and proximity links between that spacecraft and landers, rovers, and other vehicles. These activities provide opportunities for systems engineers and experts in optical communication.
Low-cost, deep space exploration missions will require highly miniaturized, yet highly capable and reliable, avionics systems. Work is currently under way on advanced packaging and miniaturization of avionics elements as well as on environmentally tolerant systems. These are required for operation in the intense radiation environment of Europa, for example, and in the extreme temperatures and very high pressures within the atmosphere of Venus.
NASA researchers are working on highly integrated avionics subsystems built as single-chip solutions, and then as potential system solutions on highly integrated microscale and possibly nanoscale modules. Such devices will be produced in high volumes on commercial fabrication lines with special design and process enhancements for radiation hardness in space. The system on a chip will include building-block cells for a modular design that includes telecommunications processing, power management and distribution, science and program storage, data collection and onboard processing, device interface and real-time system control, and navigation. The challenge is to accelerate the introduction of novel commercial technologies into space.
The development of technology for safe, accurate landing of robotic vehicles on diverse bodies is one of the challenges of solar system exploration. Three complementary technologies—precision landing, hazard avoidance, and robust landing—are being developed to improve landing accuracy on Mars by several orders of magnitude. It will enable landing within roving range of sites of interest, while avoiding hazardous regions such as craters, mountains, and canyons.
Precision landing technology encompasses advanced optical navigation methods and aerodynamic guidance. Hazard avoidance technology will enable the spacecraft to detect large rocks and steep slopes, and maneuver to avoid them. Robust landing technology aims at development of resilient landing systems, such as deployable airbags or pallets, to ensure the safety of payloads when landing on inhospitable terrain.
It takes at least three hours for a command to travel between Earth and Saturn’s moon Titan. It takes 20 minutes to Mars. The delay makes Earth-bound remote control of exploration vehicles at distances beyond the Earth’s moon impractical. Autonomy technology is a key to meeting this challenge. The Mars technology program, for example, is developing technologies that will enable a rover to travel to and sample a rock 10 meters away with a single command (instead of the five to 10 commands required by the present rovers). More powerful systems would be needed for aerial vehicles operating on Titan.
The subsurface exploration of solar system bodies, such as Mars and Europa, will require another dimension of planetary mobility. Advances will be needed in drilling, coring, or boring devices, and in sensors and the platforms that carry them.
Samples from various destinations will provide insights into the evolution of the solar system, and possibly into the origin of life itself. Technology for sample acquisition and return encompasses power and propulsion, robust landing, sensors, handling and packaging systems, ascent vehicles, and autonomous rendezvous and capture systems. Measures are needed to ensure that the samples are not contaminated during collection or the return to Earth, and that samples cause no harm to the Earth’s environment.
Future solar system exploration requires advances in instrumentation. For remote sensing, new types of laser systems will enable the detection of trace atmospheric components by molecular absorption, mineralogical identification, and elemental analysis using laser ablation. Active scanning laser systems will enable three-dimensional mapping of landing sites with centimeter-scale vertical precision, as well as detection of ices within shadowed or night-side regions. Fission-powered spacecraft will make possible the use of high-power lasers and radars.
The United States space program is about to engineer an unprecedented schedule of missions to investigate our solar system over the next few decades. They will include a detailed study of Mars, the return of samples from distances never attempted before, perhaps from the surface of a comet; an orbital exploration of Jupiter’s icy moons; flyby missions to Pluto and several Kuiper Belt objects, and construction of a permanent, habitable lunar outpost.
A number of the missions under study are intended to extend our gaze outside this system to other stars and galaxies as we look for clues to their formation. Space-based telescopes will study stars with several hundred times the capabilities of the Hubble Space Telescope. An orbiting interferometer will link multiple telescopes to detect planets of varying sizes in other star systems. Mars is the most Earth-like planet in the solar system. Its proximity to Earth permits relatively rapid access, so it has served as a natural laboratory for understanding the process of planetary evolution. Mars has been a target of scientific exploration for 40 years, and after Earth, it is the planet we have explored most in our solar system.
A series of future missions aims at a detailed study of Mars through the use of a combination of exploration tools—including airborne platforms, and surface and subsurface robotic explorers. Samples taken from Mars could be returned to Earth. Robotic exploration of Mars is expected to lead to the eventual human exploration of the planet. NASA missions for Mars include the Telesat Orbiter and Science Laboratory, and Scout missions, which will be selected from proposals by the scientific community and may involve airborne platforms or small landers. Under study for the second decade of this century are sample returns, a reconnaissance orbiter, and a network of landers, which will conduct geophysical observations of the planet. Robotic missions are likely to continue past 2020 with a possibility of a human landing on Mars during that decade.
Venus is Earth’s twin, but it evolved in a radically different manner. Its internal structure is similar to Earth's, with a metallic core, rocky mantle, and crust. Venus has a diameter equivalent to 0.95 that of Earth, and 0.82 the mass. The first two successful landers on Venus were the USSR’s Venera 9 and 10 in 1975, which sent back photographs of the planet’s surface. Venus presents a set of unique environmental challenges for obtaining samples and returning them to Earth—surface temperatures of 460°C; surface pressure 90 times that at Earth’s sea level; a greenhouse effect caused by thick, mainly carbon dioxide atmosphere; clouds filled with corrosive sulfuric acid surrounding the planet; and poorly characterized terrain.
Early studies showed that the use of return rocket launching directly from the surface was impractical. However, it was determined that a balloon deployed at the surface could lift the sample return vehicle until the atmosphere became thin enough for a launch to space.
Saturn’s moon Titan is considered to be one of the prime locations for understanding the origin of life. “Prebiotic” organic chemicals that once formed on Earth, and were a crucial stage in the development of life, may still be preserved on Titan. Titan has a combination of dense atmosphere (more than four times that of Earth), methane clouds covering the entire surface, low gravity (one-sixth of that on Earth), and small temperature variations, with an average at the surface of -183°C.
NASA’s Cassini spacecraft successfully entered Saturn orbit at the end of June. In December, it will eject the Huygens probe developed by the European Space Agency. After a 22-day coast, the cone-shaped probe will descend into Titan’s cloudy atmosphere and during a 2/2-hour descent to the surface will collect information about the atmosphere’s chemical composition and the clouds surrounding Titan. During its planned four- year tour of the Saturn system, Cassini will execute 45 close flybys of Titan as it orbits Saturn. The spacecraft carries sensing instruments to gather information about oceans and lakes that may exist on Titan.
Autonomous mobile robotic sensors are being considered for exploring Titan’s surface and subsurface. They include a battery of sondes with sample collection harpoons and an airborne platform for battery transportation and deployment. The airborne platform could be a propeller-driven aerobot that can move to specific targets on Titan, or a combined airship-rover, or a station-keeping aerobot that winches down an instrumented platform.
Various objectives are being examined for future robotic/human missions to the moon, with particular emphasis on the use of the moon as a test bed for later human and robotic exploration of Mars and destinations beyond.
Effective on-site exploration requires sophisticated miniaturized analytical laboratories, taking very sensitive measurements and possibly using new principles of investigation for evidence of life on other planets. The miniaturized laboratories will enable the selection of the most interesting samples for return to Earth.
Robotic airborne platforms can be thought of as another tool for surveying planets and moons with atmospheres. Whereas autonomous surface vehicles can provide very high-resolution data on a local scale, and space-based orbiters can provide lower-resolution data on a planetary scale, airborne platforms can provide high-resolution data on a regional scale, including direct sampling of a planet’s atmosphere. Taken all together, orbital, atmospheric, and surface vehicles provide a complete set of capabilities for planetary exploration.
A number of airborne vehicle classes are being considered for future missions, including airplanes and rotorcraft, balloons and airships. Also under consideration are hybrid vehicles, or aerovers, which could combine atmospheric flight and surface roving. Near-surface and landed operations are clearly difficult or impossible for airplane-type vehicles. Rotorcraft are better suited for landing on rugged terrain, but the power requirements for hovering and safe landing are formidable. Another heavier-than-atmosphere concept being considered is a glider with onboard decision-making software to identify the best winds, thermals, and atmospheric conditions to stay aloft as it carries out its mission or selects the best landing site.
Lighter-than-atmosphere vehicles are attractive because they use so little power for mobility. Balloons can lower sampling devices to a planet’s surface and can provide a platform for launching sample returns. Mars, Venus, and Titan missions are attractive targets for aerial exploration. For Mars, two kinds of balloons are being studied: ITelium super pressure balloons are able to maintain a constant altitude, whether it is night or day; solar Montgolfière balloons, on the other hand, are filled with ambient atmosphere that is solar-heated to provide lift. Solar-heated balloons can provide a unique near-surface platform for an extended traverse over the polar regions of Mars. The vehicle can fly slow, land, take off, and potentially hover.
For Venus, wide-altitude excursions can be achieved with a phase change balloon for which the temperature gradient in the atmosphere is used to extract energy during ascent and descent. On Titan, an atmosphere almost five times as dense as Earth’s with light winds expected near the surface provides an excellent environment for lighter-than-air vehicles. These vehicles can operate for months with very little propulsive power needed, image the surface from close proximity, and acquire samples.
Surviving the Elements
Some of the future solar system missions will experience extreme environments. The extreme cold and intense radiation around Europa, or the searing heat and crushing pressure of Venus would limit the lifetime of systems built with present technology to just minutes. Improved pressure vessels, thermal control, environmentally tolerant electronics, and low-power systems are needed to prolong the lives of vehicles and instruments for these missions.
Consider the challenges of returning soil samples from Venus. The pressure of the very thick atmosphere is nearly 100 times that of Earth. Surface temperatures are 460°C on the plains and only a little cooler on the highest mountains. All electronic components used for control, communications, and power must be provided with lightweight, corrosion-resistant, and thermally isolated pressure housing. The landing gear and sampling system must be made of materials that can withstand the local environment for several hours. The inflation system for the ascent vehicle must be protected from the thermal environment to maintain safe pressures in the gas containers. Novel techniques for high-speed drilling must be used so sampling can be done quickly and with limited power.
Robotic and human exploration of the solar system will be a fascinating challenge during the coming decades. Integrated strategies are being developed by NASA and other space agencies for technology research, demonstration, and ultimately space exploration. Answers to our profound questions about our origin and our future may be within our grasp.
But it is perhaps more important that the program will require a progressive culture of integrated perspectives, synergistic connections, and viewpoints not normally considered conventional thinking. NASA expects the new space program to reinvigorate science and engineering education in the United States, to spur innovation in critical high-technology areas, and to revitalize the U.S. aerospace industry. It should also excite our high-technology science and engineering workforce, without whose talents the job will not get done.