Although radar imaging has been a part of space exploration since the Apollo program, spacecraft power limitations have prevented wider use of radar instruments on planetary probes and orbiters. That could change, however, once NASA’s Project Prometheus nuclear power and propulsion initiative delivers the abundant onboard power it promises.

Radar mappers and sounders belong to a power-hungry class of so-called active remote sensing instruments that work by sending out a signal toward a target and detecting the intensity of the signal reflected back. By contrast, so-called passive instruments such as the cameras and spectrographs that are staples of planetary exploration, merely record the sunlight or other electromagnetic signals radiating off a planet, moon or asteroid.

NASA planetary scientist Ken Neibur said mustering the electrical power that radar instruments need is not much of a problem in Earth orbit, where sunlight is abundant and remote sensing satellites tend to be dedicated spacecraft. On power-strapped planetary probes and orbiters, however, finding room for a radar instrument can be tough or even impossible, particularly if the mission is bound for the sunlight-deprived planets.

“Radar systems are very complex,” Neibur said. “They generate a tremendous amount of data and require a lot of power. That’s one of the reasons we haven’t used them more often.”

NASA flew a lunar sounder experiment on Apollo 17 in 1972 to study the moon’s surface and interior. NASA’s Magellan orbiter used a synthetic aperture radar in the early 1990s to peer through Venus’ dense atmosphere to make what remain the most detailed maps of that planet’s surface. NASA’s Cassini probe, which will finally reach Saturn in July after a nearly seven-year journey, is also equipped with a synthetic aperture radar. Cassini will use it to map Titan, Saturn’s largest and most mysterious moon. Cassini is equipped with three radioisotope thermoelectric generators that provide sufficient power for all of its instruments.

Titan, like Venus, is obscured by heavy cloud cover.

“Synthetic aperture radar excels when you have a lot of cloud cover,” Neibur said. “You really have got to have radar that can penetrate the cloud cover and allow you to see the surface.”

Ground-penetrating radar instruments, meanwhile, are being used to explore Mars and unlock some of the secrets of the red planet’s past. “On Mars [ground-penetrating radar] is critical because we are trying to find the water,” Neibur said. “There is a lot of history buried underground.”

The European Space Agency’s Mars Express Orbiter, which has been circling the planet since December, is equipped with a subsurface sounding radar altimeter dubbed MARSIS. The instrument’s 40-meter antenna is sending low frequency radio waves toward the planet to map the subsurface structure of the planet to a depth of several kilometers.

NASA’s Mars Reconnaissance Orbiter slated for launch in August 2005 will be equipped with a more precise Shallow Subsurface Radar -- SHARAD for short -- designed to peer up to a kilometer beneath the martian surface in search of liquid or frozen water. The Italian Space Agency is providing the instrument. While SHARAD cannot penetrate as deep as MARSIS, Neibur said it will return higher resolution imagery than MARSIS, allowing scientists to study Mars’ sub-surface layers in greater detail.

Although radar already has made important contributions to planetary exploration, the future looks even brighter. Neibur is the NASA program scientist for the Jupiter Icy Moons Orbiter (JIMO), the U.S. agency’s first planned use of the nuclear power and propulsion systems it is developing under Project Prometheus. Slated to launch around 2015, JIMO is expected to spend years studying in turn each one of Jupiter’s three planet-sized moons -- Callisto, Ganymede and Europa.

The massive spacecraft NASA envisions is expected to have kilowatts of power available for science instruments -- more juice than most planetary scientists have ever contemplated having.

“The amount of power we will have for JIMO drastically changes the amount of science we can do,” Neibur said. “When you have the data rate and the power that JIMO and JIMO follow-on missions will provide, that it really custom tailored for planetary radar experiments.”

Powering radar instruments is only one part of the challenge. Another is having a communication link robust enough to handle the data-rich stream even the simplest radar delivers.

“A lot of the radar data collected by Magellan had to be thrown away because we did not have the data rate to get all the information back to Earth,” Neibur said. “On JIMO we will have orders of magnitude higher data rate.”

Neibur said some of the early radar concepts scientists are considering for JIMO entail antennas 10 or 20 meters in length. “The nice thing about JIMO is we are going to have room for 1,500 kilograms of science instruments,” he said. “If you need a large antenna, it can be accommodated.”

Paul Spudis, a staff scientist at Johns Hopkins University’s Applied Physics Laboratory near Baltimore, said JIMO’s “virtually unlimited electrical power” is a big boon for radar.

“This not only allows you to build more powerful, multi-spectral imaging radars so that you can better characterize and constrain deposit thicknesses, but also to build more robust and capable data-handling sub-systems and communications architectures, for increased bandwidth and data volumes,” Spudis said.

Oddly enough, eliminating the weight and power constraints that have kept radar in check for decades could also create new problems. Spudis said the most immediate challenge is identifying just what those problems are. “The long poles on such instruments is that not much thought has been given to these great possibilities by the planetary science community, being as how these new opportunities have popped up only within the last few months or so.”

Neibur said most space-based radar technology development of the past couple decades has been targeted toward living within serious power and mass constraints. For starters, the radar instrument would have to be extremely radiation tolerant to function properly for so long a time in orbit around Jupiter. Another area in need of addressing would be the development of a high-powered amplifier for the radar instrument’s transmitter. “We’re not exactly used to putting transmitters on spacecraft that take kilowatts,” he said.

To identify and address such potential problems, NASA has created the High Capability Instruments for Planetary Exploration program, an effort to develop new generations of instruments that can take advantage of the high power, high data rates and very long observation times promised by JIMO and its follow-ons.

Neibur said several radar concepts were among the 11 instrument concepts NASA recently selected to receive $1 million to $1.5 million in funding over the next three years.

Spudis and Neibur both said the science community has not given a whole lot of thought to where it might want to send nuclear-powered radar missions beyond JIMO.

Closer to home -- and perhaps nearer term -- Spudis said there still remains the need to better characterize the polar ice deposits on the moon and map subsurface ice on Mars. But both of those applications of radar remote sensing, he said, “will probably be carried out under the old solar-power constraints, for reasons of cost more than anything else.”

Maria Zuber, a professor of planetary geophysics at the Massachusetts Institute of Technology, said the nuclear power systems NASA envisions should enable subsurface mapping of Europa, an exciting prospect given the strong possibility that a liquid ocean lies beneath the frozen moon’s icy surface.

“But one should not get too carried away,” Zuber said. “The kinds of power available will be comparable to that for payloads in Earth orbit. So systems that are very, very power intensive are still out of the picture.”