Future planetary exploration may draw upon a rich history of aeronautical progress here on Earth -- from the Wright brothers to centuries-old hot-air ballooning.
Space engineers are charting novel ways to investigate Mars and other worlds in our solar system, with innovative research under way to design robotic aerial craft that can plumb the atmospheric depths of Titan, a moon of Saturn, even slice through the clouds of Venus, or glide over the outer-planet gas giants.
One way to take to the air on other worlds is by balloon.
First of all, balloons can lower spacecraft down onto a planet's surface. Conversely, instruments can be sent aloft to various altitudes, airlifting cameras or other forms of remote-sensing science. Balloons also offer ways to augment the surveying of a planetary surface, doing so at greater speeds while covering larger expanses of territory than today's ground-based rovers.
An engineer identifying approaches to planetary ballooning is Jack Jones, principal engineer for Advanced Thermal and Mobility Technologies at the Jet Propulsion Laboratory in Pasadena, Calif.
The laboratory is actively studying three worlds that are ripe for ballooning, a technology that can be tagged as the "light stuff": Venus, Mars and one of Saturn's moons, Titan, Jones said.
For Venus, an altitude-controlled phase change balloon is ideal. On Mars, constant-altitude helium balloons or altitude-controlled solar-heated ambient atmosphere balloons are under study. And for Titan, blimps filled with hydrogen or helium look highly promising, Jones said.
The area that Jones and his colleagues have focused on in recent years has been solar-heated balloons for Mars.
Jones said that hot-air balloons, similar to those flown by the French Montgolfier brothers in the 18th century, can provide a unique near-surface platform for an extended traverse over the polar regions of Mars. Engineers at JPL have developed a novel, hot-air-venting system that for the first time allows repeated, precision soft landing of hot-air balloons.
On Mars, during the Red Planet's polar summer, such solar-powered balloons could remain airborne for many weeks, perhaps even months. The atmospheric circulation of Mars would drive the balloons around the polar region many times before the balloon would cross the planet's terminator.
Among a myriad of tasks, balloon-carried instruments over Mars could scour the terrain for signatures of biology, even subsurface aquifers that might serve as ecological niches for hardy microbial life, Jones said.
"Planetary aircraft and air vehicles should be thought of as another tool in the exploration toolbox," said Anthony Colozza, a space engineer at NASA's Glenn Research Center in Cleveland. "They are not intended to take the place of other types of exploration vehicles, but to complement them."
Colozza explained that air vehicles are useful in conjunction with land-based exploration, of the rover and lander variety, as well as space-based orbiters. Taken all together, such craft provide a complete set of capabilities for planetary exploration.
"Whereas rovers and landers can provide very high-resolution data on a local scale, and orbiters can provide lower-resolution data on a planetary scale, air vehicles can provide high-resolution data on a regional scale," Colozza said.
And there's another advantage offered by high-flying air vehicles. They also provide a means of direct sampling of a planet's atmosphere, in specific regions and over a range in altitudes, Colozza said. Armed with the right sensors, aerial craft might probe for concentrations of biogenic gases.
"If a concentration of these are found, a future mission can be used to send a rover or lander to the site of interest to complete the investigation," Colozza said.
The technology to fly air vehicles on other planets is pretty much at hand, Colozza said. "There are engineering issues and some technology development needed … but this is no different or greater than what would be required for any other type of exploration vehicle," he advised.
Because it has never been done before, there is a perception of higher risk associated with a planetary aircraft mission, Colozza observed. "Whether justified or not, the perceived risk of a planetary aircraft mission needs to be reduced."
One risk-reduction effort, for example, involved an aerial vehicle being considered by NASA as a flagship mission for its Mars Scout program. A scale model of the plane was dropped from a balloon high above Earth to demonstrate how it could unfurl itself and perform controlled flight. Proposed by NASA's Langley Research Center in Hampton, Va., the Aerial Regional-scale Environmental Survey of Mars was not picked, however.
Colozza has worked on a futuristic insect-like "entomopter" -- a robotic Mars explorer that could be the answer to extended flight on Mars, he said.
The environment on Mars -- a low atmospheric density and reduced gravitational force -- presents the possibility of fabricating a vehicle that, by flapping its wings, could generate lift in a similar fashion as an insect. This would enable the vehicle to fly slow, land, take off and potentially hover within the atmosphere of Mars. Furthermore, this type of aircraft could be refueled by a rover, making long-duration flight on Mars possible.
"The entomopter would not be practical for any other planetary environment. No other planetary environment has the combination of low atmospheric density near the surface and the lower gravitational strength needed for the entomopter concept to work," Colozza said.
Outrun the winds
Solar-powered aircraft are a good example of utilizing environment and a renewable power source to provide a vehicle capable of long-duration flight, Colozza noted. "Solar power obviously won't work everywhere. Beyond Mars, the solar intensity is too low to power an aircraft. So solar flight on the outer planets and moons will not be feasible. That leaves Venus and Mars," he said.
Colozza said Venus provides a unique possibility. Due to the planet's proximity to the sun, there is abundant solar energy. Venus, however, is veiled in thick clouds. Add to that high wind speeds. This environmental twosome makes solar-powered flight tough to do on that planet.
But upon closer inspection, Colozza said, there is a region of atmosphere on Venus in which a solar-powered aircraft can operate and fly faster than the wind speed.
"This ability to outrun the winds and the very long day time period of Venus -- one day on Venus is equal to 243 Earth days -- means that a solar-powered aircraft flying within this altitude range can have a very long mission time. Although it wouldn't be a 'one size fits all' vehicle, it would do very well on that planet and at that altitude range," Colozza said.
Now closing in on its final destination, the Cassini spacecraft arrives at Saturn next July 1. Cassini will begin a four-year prime mission in orbit around that ringed world. As part of its exploration campaign, the spacecraft is set to release the Huygens probe for descent through the thick atmosphere of the moon Titan. The probe could impact in what may be a liquid methane ocean.
Even before Cassini begins its tour of duty at Saturn, scientists and engineers have begun looking at future craft to follow up where the Huygens probe is expected to leave off.
"The very best place to deploy a powered aerial vehicle — blimp, airplane or copter — is Saturn's moon Titan," said Jonathan Lunine, professor of planetary science and physics chair of the program in theoretical astrophysics at the University of Arizona in Tucson.
"With low gravity, a dense atmosphere of high molecular weight (nitrogen) and low temperatures, it should be an aeronaut's dream. And indeed such vehicles may be the best way to explore Titan in depth following up on the discoveries of Cassini-Huygens," Lunine told Space.com.
In any post-Cassini exploration of Titan, using airships and helicopters appears to be quite feasible, said Ralph Lorenz, a space scientist at the University of Arizona's Lunar and Planetary Lab.
Orbiters would not usefully address post-Cassini objectives, Lorenz suggested. But they would serve as communication nodes with Earth, relaying data gleaned from mobile platforms soaring about Titan, even as those vehicles flew during nighttime conditions on the moon.
Airship for Titan
"Although Titan helicopters and airplanes remain as feasible as ever -- and the detection earlier this year of radar glints suggesting lakes of ethane makes airborne platforms all the more attractive -- the leading concept seems to be an airship," Lorenz said.
An airship would move about the moon quite slowly, given the low temperature at Titan. To remain airborne as long as feasible, ballast from the craft would have to be dropped as needed. One obvious idea, Lorenz suggested, is to deploy instrumented ballast, namely "dropzondes" -- instrumented capsules ejected to Titan's surface.
No doubt, a Titan explorer probably needs to return on the order of gigabits of information to be worthwhile, Lorenz said. An impressive array of lightweight and miniature science tools would ride aboard the airborne craft.
Surface chemical assays need to be taken. A small magnetometer could help deduce more about the interior of Titan. Meteorological sensing gear should prove useful too, even though the moon sports a thick atmosphere. There are known to be time-variable cloud phenomena, Lorenz said, including the prospect of "methane rainfall" on the enigmatic world. Lastly, a future Titan explorer might also tote subsurface radar sounding gear, even seismic sending devices, he speculated.
To greatly enhance the exploration of other planets requires three-dimensional mobility, said Larry Young of NASA's Rotorcraft Division at Ames Research Center at Moffett Field, Calif. He points out that Titan is the only moon that has a substantial atmosphere, one that can support rotorcraft or powered-lift vehicles.
Young has looked into what types of rotorcraft, such as helicopters and tilt-rotor, could be developed for aerial exploration of Venus, Mars and Titan. "Such aerial explorer mobility would allow investigation of terrain features of interest that were inaccessible by any other means," he said.
At the outer gas giant planets, Young said, aerial explorers could use their mobility to scrutinize atmospheric chemistry and meteorology in a manner not achievable by simple entry probes.
"Nonetheless, it is crucial to insure that such proposed aerial explorer missions are not seen as merely technology demonstrations but the best tools available to meet the objectives of a science-driven mission. Having said that, though, the development of aerial explorers will also have a tremendous technology payoff in terms of other related fields -- including, for example, advances in power systems, microsensors and vehicle automation," Young explained.
At Ames, both fixed-wing and vertical-lift aerial vehicles for planetary exploration are being studied. Among an armada of designs: a Mars rotorcraft, rotary-wing probe decelerators and semi-buoyant hybrid airships for Venus, and robotic gliders for the outer-planet gas giants.
Meeting science objectives
Over the last several years, talk about planetary aerial explorers has tightly focused on Mars airplanes or balloons. Young is quick to point out, however, that no one single vehicle type, or design configuration for that matter, is going to meet all science objectives.
"Certain vehicle types will be better for certain missions and science objectives. Multiple missions using multiple vehicle types might well be required to conduct the comprehensive research dictated by the planetary science community." Young said.
Young advised that there are many planetary bodies in our solar system, in addition to Mars, where aerial explorers might be used to great advantage. Scientific investigation of Venus, Titan and the outer gas giant planets could all benefit from the use of aerial explorers as well.
"In each case, the planetary environment and the anticipated science objectives have to be factored into the conceptual design decisions," Young concluded.