In the long and hallowed history of flight, engineers are writing a new chapter. It’s a chapter that involves skies more distant than the stratosphere of our own world. Space flight designers today are beginning to explore the possibilities of flight in the atmospheres of other worlds.

Up, Up and Away

Forays have already begun. In 1984, the Soviet Union deployed two French-designed balloon probes in the not-so-friendly skies of Venus. It had been two centuries since the Montgolfier brothers first took to the heavens in their hot-air balloon. The descendants of the French inventors’ open-air craft were super-pressure sealed balloons designed to float high above Venus’ 900º surface. The small airships drifted in the alien skies of the blast-furnace world for about 48 hours, charting winds and tasting the sulfuric acid hazes some 30 miles above the surface, in a region where temperatures are akin to those on Earth. Each helium-filled Teflon envelope spanned 11.5 feet in diameter and carried a battery-powered instrument gondola. The gondola hosted a variety of experiments. Both balloon stations were a resounding success; the Montgolfiers would have been proud.

Michael Carroll
The Soviet VEGA balloon probes drifted for several days in the skies of Venus.Michael Carroll, for the book "Drifting on Alien Winds: Exploring the Skies and Weather of Other Worlds," Springer publishers

NASA and the European Space Agency are studying follow-on missions using more sophisticated technology. One typical variation would carry a payload of about 40 kilograms (~90 pounds) consisting of a flight computer, batteries and transmitters. A science suite could include devices for measuring gases and detecting volcanic plumes, lighting sensors, and particle analyzers. As was done with the Soviet/French balloons, ground-based radio telescopes would chart the balloons’ trajectories, mapping winds on a global scale.

Fixed-Wings to the Stars

More conventional aircraft are under study for the high frontier. NASA Langley’s ARES (Aerial Regional-scale Environmental Survey) would ply the Martian firmament powered by a hydrazine rocket engine. Although orbiters and rovers are imaging the Martian surface in unprecedented detail, ARES has the advantage of in situ measurements in a critical area: the planetary boundary layer. It's one of the regions scientists know very little about. The boundary layer is a very dynamic zone where solar heating during the day causes the air to expand, while at night the air contracts.

Mars is a difficult place in which to fly. Its air pressure is equivalent to that of Earth at 100,000 feet (about .007 bar, or about 1%). To stay aloft, ARES will need to travel at speeds up to 450 miles per hour. ARES is very light, with most materials consisting of non-metallic composites. Engineering studies carried out at 102,000 feet above the Earth’s surface have demonstrated that ARES is a capable aircraft at even such low pressures. For navigation, a tail-mounted camera sends real-time images of the landscape ahead, while a downward-looking camera images objects less than a few centimeters across.

Another craft, the “Preliminary Research Aerodynamic Design to Land on Mars” (Prandtl-m) envisions a miniature drone weighing in at just 2.6 pounds. The flying wing design would touch down on Martian soil after flight.

Michael Carroll
A NASA/Jet Propulsion Laboratory design envisions a blimp that samples the surface of Titan’s cryogenic seas from the air.Michael Carroll

Some planetary aircraft designers have set their sights higher than Mars. Saturn’s moon Titan, the size of the planet Mercury, has much to offer future air travelers. The moon’s low gravity (just 1/7 that of ours) and dense atmosphere (1.5 times that of Earth at sea level) make flight simpler than anywhere else in the solar system. Johns Hopkins University Titan expert Ralph Lorenz says, “Titan affords lots of opportunities for mobility on larger scales. You can have a hot air balloon driven by the waste heat from a radioisotope generator, you can have a light gas balloon, or you can have an airship. You can fly heavier-than-air with an airplane. Helicopters would work well. Hovercraft have even been speculated.”

One recent proposal is called AVIATR, short for Aerial Vehicle for In-situ and Airborne Titan Reconnaissance. Advanced Stirling Radioisotope Generators — nuclear batteries — would power the drone-like vehicle, enabling it to remain airborne for an entire year, exploring the liquid-methane-soaked skies and seas of this strange world.

Michael Carroll
The NASA airplane study AVIATR envisions a drone-like vehicle that would explore the atmosphere and surface of Saturn’s moon Titan. Here, AVIATR comes to the end of its mission, settling upon one of Titan’s great sand dunes.Michael Carroll, for the book “Alien Seas: Oceans in Space,” Springer publishers

Blimps and Dirigibles

Other studies propose lighter-than-air ships for Titan. A recent NASA/JPL study proposes a blimp that could drop canisters to retrieve samples of soil or liquid from Titan's hydrocarbon lakes. The advantage of a blimp is longevity and simplicity: the craft floats passively without need for course correction or wind-current adjustments. The disadvantage is that the craft wanders at the whim of the wind. Still, scientists may have a good enough bead on Titan’s global breezes to deploy an airship at just the right location for a detailed survey of this intriguing, fog-enshrouded world. If all goes well, Titan and Venus will simply be testing grounds for flights into more challenging planetary destinations like the clouds of Jupiter and Saturn. The future of aviation awaits within the skies of distant worlds!

For more on the subject, see Michael Carroll's books here.

Michael Carroll
Perhaps in the distant future, our descendants will live and work in the skies of the giant planets. Here, buoyant science stations drift on the winds of Uranus beneath super-pressure balloon systems. The planet’s rings are visible as glowing lines in the sky.Michael Carroll, for the book "Living Among Giants: Exploring and Settling the Outer Solar System," Springer publishers