Nasa’s Mars rovers, spirit and opportunity, dazzled the world with their journeys across the planet’s rusty landscape, but they have huge limitations: They can travel no more than about six miles during their entire life spans, and they are stuck on the surface. Mounting a serious search for water and signs of ancient life demands probes that can fan out over vast areas, both above and below ground, and poke into nooks and crannies. Several forward-looking NASA researchers say it is time to abandon the six-wheel-drive mind-set and design probes inspired by the original organic vehicles of exploration: bees, insects, and seeds.
MIT engineering professor Steven Dubowsky proposes creating a swarm of baseball-size probes that can be released by a mother ship several hundred feet above the Martian soil. They will then hop, bounce, and roll downhill to low ground, exploring gullies and penetrating open caves. If a probe gets stuck, a mechanical foot can pop out to bounce it off in a new direction. Because each probe will be cheap—Dubowsky estimates they can be mass-produced for $10 apiece—NASA could release 1,000 at a time and mimic the way trees scatter seeds: Only a tiny minority need to succeed.
“If you can get into the Martian caves, that’s where we’re most likely to find ice and life,” says Dubowsky, who has tested his concept by releasing prototype probes into caves on Earth. Such devices could also expand the range of manned missions: “Astronauts could carry them in backpacks and release them to explore deep ravines.”
John Manobianco, a director of advanced technology at Ensco, an engineering firm in Falls Church, Virginia, is working on a related concept in which swarms of probes waft down like dandelion seeds. He is designing featherweight, grapefruit-size hollow balls that can be dropped from a spacecraft or balloon, charting storms and other weather patterns as they descend. A related approach could prove useful on Earth as well. Manobianco has talked with Department of Defense officials about equipping similar, helium-filled devices with chemical sensors and minicameras to perform military surveillance.
NASA’s Tumbleweed concept spins another variant on the theme: 18-foot inflatable Kevlar balls that are so light they could be propelled across the ground solely by Martian winds. Smaller prototypes released at the South Pole and in Greenland traveled as far as 140 miles, about 40 times the distance each Mars rover has covered so far. When the Tumbleweeds encounter something that merits closer study, they can deflate to stop moving and deploy sensors. “We could even include generators inside that move when they roll, so you get power for the onboard instruments for free,” says Alberto Behar, a roboticist at NASA’s Jet Propulsion Laboratory.
To cover still more terrain, engineers want to mimic birds, nature’s quintessential long-range travelers. One NASA-sponsored study sketches designs for probes crafted from sheets of electroactive polymer, which can change its shape to twist and flap like a thrush’s wings. On slow-rotating Venus, a solar-powered flapping machine could fly continuously for months in the cool upper atmosphere, above the planet’s sulfuric-acid clouds. “There’s even been speculation that the upper part of Venus is where you could find life,” says Anthony Colozza of NASA’s Glenn Research Center, who is working on the robots.
Probes like these are just drawing-board concepts. Six-wheeled robots are likely to remain the mainstay of space exploration for the next few missions. “Unseating the thinking that you can do anything with something other than rovers is probably the biggest challenge right now,” Manobianco says wryly. But NASA is considering airplanes and balloons for its post-2011 Mars missions as well as for other destinations ill suited to wheels. Landers on small bodies like comets and asteroids might require tethers, while a proposed voyage to seek life on Europa might include a submarine to swim through that moon’s putative ice-covered ocean.
Robert Frisbee is a senior engineer at NASA’s Jet Propulsion Laboratory and one of the agency’s top brainstormers. He likes to examine the plausibility of futuristic propulsion systems that could accelerate space travel and even take us to the stars.
What do we need to get back to the moon and on to Mars?
F: If you’re just going to the moon, you can use chemical propulsion. For longer distances, the problem is that chemical fuels are so heavy. If we were exploring the United States of 200 years ago the way we explore space now, we’d have a mule and a wagon, but we would have to carry all the food and water for the mule with us! So we need to figure out how to make propellant along the way—like from water ice on the moon or on Mars. Then you don’t have to carry all the propellant you need to get back home.
Chemical rockets would take more than two years to go to Mars and back. What’s faster?
F: A nuclear-thermal rocket—a fission reactor that heats hydrogen to 4,000 degrees Fahrenheit—gives you an exhaust velocity twice as fast as the best chemical rocket. You could do a round-trip from Earth orbit to lunar orbit and back in 24 hours. For Mars it’s twice as fast as a chemical rocket: a round-trip in about one year. That’s short enough that you probably could get away with having everyone at zero g without any bad health effects.
Is there an even faster way to travel?
F: If we ever perfect nuclear fusion, a fusion rocket could do a round-trip to Mars in about three or four months. You could get to Jupiter in a year. All we have to do is make nuclear fusion work—and that, as they say, is left as an exercise for the students.
The Planetary Society recently tried launching a solar sail. Do sails make sense?
F: They don’t require any fuel, and they’re great for hauling cargo. Solar sails are a promising way of doing an interstellar precursor mission. If you had to do it using a regular rocket engine, you’d need an enormous amount of propellant. With a solar sail, you just fly very close to the sun, turn the sails face on toward the sun, and let the sunlight pressure blast you out of the solar system.
What about better ways of getting off the ground in the first place?
F: You could build a space elevator [a cable running from Earth’s surface 22,000 miles up]. The disadvantage is that it’s like building the entire interstate highway system before the first car could travel on it. But once it is built, the operating cost would be dollars per kilogram to put something into orbit. Today the cost is around $10,000 a kilogram. You could also have a spaceship on a maglev rail going up the side of a mountain at 45 degrees. When it gets going up to almost Mach 1, the vehicle cuts loose and rockets into orbit.
What are the really far-out propulsion ideas?
F: There’s the old Captain Kirk favorite: antimatter. A collision of matter and antimatter has an exhaust velocity of about a third the speed of light. Maybe we’ll have a breakthrough in physics that allows crazy stuff like wormholes and warp drives. Or maybe nature’s going to be a stinker and say, “No, you can’t do that—here’s why.”