Bugs on Mars: unearthly aircraft may explore the Red Planet--and beyond.

Anthony Colozza hopes to unleash flying robots on Mars. That may sound like the brainchild of a crazed, sci-fi film director, but the proposed bird-size robots are actually a technology designed for serving planetary science. Aerospace engineer Colozza and his colleagues are convinced that the machines, which will fly as insects do, may be the perfect explorers for the Red Planet.

Compared with rovers hobbling over all kinds of terra incognita, unmanned flying machines could peruse much more of a planet and do it faster, say Colozza and other proponents of flying robots. And unlike planetary orbiters that scan a lot of terrain at low resolution, autonomous aircraft would enjoy a closer view and might even be able to drop down at selected spots to examine an area in detail or to take and analyze samples.

Aeronautical engineers have been designing aircraft for missions to Mars since the 1970s. Some researchers are investigating the use of giant balloons, but most proposals are for fixed-wing fliers, like conventional airplanes. However, because there are no runways for landings and take offs on Mars' rubble-strewn, canyon-streaked surface, the first launch of a fixed-wing plane into the Martian atmosphere would also most likely be the aircraft's last.

Given those limitations, a handful of researchers, including Colozza of the Ohio Aerospace Institute (OAI) in Cleveland, have begun looking into robotic aircraft that could land and take off vertically in the extremely thin Martian atmosphere. With the right power source and an efficient design, such craft could carry out prolonged surveillance missions as well as detailed investigations of selected locations on the planet.

The insectlike machines being pursued by OAI and the Georgia Institute of Technology in Atlanta would carry out those tasks by vigorously flapping their wings--a means of flight long used by birds and insects but never mastered by human aviators. With that approach, Colozza and his colleagues are making a leap into the unknown. Although flapping as a means of flight propulsion is still in its infancy for aircraft on Earth, Mars' "strange combination of low air density and low gravity make that place perfect for flying large bugs," Colozza says.

Other researchers are pursuing more conservative designs that resemble helicopters. However, under the Red Planet's exotic flight conditions, even those more ordinary vehicles may have to assume unfamiliar forms. In one design, for instance, the rotors of the proposed Mars whirlybirds resemble the vanes of windmills.

OUT OF THIN AIR Despite his current passion for flapping wings, Colozza started out working on Mars flight as a fixed-wing guy. He was part of a now-defunct NASA project that would have celebrated in 2003 the centennial of human flight on Earth by launching a small, fixed-wing airplane into the Martian skies.

The idea of using flapping-wing vehicles came later to him and his colleagues, almost as an afterthought, given what they knew already about aerodynamics and the Martian atmosphere. The alien air is so thin--roughly equivalent to Earth's atmosphere at 30,000 meters--that a fixed-wing plane would have to fly faster than 400 kilometers per hour (km/h) to avoid crashing. Below that speed, it wouldn't be able to generate enough lift to stay airborne. For comparison, 400 km/h is about the top speed of a single engine, four-passenger plane traveling in Earth's skies.

To carry sufficient scientific gear, the Martian aircraft have to be bigger than flies or bees. However, if an aircraft's wings flapped in the manner of those of insects, Colozza and his colleagues realized, the flier's increased size and the decreased Martian air density could balance each other out.

"The size of a vehicle and the viscosity and density of the fluid [the air] are the main factors" in whether the vehicle can fly, Colozza notes. "You can play with those and, as long as the equation gives you the same final number, [the aircraft] should be the same aerodynamically."

Colozza had read in Scientific American that Georgia Tech was developing a 50-gram, flapping-wing flier for use on Earth. It has a wingspan of 15 centimeters. The Atlanta team's research, funded by the university and military agencies, was part of a broader effort in academia and industry to develop palm-size fliers called micro air vehicles (MAVs).

Unobtrusive and potentially cheap, those miniature craft are expected to be able to slip into buildings or caves and take pictures and sensor readings. Once inside such places, MAVs might also unleash weapons on holed-up enemies or help find missing persons.

Recently, findings on the aerodynamics of insect flight have convinced researchers that flapping-wing MAVs may offer an advantage over rotary or fixed-wing versions. Insects "don't fly like birds or planes--they use completely different principles of aerodynamics," Colozza says.

An insect's wing motions create vortices of air along the wings' edges that produce exceptionally strong forces for upward and sideways propulsion (SN: 6/19/99, p. 390).

"Those vertices generate huge amounts of lift," says Colozza. So, flapping-wing MAVs could potentially carry heftier payloads and outmaneuver other microfliers.

In spring 1999, Colozza bounced his idea for a Martian flier off Robert C. Michelson, leader of the Georgia Tech group that was making the 50-gram fliers. "He didn't seem to think there was any reason we couldn't `grow' the vehicle" to much larger sizes, Colozza recalls. For the past year and a half, the OAI--Georgia Tech team has been working on a feasibility study of the idea.

The NASA Institute for Advanced Concepts (NIAC), a think tank established in Atlanta in 1998, has been paying for the study. NIAC fosters innovative space-related ideas expected to prove important 10 to 40 years from now, says the institute's director, aerospace engineer Robert A. Cassanova. Recently, the institute also sponsored Ilan M. Kroo of Stanford University to look into tiny helicopters--called mesicopters--that may also have some potential for use on Mars.

Cassanova says that flapping-wing and rotorcraft fliers that could work in Martian skies look like they'll be ready for action in about 10 years rather than 40. Moreover, he predicts, "there are going to be some breakthroughs here in understanding the aerodynamics of flapping wings and very small rotary blades," advances that could change the character of Earth's own flying fleets.

BUG JUICE: There's no insect on Earth quite like the one that Michelson and his crew are putting together. They've dubbed their sparrow-size vehicle the entomopter--a name derived from the Greek words entomon for insect and pteron for wing.

The device resembles an oversize dragonfly, although one of its twin pairs of wings is at its tail. It also appears to be clasping to its abdomen a cylindrical beetle--actually, the flier's engine to which are attached four movable legs for short-distance crawls.

Although the researchers have made a plastic scale model of the entomopter, they have yet to miniaturize the motor sufficiently. In the meantime, the engineers have demonstrated the flight-worthiness of their design with a rubber band powered model of the aircraft made from paper and wood. They expect the final entomopter to cruise along about as fast as a person runs.

The team envisions the Mars-exploring versions--existing now only in designs and computer simulations--as hunkering down on the flat top of a wheeled rover like fighter jets hugging the deck of an aircraft carrier. "This whole unit will explore slowly across the surface. The entomopters fly off and come back, and they guide the rover," says Colozza.

During each of their brief flights, the entomopters might venture a couple of kilometers from their mother rovers and measure magnetic fields, take high-resolutions images, or perform other duties.

The fliers will flap their wings constantly at about 10 times per second and cruise at 50 km/h, Michelson says. However, by modifying their flying angles and other maneuvers, the vehicles will be able to fly at the much lower speeds needed for many scientific missions. "The whole intent is to try and get a slow flyer on Mars," Michelson says. "Theoretically, we'll be able to hover."

With nearly a meter wingspan, the Mars fliers will be a lot bigger--and much more massive at 2.5 to 3 kg--than the Georgia team's Earth version. However, because Mars' gravity is only 37 percent of Earth's, the entomopter should still be able to carry a payload of up to 1.5 kilograms of scientific equipment.

To make the entomopter especially efficient, the researchers are planning other innovations. In particular, they intend to recycle exhaust from the motor back through the vehicle in a half dozen ways. For instance, waste heat will be converted to electricity by a thermoelectric device. Discharged gases shot through orifices will create beams of ultrasound that the entomopter will use to detect obstacles, much as bats do with the high-pitched clicks they emit when they fly.

Also, by being vented through strategically placed openings on the wings, those gases will steer the entomopter and squeeze more lift out of the flapping-wing design. Robert J. Englar, a member of the Georgia Tech team, had demonstrated that such gas jets can boost the lift of fixed-wing aircraft and may also improve the aerodynamics of large tractor-trailer trucks (SN: 10/28/00, p. 279). However, "running gas through a flapping wing is something absolutely new," Michelson says.

WEIRD WHIRLEYS Rather than gambling on an approach with so many aerodynamic and other engineering unknowns, other researchers are exploring vertical-lift Martian fliers based on well-established helicopter technology.

To generate enough lift to raise a craft of a certain mass in the thin Martian air, conventional rotors would have to spin much more rapidly than they do on Earth. Yet, rotorcraft designers can't simply develop vehicles with very fast spins because of troubling shock waves they would produce.

The mix of gases in the Martian atmosphere makes the speed of sound 20 percent slower than it is on Earth. A rotor blade exceeding the speed of sound generates shock waves that interfere with its lifting action. Fortunately, there are other ways to generate greater lift--by increasing the area of the blade, for instance. That comes with a vexing cost, however. It increases the drag on the blade, costing more fuel to drive the aircraft.

A team of graduate aerospace engineering students at the University of Maryland in College Park has confronted these conflicting challenges.

"The cross-sections of our rotor blades do not look like helicopter blades but like [wind] turbine blades," says Anubhav Datta, who led the student team. Because wind turbines are designed to harvest wind energy, they're made to work in slow-moving air--aerodynamically the equivalent of thin air. Also, they're shaped to keep the air flowing close to more of the blade's surface than a helicopter blade does.

The students also tailored the shape and proportions of their blade to keep the airflow subsonic. The design earned the Maryland team the top award in 2000 for the American Helicopter Society's annual student-design contest.

NASA researchers, too, are exploring Martian helicopter design. This spring, Larry A. Young of the Army/NASA Rotorcraft Division at NASA Ames Research Center in Moffett Field, Calif., and his colleagues have begun testing the aerodynamic performance of other prototype Martian rotor blades. The tests take place in a vacuum chamber more than 6 meters across. The researchers have also begun building pared-down models of whole rotorcraft, which they plan to test for lift and other performance features.

A series of Mars Scout missions, which NASA has slated to begin in 2007, may use some type of aircraft. Both Young and Colozza say they expect to see fixed-wing planes fly on Mars first, but vertical-lift fliers may not be far behind. "People will see the benefits [of aircraft], and then further-out technology like the entomopter will be the next generation," Colozza predicts.

Young is looking even further out in time--and space. He has been giving talks at engineering meetings around the world promoting what he calls planetary aerial vehicles for exploring any planet with an atmosphere. He's particularly enthusiastic about fliers that take off and land vertically.

Venus is a potential candidate for such craft, he says. It's the only planet other than Earth where aircraft have already flown: Two Soviet balloons drifted above the planet in the mid 1980s.

Young is also co-organizing a NASA-sponsored contest for next fall in which university students will design a vertical-lift vehicle for Saturn's moon Titan. Scientists suspect that this body harbors molecular precursors to life (SN: 11/1/97, p. 284).

After nearly a century of artificial flight on Earth, he says, the time is right for a new age of flight on other worlds.