SCIENCE / MEDICINE : Soviet, U.S. Teams Create Mirror Image Mars Rovers : Space exploration: Prototypes show remarkable similarities despite sharply different approaches by the two nations.
One scientifically enlightening aspect of the Soviet policy of glasnost is the increasingly intimate glimpse of the Soviet space program being offered to Western scientists.
Earlier this year, visiting U.S. scientists were shown prototypes of a Soviet moon lander that was never used. Last week, the chief designer of the Soviet moon and Mars rovers made his first visit to the United States, bringing with him hitherto unseen photographs and videotape of the Soviet space program’s first attempts to design a Mars rover, as well as pictures of the latest version.
Mechanical engineer Aleksander Kemurjian of the Soviet Transport Industry Institute, an arm of the Defense Ministry, showed the pictures at a public lecture at Caltech and in a press conference sponsored by Pasadena’s Planetary Society.
The most extraordinary aspect of the Soviet design was that it closely parallels a Mars rover being developed at JPL. Caltech planetologist Bruce Murray called the resemblance a case of “parallel evolution.”
The coincidence is all the more remarkable, according to Donna Pivirotto, head of the JPL Mars rover program, because the two nations have approached the project from sharply different directions.
The Soviets have focused on building full-scale models of potential rovers and testing them in hostile environments--such as the volcano fields of the Kamchatka peninsula--to refine their designs.
In contrast, the American teams designing Mars rovers have been hampered by lack of funds and have concentrated more on less-expensive computer modeling studies. So far, the JPL team, for example, has built only a one-eighth-scale model of their design--”all we could afford,” Pivirotto said.
The fact that the two approaches have yielded very similar designs, she said, “is very gratifying.”
Both nations early on made the decision to discard tracked, Caterpillar-like vehicles because the tracks entrain a lot of dirt, are not energy-efficient and wear out fast. Both groups also have studied walking and six-wheeled vehicles. The Soviets have settled on the wheeled version, but NASA has not yet chosen between the two concepts. A walker is being developed by FMC Corp.
According to Kemurjian and Valery V. Gromov of the Soviet Institute of Transport Machine-Building, the Soviet design is unique in that it combines both the rolling function of wheels and the stepping function of legs, giving it an unprecedented ability to surmount obstacles.
The Soviet Mars rover is about 1.8 meters (5.4 feet) long and weighs roughly 440 pounds, 88 pounds of which represents its cargo of instruments. Each of the 1 1/2-foot-high wheels is independently sprung and powered, which in itself gives the rover great mobility. The JPL rover, in contrast, is expected to be about the size of a small car, weighing 1,750 pounds and carrying 330 pounds of payload.
An unusual feature of the Soviet design is that the segments between the front and middle wheels and between the middle and rear wheels can each be extended by as much as 0.6 feet. If the vehicle is having a hard time making it up a sandy slope, for example, the rear wheels are held in place and the front pairs are pushed forward.
The rear wheels can then be pulled up to join the rest of the vehicle, giving the rover the gait of a caterpillar. Because of this design, Kemurjian said, the vehicle can climb over obstacles twice the height of the front wheel. In contrast, the highest obstacle the non-extendable JPL rover can surmount is only 1.6 times the wheel height.
The advantage of the American design, Pivirotto said, is its simplicity: there are fewer moving parts to go awry a long way from home. The advantage of the Soviet design is that it may have more flexibility in the types of obstacles it can surmount.
Although the Soviets are well ahead of their American counterparts in building and testing rovers, they are apparently far behind in automating the rovers so that they can operate autonomously on a distant planet. JPL is now testing a guidance system with stereoscopic vision and artificial intelligence that would allow the rover to avoid problems. Experts say the Soviets are not nearly as far along in this area.
Both the Soviet and American vehicles will probably be powered by radioisotope thermoelectric generators, in which heat from radioactive fission is converted into electricity. Batteries are too heavy, and solar panels do not allow the rover to move at night. More important, the service lives of the Surveyor instruments on the moon were significantly shortened by the extremely cold temperatures encountered at night.
The thermoelectric generators will allow the electronic packages to be kept warm even in the absence of sunlight.
A CASE OF PARALLEL EVOLUTION Researchers in the U.S.S.R. and at the Jet Propulsion Laboratory each arrived at six-wheeled designs for a Mars rover. The Soviet vehicle’s wheels, below, are larger to spread the load more, and the front and rear axles can be extended from the center to give it a caterpiller-like gait.
