The challenge of making satellites reliable
It amounts to a $75 million set of fuses. When the Solar Max, a solar observation satellite, was rocketed into space in 1980, it was supposed to send back data for a least four years. It wobbled out of control in six months. Blown fuses in the system designed to keep it tilted toward the sun were the culprit.
The costly failure of the Solar Max -- and others like it -- underscores a continuing concern of scientists in the space age: satellite reliability.
Later this week National Aeronautics and Space Administration astronauts will try to tackle the problem for the first time from a new vantage point. Donning jetpacks, they will maneuver out form the Challenger space shuttle and attempt too repair the solar Max in space. If successful, their mission will inaugurate what many in the space community hope will be a new era of routine maintenance of satellites in orbit.
But until in-space service calls, either from vehicles like the shuttle or a space station, become commonplace, the resonsiblity for reliability will rest on clipboard-carring engineers on Earth.
In the 25 years since the first probes were sent aloft, designers have made long strides in increasing their operating life. but as satellites become more complex and as more are sent into space, new challenges are arising.
The issues isn't an idle one. Satellites are costly: Even a mundane scientific probe or communications craft usually costs $15 million to $75 million. The recent communications satellites that failed to reach proper orbit when launched from the space shuttle in February were insured for nearly $200 million.
"Satellites are not reliable like the phone on your desk," says Mark Oderman, vice-president of the Center for Space Policy Inc., a Massachusetts-based consulting firm.
Space is a hostile place to put a machine. First there's extreme heat and cold: Temperatures vacillate between near absolute zero when the craft is in the earth's shadow to serveral hundred degrees F. when it's in sunlight. The dramatic differences creat contraction and expansion problems. Parts also have to be tailored to work in a vacuum. The there is the radiation -- electrons, protons, ultraviolet rays, and more -- that bombard the probe, particularly during violent solar flares and magnetic storms. These threaten the electronic circuitry.
Satellite designers generally understand the hazards. but as new materials and equipment are put into space, questions continue to arise about how they react. Scientists at NASA and the McDonnell Center for the Space Sciences at Washington University, for instance, are looking into why some plastics are eaten away in low orbits. A few previously unexplained satellite failures have been attributed to the phenomenon.
Relatively few satellite turn out to be complete lemons. As long as they make it into the right orbit, most perform some functions even if they suffer partial breakdowns. This is due in part, to the numerous backup systems used on most probes -- extra batteries, fuel tanks, propulsion systems, and other critical components.
Lifespans of satellites vary widely, but in general, experts say, they have been increasing. There are currently some 5,200 satellies orbiting the earth. Most -- roughly 3,800 -- are no longer functioning. The first communications probes were lucky to last two years.
Today, a 10-year design life isn't uncommon.Better technology has helped. So has rigorous ground testing, in which satellites are seared in thermal vacuums to check for response to heat and rattled in vibration chambers to see if they will withstand the rigors of a launce.
But minor problems can still hobble a craft in space. A failed tape recorder can keep it from transmitting data back to Earth. Landsat 4, an earth-resources satellite, has had problems with several of its solar panel power cables. This has limited the amount of data it is sending. (It is, however, one of the satellites being eyed for in-space repair by a shuttle team.)
New concerns are arising, too. One factor making some craft more problem-prone than in the past is their complexity. Many now being sent aloft are multipurpose: They might be fitted with navigation as well as communications equipment, hence the possibility of more glitches. Even those that are built for a single purpose end up doing more than one chore. As well, early communications satellites had two "beams" for receiving and transmitting signals , while the latest ones may have as many as 48.
With greater launce capabilities, today's satellites are often bigger than in the past.The shuttle can carry a 60,000-pound payload. Early rockets could tote only 10,000 pounds. Herbert Hecht, president of SoHaR Inc., a California research firm, is conducting a reliability study for the US Air Force of some 2, 500 satellite failures that have occurred over the past 20 years. He says the breakdown rate has gone down for individual satellite functions. But because each satellite performs more tasks today, the number on which there is some kind of failure is rising.
Similarly, officials at NASA's Goddard Space Flight Center recently conducted a reliability study of some of its earth-resource, scientific, weather, and other satellites.The results showed that between 1975 and 1979 the probability of its probes lasting at least three years actually dropped (from 90 to 60 percent). There were no mission failures. But Robert Baumann, Goddard's deputy director for flight projects. was a chief reason for the downward tend was the increasing sophistication of staellites. Another factor: the agency has lately been stressing cost-control over reliability, he contents.