Magellan Hiccups, Sails On

September 21, 1990

America's space program badly needed a success story when the Magellan spacecraft reached Venus, and the $750 million radar surveyor seemed made to order. It swung by, using gravity to slow its approach, and maneuvered into orbit right on time. But then came the "hiccups": just as NASA scientists were congratulating themselves, the spacecraft stopped transmitting.

Managers at the Jet Propulsion Laboratory said Magelladropped out of contact twice last month before its signal was re-acquired for good. Computer "hiccups" blocked the contact.

Magellan is designed to pirouette back and forth as it orbits. It points its dish antenna at Venus for mapping, then turns to Earth to transmit its data. That's worked for 30 orbits, illuminating a slice of Venus comparable to mapping the Earth from North Pole to Antarctica, from Los Angeles to Albuquerque, but there are many more orbits to go. Magellan must complete such turns many times over its year-long working life, and it notes its direction by locking onto certain stars.

Here's where the hiccups come in. If the scanner picked the wrong stars, the spacecraft sees that, turns to the sun to power up its solar arrays, then hunts for the correct guide star. Then the spacecraft can re-orient itself, re-establishing contact with Earth. But a backup computer's momentary glitches keep throwing everything off. NASA's brain trust hasn't figured out what causes the hiccups, so they've programmed the system to ignore them.

It might have been better in the first place to have paid for a second antenna dedicated to communications. Three-quarters of a billion dollars is a lot to sink into a remote orbiter and risk losing its findings because of a pivoting failure. Redundancy might have been wiser, especially with only four hours' battery life. All seems to be well now, but success should not be jeopardized by hiccups.

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