SOHO Learns the Cruder Arts of Navigation

Like a 19th century ship captain forced to navigate by sextant after losing his chronometer, the Solar and Heliospheric Observatory (SOHO) is now taking its cues from the sun thanks to an innovative solution to a dilemma that threatened to end the spacecraft's mission. Earlier this week, ground controllers at Goddard Space Flight Center in Greenbelt, Maryland, beamed aboard SOHO a jury-rigged software program that has reoriented the spacecraft, which now has no functioning gyroscopes. By relying on a sun sensor instead, engineers expect that the craft should be able to keep its bearings for at least 4 more years.

This latest problem began after the last of three gyroscopes failed on 21 December ( ScienceNOW, 4 January 1999). The craft started spinning slowly and tripped into a safety mode that aimed to keep the craft's solar panels pointed at the sun. But this holding pattern was perturbed after a sensor triggered the brief firing of a hydrazine thruster to correct the spacecraft's position. It overshot, then fired again to compensate. This thrust-counterthrust duet kept repeating. "We were [wobbling from] a thruster pulse about every 7 minutes or so, rapidly depleting the hydrazine fuel," recalls Bernhard Fleck, ESA project scientist for SOHO. "It was a race against time."

But taking the craft out of safehold mode, as it's called, required input from the gyroscopes--all of which were broken. Instead, the engineers wrote software that allowed the spacecraft to ignore the gyroscopes. Next they fired the thrusters to stabilize SOHO and fine-tuned its orientation by a subtle braking and accelerating of flywheels. The craft has been working normally since 2 February. "The operation was well planned and conceived," says Alan Gabriel of France's Institute of Space Astrophysics near Paris. "It brought the spacecraft back from death."

One hitch remains: Although SOHO is pointed at the sun, the images it sends back are rotated 69 degrees, which interferes with some measurements. This roll position is normally controlled by a star tracker--but that instrument is pointed away from its beacon. Engineers will attempt to fix this particularly thorny problem without gyroscopes early next month, says Fleck.

Astronomers are now geared up to start scrutinizing the sun again as it approaches a peak of sunspot activity around 2001. They lost about 6 weeks of observation time, but consider it a small price for having the spacecraft back online.