Satellite restored to service

Satellite restored to service

By RANDOLPH E. SCHMID

WASHINGTON (February 3, 1999 5:24 p.m. EST http://www.nandotimes.com) - The Solar and Heliospheric Observatory sun-observing satellite has been returned to service more than a month after malfunctioning gyroscope red-lined it.

The SOHO satellite's controllers returned the $1 billion apparatus to service Tuesday using a new computer program to help it keep its orientation without the failed gyroscope, Bernhard Fleck, European Space Agency project scientist for SOHO, said Wednesday in a telephone interview. SOHO stands for Solar and Heliospheric Observatory.

Trouble first developed on SOHO last June when it began spinning out of control. Scientists were able to return it to service in September, but it was frozen, and two of its three gyroscopes were damaged, said Fleck, who is based at NASA's Goddard Space Flight Center in Greenbelt, Md.

The third gyro failed Dec. 21.

Without the gyroscopes to keep track of its position, the satellite had to constantly fire on-board jets to keep pointed toward the sun, a practice that threatened to rapidly exhaust its fuel supply.

Fleck said engineers in the Netherlands were able to develop a computer program that allowed the satellite to bypass the broken gyroscopes and instead use a star tracker to allow the satellite to determine its position.

With that information, three spinning momentum wheels aboard the craft can be directed to speed up or slow down to keep it properly oriented, he explained.

This was done on Saturday, the craft was maneuvered to a new position on Monday and, on Tuesday, said Fleck, "we returned to normal where the spacecraft is perfectly pointing toward the sun."

Michel Verdant, SOHO's program manager, described the effort as similar to "instructing an airplane pilot from the ground to execute very complex maneuvers and helping him make a successful landing."

This is the first time that a spacecraft equipped with gyroscopes has carried on working without them, the European Space Agency said.

SOHO was launched in December 1995, and the mission is now expected to continue until 2003, allowing it to observe intense solar activity when the number of sunspots reaches a maximum around mid-2000.