Yohkoh Loses Control

The Yohkoh spacecraft lost control when its Sun-acquiring system failed during an annular eclipse. Click on image to see what the craft saw as it failed to reacquire the Sun.
Courtesy Institute of Space and Astronautical Science (ISAS).
On December 14, 2001, the Japanese solar observatory Yohkoh began spinning out of control. Since then, all scientific operations have stopped, and it remains unclear when the craft will be operational again.

The problem began during last month's annular eclipse of the Sun. Yohkoh uses a Sun-centering system to determine its position at any given time. During the eclipse, the craft lost contact with the Sun, put itself into a "safe mode," and slowly began to drift off track and rotate. Normally this wouldn't have been a problem — during its decade in orbit, Yohkoh has seen its share of eclipses. However, this event occurred during a rare period of the craft's orbit (known as an invisible orbit) when the craft was out of communication with Earth. Thus controllers on the ground couldn't detect (or compensate for) the craft's sudden roll.

Problems only got worse from there. Because of its slow roll, Yohkoh's solar panels no longer received direct sunlight. By the time ground controllers at the Kagoshima Space Center regained contact with the observatory, its batteries were very low and the craft had lost attitude control.

To fix the problem, scientists first established contact and turned off all the craft's science instruments in order to conserve power. Currently the craft is rotating slowly, about one rotation per minute. According to Loren Acton (Montana State University), head scientist of Yohkoh's solar X-ray telescope, in the spacecraft's current state, its solar panels only receive sunlight in spurts. "During flashes of illumination, electricity is produced," says Acton. Thus the first step toward recovery is for scientists to wait until the craft can charge up.

It's currently unclear when, and even if, scientists will regain control of the craft. But astronomers are hopeful. "It will take clever work to stop the roll and reacquire the Sun," says Acton.

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