Yohkoh Sun-watching craft remains out of commission

Posted: January 28, 2002

Engineers continue to pursue attempts to recover the decade-old Yohkoh solar probe after a chain of events last month halted science observations.

The troubles began on December 14, when Yohkoh began passing through a deep eclipse of the Sun, momentarily upsetting the spacecraft's attitude control system. Because the observatory was passing out of range of ground stations, controllers were unable to immediately get the satellite's systems back to normal.

The delay in contacting Yohkoh resulted in the spacecraft drifting off the correct pointing to the Sun, causing the solar panels to stop charging correctly. When the ground team was finally able to communicate with the satellite again, they found it drifting and with low power levels of around 15 volts in its batteries. Even switching off the craft's intruments did not help the electrical situation. The batteries lost even more voltage in the days following the occurrence, with levels dropping to around 4 volts on December 17.

Yohkoh project manager Takeo Kosugi characterized the events as a "chain of unlucky troubles" in a presentation at the Yohkoh 10th Anniversary meeting in Hawaii last week.

The episode that has befallen Yohkoh is similar to what happened to the SOHO observatory in 1998, when a full recovery was made.

"We are now waiting for warmth to return to the batteries, monitoring the conditions and studying ways and means of getting the batteries charged once again, and -- if successful at that, a big "if" indeed -- then re-orienting Yohkoh and establishing normal operations," said a statement issued shortly after the incident.

Over six weeks have passed since the initial event that triggered the barrage of events leading to the current status of the now-crippled Yohkoh spacecraft, and very little progress has been made.

The team is "still struggling" with recharging the craft's batteries, Kosugi reported. At least 7 volts of electrical output are required to get the restoration process moving along. If the electrical side of the recovery is corrected, next would come the re-activation of the attitude control system, allowing the spacecraft to resume pointing toward the Sun. Other systems and instruments aboard Yohkoh would then be switched on, completing the revival of the international solar studies spacecraft.

However, Kosugi's presentation said that there are no good expectations for the current recovery attempts to succeed.

With no luck thus far in bringing the satellite back to life, officials may eventually be forced to make a decision to end the efforts to bring Yohkoh back on-line. "We'll continue attempts at least one more month," Kosugi told Spaceflight Now.

A joint mission between Japan, the United States, and the United Kingdom, Yohkoh was launched in August 1991 on a mission to study X-ray and gamma ray emissions from the Sun. Orbiting at an average altitude of 650 kilometers, Yohkoh spends around two-thirds of each 90-minute trip around the Earth bathed in sunlight to afford the best data.