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Former glory: SOHO's view of a coronal mass ejection from the Sun last April. Credit: NASA

Ground controllers regained command of the errant Solar and Heliospheric Observatory (SOHO) last week for the first time in almost three months. Their achievement has raised hopes that scientific use of the European-US satellite could resume within the next two months.

After weeks of slowly thawing fuel lines connected to SOHO's onboard thrusters, a team of European and US engineers working at NASA's Goddard Space Flight Center in Maryland successfully commanded the spacecraft to point its onboard solar power arrays toward the sun on 16-20 September.

Next will begin check-outs of key engineering subsystems and SOHO's 12 scientific instruments, which suffered extreme heat and cold during the period in which the spacecraft was uncontrolled. Mario Acuña, project scientist for the International Solar-Terrestrial Physics programme at Goddard, says that temperatures on some instruments went beyond the range that onboard sensors could record.

Still, he says, “the mood is one of optimism” that SOHO will resume scientific observations. If all the instruments work properly, that could happen in as little as four to six weeks, according to project engineers.

An accident investigation board determined earlier this month that the loss of SOHO in June was due to mistakes made by ground controllers, including an erroneous decision that disabled part of the spacecraft's autonomous failure detection system. Ironically, the day after SOHO was recovered, the Mars Global Surveyor (MGS) currently in orbit around Mars experienced its own brief shutdown caused by a faulty computer instruction sent up from the ground.

Engineers at NASA's Jet Propulsion Laboratory had been preparing for a propulsive manoeuvre to begin lowering the Surveyor's orbit around the planet when a bad software command turned the craft's solar power panels at the wrong angle to the sun. That made onboard batteries start draining power. In this case, however, the spacecraft's automatic fault protection system caught the mistake and aborted the manoeuvre when power fell below 50 per cent.

The fact that a faulty software command survived several levels of review on the ground before being transmitted to the spacecraft is “a little disconcerting”, admits MGS project manager Glenn Cunningham. But — unlike SOHO — the MGS's fault protection system saved the day. “It did exactly what it was supposed to do,” says Cunningham. As a result, the Mars spacecraft is safe and stable, and is due to resume lowering its orbit as planned this week.