Cold Peril: The Ulysses Spacecraft

Engineers quickly figured out the problem. "All of Ulysses' science instruments had been activated to study Jupiter," explains McGarry, "and this was straining the RTG to its limit." Ulysses would have trouble supporting even one more device. But when a signal arrived from Earth, another device did turn on, automatically: the decoder, which translates radio signals into a stream of binary ones and zeros understood by Ulysses' computers. "The decoder was stealing power from the heaters."

An X-ray image of the sun overlaid with an 11-year plot of sunspot numbers.

Since then ground controllers have learned to keep their transmissions to Ulysses brief, so the temperature can't fall very far.

Ulysses is about to turn away from Jupiter and head back to the sun. Eventually solar heating will keep the hydrazine warm, and onboard heaters can be turned off, "but that won't happen until 2007," says Angold. Meanwhile, engineers at JPL keep a constant watch on the spacecraft.

Mission scientist Steve Suess at the NASA Marshall Space Flight Center believes it's worth the effort. "The extended mission gives us a chance to learn a lot more about the sun." Of special interest is the Solar Minimum. Solar activity waxes and wanes every 11 years, he explains. Ulysses studied the sun's quiet phase, Solar Minimum, between 1994 and 1995. Now Ulysses gets to do it again. "The next Solar Minimum is due around 2006," says Suess, "but it won't be the same as before." In 2001 the sun's magnetic field flipped. The north pole shifted south, and vice versa. Magnetically speaking, the sun is now upside down. How will that affect Solar Minimum?

Perhaps Ulysses will find out … if it doesn't freeze to death first.

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