Cold Peril: The Ulysses Spacecraft

The NASA/ESA Ulysses spacecraft is perilously cold as it begins a newly extended mission to study the sun.

Deep space is cold. Very cold. That's a problem - especially if you're flying in an old spaceship. And your power supplies are waning. And the fuel lines could freeze at any moment. Oh, and by the way, you've got to keep flying for thirteen more years.

It sounds like a science fiction thriller, but this is really happening to the NASA/European Space Agency spacecraft Ulysses.

Ulysses was launched in 1990 on a five-year mission to study the sun. The craft gathered new data about the speed and direction of the solar wind. It discovered the 3D shape of the sun's magnetic field. It recorded solar flares on the sun, and super-solar flares from distant neutron stars. Ulysses even flew through the tail of comet Hyakutake, an unexpected encounter that delighted astronomers.

The mission was supposed to end in 1995, but Ulysses was too successful to quit. NASA and the ESA have granted three extensions, most recently in Feb. 2004. Ulysses is scheduled to keep going until 2008, thirteen years longer than originally planned.

Ulysses' extended mission, as before, is to study the sun. But at the moment Ulysses is far from our star. It's having an encounter with Jupiter, studying the giant planet and its magnetic field. Sunlight out there is 25 times less intense than what we experience on Earth, and Ulysses is getting perilously cold.

An artist's rendition of Ulysses' encounter with Comet Hyakutake.

Back in the 1980's, when Ulysses was still on Earth and being assembled, mission planners knew that the spacecraft would have to endure some low temperatures. So they put dozens of heaters onboard, all powered by a Radioisotope Thermoelectric Generator, or "RTG." These heaters have kept Ulysses comfortably warm.

But there's a problem: the RTG is fading.

more Ulysses' long looping orbit carries it far from the sun. Every six years the spacecraft travels from the inner solar system to the realm of Jupiter and back again.

"The power output of the RTG has been dropping since the spacecraft was launched," says Nigel Angold, the Ulysses ESA Spacecraft Operations Manager at JPL. RTG power naturally fades as its radioactive source decays. That's as expected. What planners didn't expect was 13 years of extra operations.