Gravity Probe B

science - superfluid helium. The dewar for Gravity Probe B - 10 feet long and 7 feet wide - is the largest ever designed for space flight.

"They need an extremely low magnetic field, lower than anything achieved on Earth. The temperature has to be held constant to ten 1/1,000th of a degree Kelvin," says Richard T. Parmley, who is guiding Lockheed's work on the dewar. "Everything has to be incredibly stable. As temperatures change, materials change. When you get down to these temperatures, things are incredibly stable."

But every solution creates a problem of its own.

When Stanford researchers conceived the idea of supercooling their probe, no one had ever used liquid helium in a satellite. To control so much liquid helium in orbit is yet another demanding challenge.

The helium is also used to power the 16 thrusters that keep the probe from rolling or pitching in orbit. If uncorrected, even the faint breeze stirred by the solar wind could blow the spacecraft off course. That is why it must be kept so precisely.

"We can't have any distur-bances," Parmley says. "The helium gas is a very smooth and undisturbing method of control."

There was simply no way to start with the kind of engineering sophistication demanded by the relativity mission. While the concept of the experiment was sound, in 1960 the technology did not exist to implement it. So the Stanford team worked up to it by designing, building and testing a series of prototypes of all the key elements of the spacecraft, in order to master their complexity.

"The real challenge," says Everitt, "is putting together all these different technologies and making them play together."

Asked to pick out the single most important innovation among the dozens that have gone into Gravity Probe B, Parkinson was momentarily at a loss for words.

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