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.