Stanford University News Service



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Gravity probe B project receives first flight hardware

It stands 10 feet tall and 7 feet in diameter and weighs 1,800 pounds. It cost about $20 million. It looks like a giant's coffee carafe fabricated out of aluminum, but in actuality it's the flight dewar for the Gravity Probe B spacecraft that is currently under construction. Delivered to campus on Nov. 13 from its manufacturer, the Lockheed Martin Missiles and Space Co. in Palo Alto, the dewar, which acts like a giant thermos bottle to keep the spacecraft's instruments cooled to a few degrees above absolute zero, now sits in the Hansen Experimental Physics Laboratory (HEPL).

Dave Murray, right, a cryogenic engineer from Lockheed Martin, and Mike Taber, Stanford's cryogenic test director, inspect the flight dewar for the Gravity Probe B spacecraft currently under construction. The dewar, delivered to campus last month, acts like a giant thermos bottle to keep the spacecraft's instruments cooled. Gravity Probe B is a longstanding Stanford experiment to test Einstein's general theory of relativity.

Photo by L.A. Cicero

Gravity Probe B is a long-standing Stanford experiment designed to provide the toughest test yet of Einstein's general theory of relativity, which explains how gravity works. After more than 30 years of development, the GPB spacecraft is scheduled to launch from Vandenberg Air Force Base near Santa Barbara in October 2000, reports John Turneaure, the research professor who is in charge of the GPB science payload team. When the spacecraft flies, the gleaming new dewar will be one of its major structural elements.

The basic idea for the GPB experiment is surprisingly simple, but making it work has taken an extremely ambitious research and development program. According to Newtonian physics, the axis of a gyroscope orbiting Earth should remain pointing in the same direction. The equations of general relativity, on the other hand, predict that its axis should drift by an infinitesimal amount. To detect this minuscule drift, measured in thousandths of an arc-second, GPB researchers have been forced to make the world's most perfectly round objects ­ quartz spheres ­ to serve as gyroscopes. And the near-perfect gyros must be protected from interfering magnetic fields and thermal agitation, hence the need for the dewar.

The giant thermos bottle is insulated with 149 layers of Mylar separated by layers of silk net. It will be filled with about 600 gallons of superfluid helium. For the experiment to be successful, it must maintain an interior temperature of just 1.8 degrees Celsius above absolute zero (minus 456 degrees Fahrenheit, far colder than outer space) for two years.

Over the next year, GPB scientists and technicians will cool the dewar down to cryogenic temperatures, attach electronic components to its exterior, blow up a series of lead balloons in its interior to reduce the magnetic field inside, and insert a backup version of the long probe that holds the quartz gyroscopes and star-pointing telescope.

After the Stanford team has finished decking out the dewar, it will be shipped back to Lockheed. There engineers will subject it to a shake test to ensure that it can withstand the rigors of a rocket launch, says Michael Taber, the senior scientist who is in charge of the low temperature tests. Afterward the dewar will return to HEPL for a "post-shake" evaluation and installation of the flight science instrumentation and probe. Then it's back again to Lockheed, where the rest of the spacecraft hardware will be added, except the solar panels. After assembly the spacecraft will be put into a large vacuum chamber to simulate the space environment.

Finally, the solar panels will be bolted on and the completed spacecraft shipped to Vandenberg, where it will await the Delta rocket that will blast it into orbit and, quite possibly, into the history books.


By David Salisbury

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