Tim Furniss/LONDON

Lockheed Martin delivered the second major piece of NASA's $550 million Gravity Probe B (GP-B) spacecraft to Stanford University in Palo Alto, California, on 27 May. The delivery marks a major milestone in the protracted development of a spacecraft which, in 2000, will attempt to verify two predictions of Albert Einstein's general theory of relativity.

The flight probe will provide an enclosure within which Stanford's scientific instrument can operate at a temperature near absolute zero and at a pressure 10 times lower than that of the vacuum of space in which the craft will operate.

Built by Lockheed Martin Missiles & Space's Advanced Technology Center (ATC), the probe, a 3m-long cigar-shaped vacuum chamber, will house the delicate experimental apparatus, comprising four gyroscopes. The probe and gyroscopes will be mounted inside the Science Mission Dewar, a large thermos container holding 2,000 litres of superfluid liquid helium, to keep the instruments at a temperature of 1.8K, just above absolute zero. A minute temperature rise could corrupt readings from the gyros, Stanford says.

The dewar, the key structural component around which the GP-B spacecraft will be built, was designed and fabricated at ATC, and was delivered in November 1996. Lockheed Martin was selected by Stanford in 1984 to build the GP-B payload and by Stanford and NASA in 1993 to build the GP-B spacecraft.

"Completion of the flight probe is an exceptional engineering achievement and a key milestone for Gravity Probe B," says Rex Geveden, GP-B programme manager at NASA's Marshall Space Flight Center.

The GP-B will be launched aboard a Boeing Delta II booster from Vandenberg AFB, California, in October 2000. The mission could have far-reaching implications for theories about the nature of matter and the structure of the Universe (see box).

"The design and construction of the GP-B probe was an enormous engineering challenge because the experiment to be carried out will involve measuring extremely minute changes in the orientation of gyroscopes housed within it," says Gary Reynolds, an ATC probe engineer.

"We had to work hand in hand with vendors to acquire the right materials and we developed new construction methods and engineering processes to meet the very strict demands for this important experiment," he says.

The requirements of the GP-B experiment for stability and freedom from outside forces are extremely demanding. The probe's magnetic field is less than one-millionth that of the Earth and the experimental apparatus inside will operate in a quiet low acceleration environment.

"The flight probe was designed and built to very demanding requirements for which there was little flight engineering heritage. The delivery of this critical piece of hardware is a big step forward as we proceed to the integration and test of the science payload," says John Turneaure, Stanford co-principal investigator and hardware manager for the GP-B.

The experimental apparatus, under development at Stanford, consists of a quartz-block structure, including the reference telescope and four reconducting gyroscopes, all to be enclosed in the heart of the probe - a cylindrical chamber 250mm in diameter and 2.3m long. As an extra precaution against data corruption, the golfball-sized gyroscopes will be levitated on an electrostatic cushion above the 530mm-long block of solid quartz.

Installed around the inner circumference of the probe, to provide a clear line of sight for the reference telescope, are the 81 electrical cables, 85 instrumentation wires and 15 plumbing lines that together operate and monitor the experiment. All had to be custom-designed to operate flawlessly in near absolute-zero temperatures for the life of the Gravity Probe B mission.

Source: Flight International