Thumbs partly up for Gravity Probe B.

Scheduled for launch in 1999, Gravity Probe B (GP-B) represents an ambitious, space-based attempt to test the general theory of relativity. Conceived more than 30 years ago, the experiment has survived a number of controversies over its cost, scientific value, and design (SN: 3/18/95, p.167). So far, NASA has spent about $240 million on the project, and the agency estimates that another $340 million will be needed to complete it.

Now, in response to a request last fall by NASA Administrator Daniel Goldin, a National Academy of Sciences panel has reviewed progress on the GP-B experiment and rendered its verdict. Because this space mission offers the possibility of a unique, direct test of general relativity, a majority of the 12 panel members judges the experiment "well worth its remaining cost to completion." Physicist Val L. Fitch of Princeton University headed the panel.

A significant minority of the panel, however, argues that the project's purpose is too narrow in comparison to alternative scientific space missions, such as the proposed Stratospheric Observatory for Infrared Astronomy (SOFIA). With GP-B, "it does not appear that unexpected new knowledge will be gained," the minority contends.

The panel commends the work done to date by Francis Everitt and his colleagues at Stanford University and the team at the Lockheed Missiles and Space Co. in Sunnyvale, Calif., in developing and building the GP-B spacecraft. For example, the group's report notes several innovations made in designing an apparatus for storing liquid helium aboard the spacecraft. This technology may prove useful to other researchers conducting low-temperature experiments.

The panel members identify no major technical shortcomings likely to hinder GP-B from functioning properly. Nonetheless, "the extraordinary experimental requirements and the impossibility of ground tests of some critical systems at the necessary level of accuracy introduce significant risks," the report points out.

At the same time, a few panel members express concern about the mission's complexity. "This minority believes it likely that some as yet unknown disturbance may prevent GP-B from performing as required," the report states. In any event, should the experiment yield results different from those predicted by general relativity, the scientific community would almost certainly not accept them unless a repeat mission confirmed them.

With the review panel's report in, the decision on whether to continue the project rests with Congress.