A ‘senior review’ of NASA’s astrophysics missions has concluded that a satellite that is trying to measure gravitational effects predicted by Einstein’s theory of general relativity should receive no additional funding after this September.

The decision is quite surprising, as we believe we were making good progress in the data analysisBill Bencze, Stanford University

The 15 member panel report — obtained by physicsworld.com — was commissioned by NASA to analyse 10 of its astrophysics missions that are currently in orbit around Earth. Although the review concludes that 9 out of the 10 missions should be extended as long as “sufficient funding were available”. The panel noted that Gravity Probe B (GP-B), which was ranked bottom, “failed to reach its goals” and therefore should not receive anymore money.

Geodetic and frame-dragging effects

Initially conceived in the 1960s, the mission was launched in 2004 and has cost around $750m. The probe is a collaboration between NASA and Stanford University that aims to measure — by using four spherical quartz gyroscopes — two principles predicted by Einstein’s theory of general relativity: the ‘geodetic’ effect — the amount Earth warps the local space-time in which it resides and the more subtle ‘frame-dragging’ effect — the amount by which the rotating Earth drags its local space-time around with it.

The GP-B team led by Francis Everitt of Stanford University, last year reported only successful measurements of the geodetic effect, with no evidence for the frame dragging effect. However, not only had NASA’s Cassini mission also measured the geodetic effect, but the report concluded that “the GP-B experiment has been overtaken by events and now only occupies a diminished niche in the field.”

The report says that future missions such as LISA — that will search for gravitational waves as predicted by Einstein’s theory of general relativity — will be more powerful and therefore it is “difficult to determine whether GP-B can improve our understanding of gravity”.

Unexpected torques

However, Bill Bencze programme manager for GP-B who is also based at Stanford University says “the decision is quite surprising, as we believe we were making good progress in the data analysis.” Although noise due to solar flares interrupted the satellites observations in 2005, as well as unexpected torques on the gyroscopes that changed their orientation, Bencze is hopeful they can get good results and to find “firm evidence” of the frame dragging effect. This would be the first direct evidence of the effect rather than indirect evidence as provided by NASA’s LAGEOS satellite. However, the report disagrees that further data analysis will yield results “it will be difficult if not impossible to rule out overlooked systematics at the level they are trying to reach,” the report states.

Indeed, according to Bencze, the team of 10 or so people who are looking through the data will need funds of around $3m to complete the project. “This is trivial compared to other missions on the list,” says Bencze. It is estimated that the Chandra X-ray observatory, placed second in the list (see below), will cost around $50m to keep it operational.

The final ranking of the missions is:

1. SWIFT
2. Chandra
3. GALEX
4. Suzaku
5. Warm Spitzer
6. WMAP
7. XMM-Newton
8. INTEGRAL
9. RXTE
10. Gravity Probe B