A NASA review appears to spell the end for Gravity Probe B, the project conceived in the 1960s to measure how the Earth warps the fabric of nearby space-time.
A panel of about 15 experts commissioned by NASA analysed the performance of 10 NASA astrophysics missions that are currently operating in Earth orbit. A copy of the "senior review" obtained by New Scientist concludes that extending the lifetimes of the top nine missions "would be certain to deliver unique data of high scientific value". But Gravity Probe B didn't make the cut because the panel doubted further analysis of its results would yield significant new information.
Building and launching spacecraft is expensive, so maximising the use of those already in orbit can offer big benefits. But the expert panel said NASA didn't have the budget to support all the worthwhile missions.
It recommended that Gravity Probe B receive no additional funding after its current funding runs out in September.
Launched in 2004, the satellite used four precision-engineered gyroscopes to measure two effects - called the geodetic effect and frame dragging - predicted by Einstein's theory of general relativity.
The probe's data was unexpectedly noisy due to solar flares in March 2005 that interrupted the satellite's observations, as well as unexpected torques on the gyroscopes that changed their orientation, mimicking relativistic effects. Additional interruptions ultimately chopped the observing time into seven intervals, which reduced the precision the probe could attain.
The noisy data meant that GPB could not measure the effects as precisely as astronomers had by firing laser beams at mirrors left on the Moon by the Apollo astronauts.
GPB managers had asked for additional funding to March 2010 to try to extract more information from the data, but the review panel doubted they could reach their goals.
It warned that the reduction in noise needed to test rigorously for a deviation from general relativity "is so large that any effort ultimately detected by this experiment will have to overcome considerable (and in our opinion, well justified) scepticism in the scientific community". Gravity Probe B's principal investigator, Stanford University physicist Francis Everitt, could not be reached for comment.
The panel gave top ranking to SWIFT, which launched in 2004 to study fleeting cosmic explosions called gamma-ray bursts.
Chandra is still in good shape after nine years exploring the X-ray universe, and the panel ranked it second, but urged programme managers to try to shave operating costs by a few percent in the tight budgetary environment.
The Spitzer infrared telescope came in fifth. It is expected to run out of liquid helium, used to cool the telescope to within 5.5 °C of absolute zero, early in 2009. That would shut down observations at long infrared wavelengths.
But passive cooling in space should keep the telescope within 30 °C of absolute zero, and that would still make it far more sensitive at the infrared wavelengths of 3.6 and 4.5 microns than anything on the ground. Spitzer planning documents had sought $110 million for a three-year extension, but the panel, which says the science is promising, urges finding a somewhat less expensive plan.
The full ranking (which first appeared here), with launch dates, follows:
1. Swift, 2004
2. Chandra, 1999
3. Galex (Galaxy Evolution Explorer, ultraviolet), 2003
4. Suzaku (X-ray), 2005
5. Spitzer, 2003
6. WMAP (Wilkinson Microwave Anisotropy Probe), 2001
7. XMM-Newton (X-ray Multi-Mirror Mission), 1999
8. Integral (INTErnational Gamma-Ray Astrophysics Laboratory), 2002
9. Rossi X-ray Timing Explorer, 1995
10. Gravity Probe B, 2004
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