Washington

Getting warm: the dewar housing the gyroscopes may lose its cryogenic helium too quickly. Credit: STANFORD UNIV.

A controversial project by the US space agency NASA to test Einstein's general theory of relativity in Earth orbit has run into a technical snag that is likely to increase costs and delay its launch. Some $21 million or more could be added to the $535 million price tag, and the launch — originally planned for next October — could be put back by several months at least.

Project managers at Stanford University in California believe that thermal problems with the Gravity Probe B (GPB) spacecraft should be relatively easy to fix, but the uncertainty is worrying NASA science officials. “We're colouring it red,” says Alan Bunner, who heads the agency's programme on the structure and evolution of the Universe.

GPB, also known as the Relativity Mission, will try to measure precisely the ‘frame dragging’ effect, whereby a massive rotating body such as the Earth is thought to drag space and time along with it as it turns. The effect is expected to show up as a slight deviation in the alignment of four exquisitely sensitive gyroscopes spinning inside a supercooled helium dewar.

During recent ground tests, however, engineers discovered that heat is not being conducted out from the interior of the spacecraft as it should. If the problem were left unfixed, cryogenic helium in the thermos-like dewar would boil off in orbit within a few months, ending the 16-month mission prematurely.

The culprit appears to be an epoxy resin that has come unbonded from a titanium liner inside the probe, severing an important pathway for conducting heat. Stanford engineering professor Brad Parkinson, one of three co-investigators for GPB, says “the probability is 98 per cent that this is the problem,” and that “we can easily see a solution” — adding copper pins and epoxy to restore the thermal pathway.

But this requires removing the probe from the dewar. This would delay the launch from next October to at least May 2001, and probably later if an additional margin is built into the schedule as the Stanford team would like. NASA expects to decide the new launch date this month.

First conceived at Stanford University in 1959, GPB has already had a long and turbulent history. NASA commissioned the first mission study in the mid-1960s, and began funding substantive work on the project in the mid-1980s.

Often criticized as a technologically risky experiment with little support from the wider scientific community, GPB has been threatened with cancellation several times. The most recent hurdle was a make-or-break 1995 recommendation to NASA by a National Research Council committee that the project should continue.

But the committee was deeply divided as to GPB's merit compared with other space projects. And it cautioned that a surprising result — Einstein shown to be wrong — would probably not be accepted without a repeat mission. NASA has no plans to fly a follow-up mission.

Parkinson says GPB is a target for criticism because fundamental physics “doesn't appear to have a major constituency” at NASA. And he defends the project's four- to six-per-cent budget overrun as not unusual, especially given the advanced technology that had to be developed. This includes extremely precise pointing and manoeuvring and the use of a ‘drag-free’ orbit.

But the project has overspent its budget by $20 million in the past two years, says Bunner, which has raised the level of concern at NASA headquarters. An earlier launch delay from March to October 2000 prompted the agency to add oversight staff at Stanford. Parkinson counters that the $20 million had actually been taken from programme reserves in previous budget years and was never restored.

And he says the new launch delay, if approved, will allow other minor problems to be fixed, including one that would have made one of the four gyroscopes unusable.

At another time, a $21 million overrun might not have made a dent in NASA's budget. But the problem comes at a bad time for the agency's science office, which is struggling to contain costs in other areas, including its Mars exploration programme.

Ed Weiler, head of NASA's space science, decided last summer to cancel the Champollion comet mission, in part because of budget pressures caused by expensive delays with the Chandra X-ray observatory (see Nature 400, 99; 1999). The Congress also recently earmarked more than $70 million worth of pet projects to be taken out of the general science budget in the coming year.

Congress also gave Weiler a $10 million increase in the ‘fundamental physics’ account. Instead of funding new missions, the money is likely to cover the latest delay with GPB, the longest-running development project in NASA's history.