Published online 22 April 2010 | Nature | doi:10.1038/news.2010.198

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Urgent refit for space magnet

Particle physicists in a race against time to overhaul US$1.5-billion cosmic-ray detector.

AMSWill the Alpha Magnetic Spectrometer be able to detect dark-matter particles?NASA

A seven-tonne particle detector due to fly to the International Space Station in July will spend the summer on the ground owing to a technical fault in the superconducting magnet at its core. The Alpha Magnetic Spectrometer (AMS) will be refitted with a conventional permanent magnet, a change that could mean the detector is less likely to make the breakthrough that would seal its place in scientific history — the discovery of dark matter.

The AMS is designed to measure the properties of cosmic rays — high-energy particles reaching Earth from space — which may include particles produced by the annihilation of dark matter. From a perch on the space station, the detector would be able to study these particles before they are absorbed by Earth's atmosphere, determining their charge, momentum and trajectory, and so their source. "The AMS is by far and away the most important science currently planned for the space station," says particle theorist John Ellis at CERN, Europe's particle physics laboratory near Geneva, Switzerland.

The AMS won the support of NASA administrator Dan Goldin in 1995, when it seemed like the ideal project to boost the space station's flagging scientific credibility. But since then it has endured a bumpy ride. It drew criticism from many astrophysicists who claimed that it avoided the peer-review process normally required of large infrastructure projects, and was jeopardized by the Columbia shuttle disaster in 2003, after which NASA decided that subsequent shuttle launches would be dedicated to completing the space station.

In trouble again

Having been given a berth on the space shuttle under former US president George W. Bush, the AMS is now in trouble once again. This time, the problem concerns the nearly 0.9-tesla doughnut-shaped superconducting magnet that was to have formed the heart of the device, its field designed to bend the trajectories of incoming charged particles according to their charge and momentum.

“The AMS is by far and away the most important science currently planned for the space station.”


About two months ago, while testing the detector with particle beams at CERN, physicists noticed the magnet warming up unexpectedly during operation, a potentially major problem given that it is designed to run at just 1.8 degrees above absolute zero. The problem was confirmed by tests to establish the AMS's space-worthiness at ESTEC, the European Space Agency's research and technology centre in Noordwijk, the Netherlands. The tests showed that the magnet would consume some 30–40% more liquid-helium coolant than had been calculated in theory, shortening the lifetime of the experiment from between three and four years to less than two.

Advised by engineers that the problem would take some time to solve, the AMS researchers have decided to play safe and reinstall the permanent magnet that formed part of the detector when it underwent a ten-day test run on the space shuttle in 1998. According to project team member Martin Pohl at the University of Geneva, this switch allows the experiment to take full advantage of the full lifetime of the space station — which may be extended by the station's member nations — because a permanent magnet requires no coolant. He acknowledges that the permanent magnet will have a lower momentum resolution than its superconducting counterpart, so will require more data to identify particles with the same level of precision, but that this loss of resolution will be more than made up for by the longer running time.

Time pressure

Pohl admits that he and his colleagues will be pushed for time. But he maintains that the detector can be sent back to CERN, refitted with the old magnet and then subjected to fresh beam tests by the end of September, a process that the team is familiar with having carried it out twice before.

NASA spokesman John Yembrick confirms that the AMS will no longer take off on 29 July. The new date, expected to be in November, will be decided at the end of this month.

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But even if the AMS does get off the ground, its scientific potential remains questionable, according to astrophysicist Gregory Tarlé of the University of Michigan at Ann Arbor. "The real value of AMS," says Tarlé, would be to establish whether the excess of high-energy positrons first seen in 1997 by the HEAT balloon experiment, of which Tarlé was a member, extend to higher energy, as suggested by the findings of the Italian-led PAMELA spacecraft in 2008. These extra positrons may be the result of annihilation of dark-matter particles in the galactic halo or may simply be produced by local astrophysical sources. "With the weaker permanent magnet, its energy reach will be severely impacted and it is unclear whether they will be able to resolve these issues," he says.

Pohl points out that the AMS should have no problem detecting the positron signals associated with dark-matter particles with energies of about 150 gigaelectronvolts, as suggested by PAMELA. But he concedes that it would struggle if the dark-matter particles had a mass nearer 1 teraelectronvolt. As to what that mass is likely to be, he says, "your guess is as good as mine".

However, Roberto Battiston of the University of Perugia in Italy and the project's deputy spokesperson takes a firmer line. He maintains that by suitably rearranging the silicon tracking devices inside the detector, the sensitivity of the permanent magnet to high-energy dark-matter signals could equal that of the superconducting magnet. 

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  • #61417

    I think there are only two forces that can have stable orbits: elastic (F=kx) and Coulombic (F=k/x2 ). Magnets attract each other with a dipole force that is k/x3 , so you couldn't set up a stable orbit. As for why only these two, check out Bertrand's theorem.

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