Published online 28 April 2011 | Nature | doi:10.1038/news.2011.260

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A heavyweight, and controversial, cosmic-ray detector is set to head for the International Space Station.

The Alpha Magnetic Spectrometer will seek antimatter in deep space, and measure cosmic rays closer to home. MIT

A couple of days before lift-off, Mark Sistilli went down to the space-shuttle launch pad in Cape Canaveral, Florida, to meet researchers working on the Alpha Magnetic Spectrometer (AMS) and to sneak a last nervy glimpse of their 7-tonne cosmic-ray detector before the shuttle's cargo doors closed.

"The AMS was tucked in and ready to go," says Sistilli, NASA programme manager for the mission. On Friday 29 April, if all goes to plan, the AMS will leave on board the Endeavour shuttle, bound for the International Space Station (ISS). It is arguably the station's most important scientific payload so far.

The AMS is a cylindrical magnet, which has already flown on a 1998 shuttle flight, surrounded by a suite of new instruments for detecting cosmic rays. It is the result of former NASA administrator Dan Goldin's quest to find meaningful science projects for the ISS, and of Nobel-prize-winning physicist Samuel Ting's unorthodox ideas about antimatter.

The US$2-billion experiment has been sold partly as a search for regions of the Universe containing gas, stars and planets made exclusively of antimatter. This has raised eyebrows among those high-energy and particle physicists who doubt that such regions exist.

Ting, the mission's spokesman, acknowledges the scepticism, but says that there's no such thing as mainstream in physics. "Science doesn't depend on a vote," he says. Leading up to the launch, he and other scientists have also emphasized the unprecedented sensitivity of the AMS to the cosmic rays that rain down on Earth, which should yield more dependable science than the mission's headline might suggest.

Detecting cosmic rays

Alongside detecting any heavy antimatter nuclei — which would be a smoking gun for regions of antimatter in the Universe — the AMS will produce definitive data on the energy, charge and composition of cosmic rays from the Sun and from astrophysical sources such as supernovae and gamma-ray bursts.

"We'll be able to measure cosmic-ray fluxes very precisely," says AMS team member Fernando Barao, at the Laboratory of Instrumentation and Experimental Particle Physics in Lisbon. "The best place to be is space because you don't have Earth's atmosphere that destroys cosmic rays."

Theoretical physicist John Ellis at CERN, Europe's particle-physics lab near Geneva, Switzerland, who is not working on the mission, agrees, adding that AMS will be a big advance over previous space-based cosmic-ray detectors, such as the Russian–Italian Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA), launched in 2006.

PAMELA detected positrons from the halo of the Milky Way that may have been a signal of dark-matter particles annihilating there. With 200 times the collecting area of PAMELA, the AMS should be vastly more sensitive to the same signal, if it's real. "It'll be far and away the most detailed measurement of cosmic-ray flux we've been able to get," Ellis says, "it has way more scientific interest than any other experiment on the space station."

That is a crucial part of the experiment's politics. Astronauts' role in scientific discovery helps to maintain public support for the US space programme, says NASA astronaut Michael Massimino. He points to the Hubble Space Telescope, serviced five times by astronauts on shuttles, as a prime example. "The AMS has that potential as well," he says.

Fast-track into space

Astronauts on Endeavour, which will be captained by Mark Kelly — husband of Gabrielle Giffords, the Arizona congresswoman shot in the head at a constituency event in January — will use robotic arms on the shuttle and the ISS to ease the AMS into position. Once operational, the AMS will be controlled from Earth.

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Compared to an independent instrument, being on board the space station will increase the AMS's power and data-transfer rates. It will also be easier to fix if something goes wrong. "NASA is very proud that this is on the International Space Station," says Sistilli. "We always hoped we would have science payloads."

Apart from its mission, AMS is also controversial for bypassing the peer review that NASA normally requires of science missions. But Sistilli emphasizes that the project was endorsed by committees convened by the US Department of Energy, which is supplying $50 million of the funding. NASA is providing $85 million, and the remainder comes from a consortium of 16 countries including France, Portugal, China, Taiwan and Spain.