Published online 4 May 2011 | Nature 473, 13-14 (2011) | doi:10.1038/473013a

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A controversial cosmic-ray detector destined for the International Space Station will soon get to prove its worth.

The Alpha Magnetic Spectrometer will ride to orbit in the space shuttle Endeavour ’s cargo bay. M. Famiglietti/AMS Collaboration

The next space-shuttle launch will inaugurate a quest for a realm of the Universe that few believe exists.

Nothing in the laws of physics rules out the possibility that vast regions of the cosmos consist mainly of antimatter, with antigalaxies, antistars, even antiplanets populated with antilife. "If there's matter, there must be antimatter. The question is, where's the Universe made of antimatter?" says Samuel Ting, a Nobel-prizewinning physicist at the Massachusetts Institute of Technology in Cambridge, Massachusetts. But most physicists reason that if such antimatter regions existed, we would have seen the light emitted when the particles annihilated each other along the boundaries between the antimatter and the matter realms.

No wonder, then, that Ting's brainchild, a US$2-billion space mission sold partly on the promise of looking for particles emanating from antigalaxies, is fraught with controversy. But the project has other, more mainstream scientific goals. So most critics held their tongues last week as the space shuttle Endeavour prepared to deliver the Alpha Magnetic Spectrometer (AMS) to the International Space Station, in a flight delayed by shuttle problems until later this month.

Pushing the boundaries

Seventeen years in the making, the AMS is the product of former NASA administrator Dan Goldin's quest to find remarkable science projects for the space station and of Ting's fascination with antimatter. Funded by NASA, the US Department of Energy and a consortium of partners from 16 countries, it has prevailed despite delays and technical problems, and the doubts of many high-energy and particle physicists.

"Physics is not about doubt," says Roberto Battiston, deputy spokesman for the AMS and a physicist at the University of Perugia, Italy. "It is about precision measurement." As their experiment headed to the launch pad, he and other scientists were keen to emphasize the AMS's unprecedented sensitivity to the gamut of cosmic rays that rain down on Earth. That should allow it not just to detect errant chunks of antimatter from the far Universe, but also to measure the properties of cosmic rays, the high-energy, charged particles flung from sources ranging from the Sun to distant supernovae and γ-ray bursts.

On Earth, cosmic rays can only be detected indirectly, from the showers of secondary particles they produce when they slam into molecules of air high above the ground. From space, the AMS will get an undistorted view. "We'll be able to measure cosmic-ray fluxes very precisely," says collaboration member physicist Fernando Barão of 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 is going to destroy those cosmic rays." No matter what happens with the more speculative search for antimatter, the AMS should produce a definitive map of the cosmic-ray sky, helping to build a kind of astronomy that doesn't depend on light.

The AMS consists of a powerful permanent magnet surrounded by a suite of particle detectors. Over the ten or so years that the experiment will run, the magnet will bend the paths of cosmic rays by an amount that reveals their energy and charge, and therefore their identity. Some will turn out to be heavy atomic nuclei, and any made from antimatter will reveal themselves by bending in the opposite direction from their matter counterparts (see 'Looking for cosmic curveballs').

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By counting positrons — antimatter electrons — the AMS could also chase a tentative signal of dark matter, the so-far-undetected stuff that is thought to account for much of the mass of the Universe. In 2009, researchers with the Russian–Italian Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics, flying on a Russian satellite, published evidence of an excess of positrons in the space environment surrounding Earth (O. Adriani et al. Nature 458 , 607–609; 2009). One potential source is the annihilation of dark-matter particles in the halo that envelops the Galaxy.

Another speculative quest is to follow up on hints of 'strange' matter, a hypothetical substance, perhaps found in some collapsed stars, that contains strange quarks along with the up and down quarks in ordinary nuclei. NASA's AMS programme manager, Mark Sistilli, says that hints of strange matter were seen in a pilot flight of the AMS aboard the shuttle in 1998, but that the results were too tentative to publish.

Thanks to its status as an exploration mission, the AMS did not need to go through the peer review that NASA would normally require of a science mission. But Sistilli emphasizes that it earned flying colours from committees convened by the energy department, which is supplying $50 million of the funding. Now their confidence will be put to the test.