麻豆传媒

See more: Tour the mystery matter detector destined for space

Does the universe really hold hidden seams of primordial antimatter, dark matter and even 鈥渟trange matter鈥�? A massive orbital detector is set to find out

WHEN the off on its final flight to the this month, it will be carrying a very important payload. Secure in the cargo bay will be the , a 6.9-tonne leviathan designed to sift through the perpetual sleet of particles from deep space.

Shortly after Endeavour鈥檚 arrival, astronauts will command the shuttle鈥檚 robotic arm to from the cargo bay and hand it over, space-relay style, to the ISS鈥檚 robotic arm. Once AMS is lowered onto its final resting place on the station鈥檚 exterior, the astronauts will undertake a lengthy spacewalk to plug in and power up the space station鈥檚 greatest scientific experiment.

If AMS lives up to its billing, this $1.5 billion particle detector will change the way we think about the universe. It could tell us whether whole stars or even galaxies of antimatter exist somewhere out in space: it could tell us about the nature of the dark matter thought to pervade the universe: and it could reveal where the most powerful particle accelerators in nature are hiding.

Breakthroughs in any one of these areas would make the mission a roaring success. But AMS has some unfinished business too. Back in 1998, a prototype found hints of what seems to be an entirely new form of nuclear matter known as 鈥渟trange matter鈥�. If the latest mission confirms those findings, it will change our understanding of the stuff of the universe and even of our ultimate fate in the cosmos.

The detector鈥檚 arrival in space will mark the end of a long and bumpy journey that almost didn鈥檛 happen. It began in 1994 when the AMS team carried out the first feasibility study for NASA. Project leader and Nobel laureate Sam Ting, a particle physicist at the Massachusetts Institute of Technology, entranced NASA鈥檚 then-administrator Dan Goldin with one of AMS鈥檚 main goals 鈥� to hunt for primordial antimatter.

One of the biggest unanswered mysteries is why the universe is predominantly made of matter. Laboratory experiments show that every time a particle of matter is created out of energy, so is its antimatter counterpart. If this were a perfect process, the fireball of the big bang should have produced a universe with as much antimatter as matter. So where is it hiding?

The most likely possibility is that this process was not perfect, and ended up producing more matter than antimatter. In this scenario, the antimatter no longer exists because every bit of it went up in a puff of energy as it encountered matter during the universe鈥檚 early days. But our understanding of this process is far from complete, and that leaves open another possibility.

AMS could see a nucleus of anti-helium. That would be significant because most of the helium in the cosmos today was formed during the first 3 minutes after the big bang rather than inside stars, and we would expect the same to be true of any anti-helium out there. Any anti-helium AMS sees would therefore most likely have survived the great primordial annihiliation with matter, suggesting there must be other surviving fragments. 鈥淚f AMS sees even a single piece of anti-helium, we will know that there is a concentration of antimatter somewhere in the galaxy,鈥� says Martin Pohl, who heads the AMS group at the University of Geneva in Switzerland. 鈥淚f it sees a particle of anti-carbon, we鈥檒l know there鈥檚 an anti-star out there, because you need anti-stars to cook it up from the primordial antimatter.鈥�

While some believe the search will be in vain (see 鈥淗unt for the first antimatter鈥�), Goldin was impressed with the simple elegance of such a quest, and his enthusiasm helped to bankroll the AMS project.

Strange signals

To find antiparticles, the AMS design called for a detector much like the ones used in today鈥檚 experiments at the Large Hadron Collider at CERN. Like them, it is built from a large magnet and several component detectors each designed to measure a different aspect of the 10,000 particles passing through every minute, including their energy, direction of travel, electrical charge and mass (see diagram). At any given moment, AMS will generate 300,000 pieces of information, which 650 microprocessors will process and record for transmission to Earth, where researchers from 16 countries will pore over it.

Progress in the late 1990s was fast. To prove the experiment was feasible, the collaboration built a prototype detector, which flew on the space shuttle Discovery in June 1998. The prototype operated for 100 hours 鈥� not long enough to find any anti-helium, though the results did allow the AMS team to set some punishingly small upper limits on how much primordial antimatter there could be in the galaxy. They showed that for every million regular helium nuclei, there had to be less than one anti-helium nucleus. 鈥淲e鈥檇 like to push that to one in a billion,鈥� says Pohl of the new version of the detector. 鈥淭hen we鈥檒l know for sure that there is no primordial antimatter in the galaxy.鈥�

The prototype did find something mysterious, though. Theorist at Aarhus University in Denmark and colleagues had been studying the possibility that the most stable form of matter is something called strange matter. This heavier alternative to ordinary nuclear matter mixes exotic strange quarks with the quarks found at the heart of protons and neutrons.

If strange matter exists, Madsen reasons, it will gather in the cores of stellar corpses known as neutron stars. 鈥淎nd strange matter is what you and I will eventually decay into,鈥� he says, referring to what will happen to all atoms after cosmological sweeps of time.

Madsen and colleagues asked the AMS researchers if their prototype had found anything out of character, in particular weird cosmic rays 鈥� energetic charged particles whizzing through space 鈥� that appeared too heavy for their electrical charge. The collaboration turned up two candidates: a particle weighing as much as an oxygen nucleus but carrying the charge of a helium nucleus; and another as heavy as an iron nucleus with only the charge of an oxygen nucleus. Madsen knew at once that these atomic chimeras could be strange matter. But with just two examples, the evidence was too scant to back up such a claim.

鈥淭wo particles that might be strange matter turned up, though the evidence was too scant to publish鈥�

So Madsen and the AMS team had to wait for the full experiment. They were content to do so, though, because everyone expected it to fly in short order. 鈥淚t can provide a very clean experiment for strange matter,鈥� says Madsen. 鈥淚t will prove or disprove whether it exists.鈥�

Then disaster struck. In 2003, the space shuttle Columbia disintegrated while re-entering the Earth鈥檚 atmosphere. In the wake of the tragedy, NASA decided to refocus its human space flight efforts on returning to the moon and going on to Mars. The space station was now seen as a money pit that needed to be retired as soon as possible. That meant junking AMS as well. It was removed from the shuttle launch schedule and looked as though it would spend the rest of its life slowly corroding in a laboratory clean room somewhere.

But AMS was not being built with NASA money, so the team soldiered on. Ting managed to keep the interest going by sticking to the message he had been using since the beginning, namely that the ISS can do invaluable science and that AMS was the instrument of choice. Nevertheless, it was a nail-biting wait. It wasn鈥檛 until just after President Obama鈥檚 inauguration in 2009 that Congress ordered NASA to mount an extra shuttle flight to put AMS on the space station.

In the intervening years, the physics case for AMS has actually strengthened. It鈥檚 all down to dark matter, the hypothetical stuff that astronomers believe acts as gravitational glue in a galaxy. No one knows what dark matter is made of, but the chief candidates come from a family of particles called neutralinos, which fall naturally out of the as yet unproven theory of supersymmetry. The beauty of neutralinos, which have not so far been detected, is that they annihilate each other if they collide, producing positrons 鈥� the antimatter counterparts of electrons 鈥� which would stand out against the other particles that make up cosmic rays.

The dark stuff

Previous measurements have shown that about 89 per cent of cosmic rays are hydrogen nuclei (protons), 10 per cent are helium nuclei and the remaining 1 per cent is a hotchpotch of everything else. By sifting through these particles, AMS will have the best chance of surprising us, as experience with another space-based detector called shows.

PAMELA launched in 2006 attached to a Russian reconnaissance satellite, and will continue to operate until the end of the year. One of its main aims is to understand the mysterious origins of , whose energies can far exceed anything we can create in our most powerful particle accelerators. What whips cosmic rays up to such high energies remains a mystery. 鈥淏y detecting the cosmic rays, we can study the mechanisms responsible for their production and acceleration,鈥� says Piergiorgio Picozza of the Italian Institute for Nuclear Physics (INFN) in Rome, and the principal investigator for PAMELA.

That鈥檚 a laudable aim, but cosmic rays may reveal far more. 鈥淪tudying them is growing in importance because of their potential to help us find dark matter,鈥� says Picozza.

In fact, PAMELA may already have seen evidence for dark matter neutralinos. In November 2008, the team announced that it had found more positrons in cosmic rays than could be accounted for by known astrophysics. They spotted one positron for every 100 electrons they were detecting, whereas they were expecting around one positron for every 10,000 electrons.

One explanation, they suggested, was that these extra positrons could be coming from the annihilation of neutralino dark matter. But in that case, PAMELA should have seen a similar rise in the number of antiprotons 鈥� and therein lay a problem. 鈥淲e saw a very strange thing,鈥� says Picozza. 鈥淟ots of positrons but no antiprotons.鈥� The antiprotons were still at the level of 1 in every 10,000 protons .

Picozza suggests that either the positron excess is coming from another source, or the antiprotons are too energetic for PAMELA to detect. Crucially, AMS will be able to spot antiprotons with much higher energies than PAMELA because it has a greater volume of detectors that are much more sensitive. 鈥淧AMELA did a good job and now AMS can continue,鈥� says Picozza.

Even with AMS back in the shuttle launch schedule, though, the project was not quite out of the woods. With the launch set for November 2010, the detector was dispatched to the 鈥榮 test facilities in Noordwijk, the Netherlands, to be put through its paces in a space simulator. The results were a little worrying. Heat was somehow working its way into the instrument. That was bad because, in order to work, the superconducting magnet at the heart of AMS has to be cooled to almost absolute zero using liquid helium. To cope with the additional heat, the cooling system had to use up the refrigerant more quickly. According to some estimates, the three-year supply of liquid helium could be gone in just over two. And there were no schemes for replacing it.

Suddenly NASA鈥檚 plan to extend the lifetime of the ISS by another 10 to 15 years seemed pointless, as far at the AMS team were concerned. Without the magnet, which bends oppositely charged particles in different directions, the team would have problems telling electrons and positrons apart.

鈥淧roblems with the AMS magnet made plans to extend the lifetime of the space station by 15 years seem pointless鈥�

Undeterred, Ting and colleagues began considering the options. One was to replace the superconducting magnet with the permanent magnet from the AMS prototype. Its weaker field meant that AMS would not be quite as sensitive as they had planned, but the magnet needed no cooling and would last for as long as the station remained in operation.

鈥淚f you were given the opportunity to turn a three-year experiment into one that could run for a decade, you鈥檇 do it too,鈥� says Pohl. He uses the two suspected strange-matter particles as an example. 鈥淚f those were strange matter particles, AMS will record hundreds or thousands of similar events during its extended mission.鈥�

At the last minute, NASA pushed the launch back from February to April, giving the team enough time to pull the instrument to bits and replace the magnet. 鈥淲e took out the only limiting piece of equipment in the experiment,鈥� says Pohl. 鈥淭his is state-of-the-art technology. AMS is a completely modern particle detector.鈥�

And it is going somewhere no particle detector of its size has been before. Whatever it tells us should make all the years of waiting worthwhile.

See more: Tour the mystery matter detector destined for space