Tracking stardust in Geneva

AMS-02 is attached to the International Space Station, which hovers some 400 kilometres above the Earth. NASA

They come from the Sun or from the depths of outer space. Astroparticles, tiny specks of matter packed with energy, can help scientists unlock the secrets of the origin and evolution of the universe. Two detectors – one international and one Chinese – are tracking them directly in the Earth’s orbit. Scientists in Geneva helped design these devices, and today they analyse the data they provide.

This content was published on October 19, 2016 - 11:00
Marc-André Miserez, Geneva,,

The first detector, called AMS-02, has been operating for more than five years on the International Space Station (ISSExternal link), a joint project run by Americans, Russians, Europeans, Japanese and Canadians. The second detector, named PolarExternal link, was launched last month together with the Chinese space station Tiangong 2External link.

The goal: To detect and analyse the particles, emitted mainly by stars, that whizz around non-stop in space at tremendous speeds. The principle is similar to the detectors at CERNExternal link (the European Laboratory for Particle Physics), only in this case the particles are not being created by acceleration or collision, but are collected as they are naturally found. AMS and Polar moreover owe a lot to CERN, and to the Department of Nuclear and Corpuscular Physics (DPNCExternal link) of the University of Geneva.

For specialists, but not only ...

Let’s look at AMSExternal link for starters. Designed by Nobel laureate physicist Samuel TingExternal link, it is a super magnet comprising 6,000 pieces of metal alloy bonded together. With a hole through the middle, it is like a giant doughnut 1.10 metres wide and 80 centimetres high. It separates particles according to their electric charge before sending them through a series of detectors to identify their properties.

AMS should be able to capture protons, and even atoms of antimatter, and put physicists on the trail of the very mysterious dark matterExternal link, whose links with antimatter are still the subject of debate. Over five years, however, and after detecting nearly 100 billion particles, it has spawned no major discoveries. The results have above all helped physicists to fine-tune their theoretical models, and made them rack their brains even further.

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Are the results disappointing? Not for Sonia Natale, a CERN physicist who helped design AMS. As she points out, in the world of science outcomes are never certain, and luck also plays a role. As a case in point, gravitational waves were detected the very day that the LIGO observatoryExternal link in the United States re-opened after a technical shut-down of several years.

“In any event,” says Natale, “AMS will achieve something that has never been done before: continuous monitoring of all particle flows around the Earth, most of which come from the Sun – and this throughout the entire lifespan of the ISS, that is until 2024, and we hope even longer.” Scientists will thus obtain valuable data on the variations in the Sun’s activity during a complete solar cycle (11 years), helping them to fathom its influence on human life and activities (climate change, radio communications, etc.).

The data arrive at CERN’s Prévessin site, which was built in the 1970s on French soil. Here, we already have a foot inside NASA. Even if the place does not belong to the US space agency, AMS is mounted on “its” space station. “They manage the infrastructure, so we have to abide by their rules,” explains Natale.

Chinese discipline

Nicolas Produit has also come up against the constraints of regulations. Not in Geneva, nor in Houston, but in China. This former CERN physicist, who now works for the Geneva Observatory, designed Polar, the first non-Chinese experiment to be mounted on a Chinese space station, which is soon to receive astronauts on board.

Though duly accredited to attend the launch at the Jiuquan base in the Gobi desert in September 2016, Produit was confined to his hotel at the last minute, together with two colleagues. He had to watch the lift-off of the Long March rocket from the hotel roof. The reason: the three men had wandered into a part of the city where they should not have been because a government official was passing through. And the Chinese military, who are in charge of manned space flights, take discipline very seriously.

Lift-off of the Long March rocket that put Polar into orbit on September 15, 2016. The sensor is attached to China’s second space station. Reuters

Today, Produit tends to laugh off the incident. Because, this hiccup aside, he is very pleased to have succeeded in persuading the Chinese to put his detector into orbit.

Polar is by no means a lightweight version of AMS. This aluminium box, measuring 40cm by 40cm, contains 1,600 scintillators – that is, crystals that react to bombardment by very high-energy photons (grains of light). And while AMS is a multi-purpose tracker, Polar is designed to answer a single question: “Are the photons emitted during the most intense phase of gamma-ray bursts polarised or not?”

Galactic cataclysms

What exactly does this mean? Gamma-ray burstsExternal link are the most violent and brightest phenomena that can be observed in the universe. For several seconds to several minutes, a huge mass of particles, which can be as large as the Earth, is projected into space at the speed of light. Such cataclysms are generally believed to be caused by the collapse of a super giant star – several hundred times the size of our Sun – which then ends its life as a black hole.

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These colossal flashes – which can light up an entire galaxy – are very rare. It is estimated that around one per galaxy occurs every 100 million years. But as there are many, many galaxies, the chances of seeing one are roughly one a day. Polar has been designed to “watch” one quarter of the sky continuously, and should therefore capture on average one gamma-ray burst every four days.

And polarisation? In simple terms, we can say that “normal” light travels in vertical waves, like those of the ocean. If the light is polarised the wave will be rotated, for example by 45°, and no longer move vertically but flatten out.

So how do the waves of gamma-ray bursts spread? “We have been studying them for 50 years, but we still have no idea,” Produit replies. Estimates range from 0 to 100% of polarisation. For light to be polarised, there must be an organised magnetic field and a relatively small emitting area. The data coming in will therefore help us understand what gamma-ray bursts really are.”

Death from the sky

Gamma-ray bursts are triggered in other galaxies millions if not billions of light years from our planet. “And fortunately for us,” the physicist stresses, “for if one were to occur in our Milky Way, it would be absolutely disastrous for life on Earth.”

Indeed, according to an article published in the journal NatureExternal link in 2013, if a gamma-ray burst were to hit the Earth, it could destroy at least one third of the ozone layer in a matter of seconds. UV rays from the Sun would then become deadly for most plants and for plankton, the basis of the oceanic food chain. Moreover, the atmosphere’s new chemical make-up would spark the formation of a layer of toxic clouds of nitrogen oxide that would obscure the Sun, colour the sky yellow-orange and set off torrents of acid rain.

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