Résumé

À bord d’Endeavour, des années de passion, de précision et de patience

L’idée d’envoyer un spectromètre magnétique dans l’espace trottait dans la tête de Samuel Ting depuis 20 ans. Après l’abandon du projet de supercollisionneur suptraconducteur en 1993, il s’agissait de concevoir des expériences sans accélérateur. Avec le soutien de la NASA et du ministère de l’Énergie des États-Unis, un consortium international a commencé à travailler sur le Spectromètre magnétique alpha (AMS), et un prototype a été envoyé à l’occasion d’une mission de la navette spatiale en 1998. Cette année, Samuel Ting était présent en Floride pour assister au lancement d’AMS-02. Il a fait part à Paola Catapano de ses espoirs concernant cette mission, et de sa conception de la recherche scientifique en général.

"Most of our understanding of our cosmos up to now comes from measuring light. Besides light rays, there are charged particles, which have not been used nearly as much as light to understand the universe." Samuel Ting, the principal investigator for the Alpha Magnetic Spectrometer (AMS-02) experiment, is talking to the assembled press at the 2.00 p.m. briefing in the crowded auditorium at the Kennedy Space Center, Cape Canaveral. His experiment is soon to fly on board the space shuttle Endeavour, prior to installation on the International Space Station (ISS). "AMS is the first detector to study charged particles from cosmic rays directly in space, thanks to its magnet – the first magnet in space – and it will do so for the next 20 years, for the space station’s entire lifetime," he explains. "It is the only fundamental science experiment in the space station."

This was the main argument that Ting, well known as a leading particle physicist and Nobel laureate, used to convince the US Congress in 2008 to request that NASA "shall take all necessary steps to fly one additional space shuttle flight to deliver the Alpha Magnetic Spectrometer and other scientific equipment and payloads to the International Space Station prior to the retirement of the space shuttle" (US government 2008). AMS-02 had been grounded in 2005, in response both to the accident of the space shuttle Columbia on re-entry in 2003 and to the decision to retire the shuttle by late 2010. Were it not for its primary payload, the 2011 launch of Endeavour might never have been scheduled.

Unexpected discoveries

The principal scientific goals of AMS-02 are to search for dark matter and antimatter (AMS: the search for exotic matter goes into space). However, AMS’s biggest discovery might come in a totally unexpected area. Ting points out that the major discoveries in particle physics over the past 50 years were made in areas of physics that had not been anticipated when building the facilities where the discoveries were made. Take, for example, neutral currents, which Ting regards as the first major discovery made at CERN’s Proton Synchrotron (CERN Courier September 2009 p16). "When the Proton Synchrotron was built nobody thought about neutral currents," he says. Likewise for the Brookhaven National Laboratory, which built the Alternating Gradient Synchrotron (AGS) during the same period. "The purpose was to study the nuclear force; instead it discovered a second kind of neutrino, CP violation and the J particle," he explains. "So what you predict and what you discover are often very different things. With AMS, we are going to explore new territory with a precision instrument, and that is the key to discovery. What we will really see, nobody knows. This is how science advances." This philosophy was also a key to his Nobel-prize-winning experiment at the AGS, which opened up a new world of particle physics based on a fourth kind of quark, charm, in what has become known as the "November revolution" of 1974.

The story of the AMS experiment goes back nearly 20 years, to the cancellation of the Superconducting Supercollider project in 1993. It was then that Ting first had the idea to send a relatively large-scale particle detector into space. "I began to think maybe I should do something different, not necessarily with accelerators, and then I remembered that in early 1964 I did an experiment together with Professor Leon Lederman to show how an antiproton and an antineutron form an antideuterium. A similar experiment was also done by Professor Zichichi’s group at CERN (CERN Courier March 2009 p17). So I began to think, maybe I should do an experiment in space. In the 1990s, together with a group of colleagues, we saw the ISS as an opportunity to mount an experiment to study cosmic rays. With support from NASA and the US Department of Energy, an international consortium started work on AMS, and we flew a precursor instrument on the STS-91 shuttle mission in 1998."

The key component of AMS-01 was the magnet, as in its successor, but the detector was much simpler. "It was intended as a proof of principle, proof that a magnet could go to space and it did so by flying 10 days on the space shuttle Discovery," explains Ting. The test flight in June 1998 not only showed that everything worked, but also made some initial intriguing measurements of cosmic rays in space (CERN Courier July/August 1999 p6). This provided the ground work for AMS-02, which is intended to operate for 20 years. "It has greater, more precise subdetectors, with many channels, whose size, scope and precision are totally different from AMS-01," he says. "We made them as precise as we could manage."

So how does Ting feel to see AMS-02 finally being about to launch after the long journey that began with AMS-01? "I am actually very calm; I am confident everything will be OK. This detector spent two decades in the workshop: at CERN, we tested the detector twice with a beam from the SPS accelerator, then we tested it in the thermovacuum chamber at ESA-ESTEC. We took it apart and re-assembled it three times, so we’re quite familiar with what’s going on inside. All of the subdetectors measure energy in a repetitive way, so I think everything will work."

It is not surprising that CERN’s facilities were used in testing AMS-02. Ting’s relationship with the laboratory goes back nearly half a century, his first day at CERN being on 13 March 1963, as a Ford Foundation Fellow. "There, I had the good fortune to work with Giuseppe Cocconi at the Proton Synchrotron, and I learnt a lot of physics from him," he recalls. Particle physics and CERN have certainly both evolved a great deal since then. "When I first came to CERN, high-energy physics was dominated by the US," he says. "Most people at CERN were looking at what was done at Brookhaven and tried to do similar experiments. Now the picture has completely changed. Most US particle physicists come to CERN, and CERN now really has become the centre of high-energy physics in the world."

With all of the current interest in CERN and particle physics, Ting has some serious, practical advice for young people aspiring to become physicists. "If you want to be a scientist, whether it is a physicist, mathematician or biologist, you need to remember that you’re doing this only for interest, not for fame or glory, because only very few people in their lifetime accomplish what they really want," he explains. "Physics is a very difficult thing; particle physics involves large groups of people working together. Unless you think that physics is the most important thing in your life, you should not do it. It takes passion, precision, patience."

Patience is a quality that Ting certainly has, waiting for this "last Endeavour" for his AMS project and never giving up hope. So when does he expect the first important results? "We have no competition," he says. "We are going to do this very slowly, very carefully. We won’t publish any preliminary results; we’ll only publish the data that we’re absolutely sure about." Whatever AMS discovers, the final answers, like much that Ting has achieved, will be the result of passion, precision and patience.


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Eyewitness: the final countdown for Endeavour

Thursday 28 April, LD–1. It’s launch-minus-one day at the Kennedy Space Center (KSC) in Cape Canaveral and so far it’s "go" for tomorrow’s 10-minute launch window at 3.47 a.m. EDT, the time set for space shuttle Endeavour’s final lift-off. I am one of 1500 members of the international press accredited at the KSC and one of the expected half-a-million viewers to witness the launch.

Much of the attention surrounding this mission has focused on the fact that this will be the final flight of Endeavour and the penultimate mission of the entire space-shuttle programme, as well as that the mission commander, Mark Kelly, is married to Congresswoman Gabrielle Giffords, who is recovering from a shooting more than four months ago. And, according to the latest rumours among the press at KSC, the "first family" is expected to attend tomorrow.

It’s T–11 hours and holding, one of the longest pauses (around 14 hours) built into the countdown procedure. I join the media registered for witnessing the removal of Endeavour’s Rotating Service Structure (RSS). This is one of the important milestones performed in the T–11 hold in the countdown. Around midnight, the metallic gantry around the shuttle starts to move away under the enthralled gaze of the press representatives who were brave enough to stay, revealing Endeavour in all of its splendour. Once the operation has been performed, there are still 11 hours and so many unknowns before lift-off. Moreover, the weather does not seem promising, with lightning threatening NASA’s Vehicle Assembly Building and launch pad 39.

Friday 29 April, T–3 hours and… scrub. After a short night (the RSS removal took place after midnight), we wake up early not to miss another milestone in the countdown schedule – the astronauts’ "walk-out" and departure for launch pad. We have to be early at the media centre for the usual "K-9" controls (dogs checking for explosives). On the way, I stop at AMS’s premises at KSC, which happen to be close to the Operations and Checkout building where all astronauts spend the night before launch, since the time of the Apollo missions. We see three of them jogging – their last chance for a while.

Walk-out takes place at 11.58 a.m. as planned. I barely manage to shout "Forza Roberto" to my compatriot Roberto Vittori, before my voice is drowned in the crowd of media and NASA staff cheering the STS-134 crew, as they proceed to the Airstream van (also used by all crews since Apollo times).

However, in the media bus taking us back to the press centre, we see the Airstream van backing up – a clear sign that something has gone wrong. At a press briefing we learn that, while the astronauts were on their way to the pad, the launch team identified a fault in the heaters of the auxiliary power unit that prevents the shuttle’s fuel from freezing. This is enough to scrub the launch window.

Sunday 15 May, T–11 hours. I’m back at KSC for the second launch attempt and the legendary countdown clock is again on T–11 hours and holding. The faulty box in the shuttle’s aft compartment that resulted in the launch postponement has been replaced and the entire system re-tested. The weather forecast for tomorrow’s slot is "70% go". Countdown will resume soon.

Monday 16 May,T–9 minutes and counting. The Mission Management team has just given the final "go" for launch. In less than 9 minutes Endeavour will lift off with AMS cradled in its cargo bay. I am on the media-centre lawn, less than 5 km from the launch pad, one of the closest points to watch a launch at KSC. I’m grateful to Prof. Ting for the invitation. No words can convey the emotion; it’s a lifetime experience not to be forgotten.