The long, strange tale of one of the more ambitious particle physics experiments ever conceived just got a bit stranger. Just 3 months before it was
scheduled to lift off aboard the very last space shuttle flight and be installed on the International Space Station (ISS), physicists working on a
particle detector called the Alpha Magnetic Spectrometer (AMS) have decided to swap the 2350 kilogram doughnut-shaped superconducting magnet at the
heart of the experiment for a weaker permanent magnet that was used in a test run on the space shuttle in 1998. The change will delay by several months
the launch of AMS, which will look for antimatter lingering from the big bang, particles of dark matter, and other oddities. However, it will enable
AMS to run much longer in space, says Samuel Ting, a physicist and Nobel Laureate at the Massachusetts Institute of Technology in Cambridge, who dreamt
up the experiment in the 1990s.
The change is being made in part because tests this month suggest the magnet will generate more heat than expected.
The magnet operates at 1.8° above absolute zero, and to keep it that cold for 3 years the plan was to send along 2500 liters of liquid helium, which
would be vented to space. But tests suggest that AMS would exhaust its helium in about 20 months, Ting says, or in 28 months if parts of the cooling
apparatus were upgraded. Originally, the space station was to be "deorbited" in 2015—so a lifetime of 3 years for the device was sufficient, Ting
says. But under President Barack Obama's new plans for NASA, officially announced in March, ISS will keep circling the globe until 2020 and perhaps
beyond, Ting says. That means AMS could run for 10 years or more, but only if it uses the permanent magnet, which requires no coolant. So Ting made the
audacious decision to switch.
The modification will reduce the spectrometer's mass resolution slightly and will require a lightning-fast reconfiguration of some components. However,
Trent Martin, an engineer and AMS project manager at NASA's Johnson Space Center in Houston, Texas, says he thinks the AMS team can pull it off. "I
spent the last 3 weeks in Europe looking at whether this is possible, and I'm comfortable that they can make the change."
This is hardly the first change of plans for AMS. Physicists had originally hoped to launch it in 2005, only to have those plans derailed by the loss
of the space shuttle Columbia in 2003 and NASA's subsequent rethinking of shuttle missions. From 2005 to 2008, NASA had no plans to launch AMS at all.
Now, AMS will have to wait even a few months more to make it into space. No new launch date has been set.