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August
14, 2009: NASA's space shuttle program is winding
down. With only about half a dozen more flights, shuttle crews
will put the finishing touches on the International Space
Station (ISS), bringing to an end twelve years of unprecedented
orbital construction. The icon and workhorse of the American
space program will have finished its Great Task.
But,
as Apple's CEO Steve Jobs might say, there is one more
thing...
An act
of Congress in 2008 added another flight to the schedule near
the end of the program. Currently scheduled for 2010, this
extra flight of the shuttle is going to launch a hunt for
antimatter galaxies.
 The
device that does the actual hunting is called the Alpha Magnetic
Spectrometer--or AMS for short. It's a $1.5 billion cosmic
ray detector that the shuttle will deliver to the ISS.
Right:
The Alpha Magnetic Spectrometer. Image courtesy MIT. [larger
image]
In
addition to sensing distant galaxies made entirely of antimatter,
the AMS will also test leading theories of dark matter, an
invisible and mysterious substance that comprises 83 percent
of the matter in the universe. And it will search for strangelets,
a theoretical form of matter that's ultra-massive because
it contains so-called strange quarks. Better understanding
of strangelets will help scientists to study microquasars
and tiny, primordial black holes as they evaporate, thus proving
whether these small black holes even exist.
All
of these exotic phenomena can make their presence known by
the ultra-high energy cosmic rays they emit--the type of particles
AMS excels in detecting.
"For
the first time, AMS will measure very high-energy cosmic rays
very accurately," explains Nobel laureate Samuel Ting,
a physicist at the Massachusetts Institute of Technology,
who conceived of the AMS and has guided its development since
1995.
Antimatter
galaxies, dark matter, strangelets--these are just the phenomena
that scientists already know about. If history is any guide,
the most exciting discoveries will be things that nobody has
ever imagined. Just as radio telescopes and infrared telescopes
once revealed cosmic phenomena that had been invisible to
traditional optical telescopes, AMS will open up another facet
of the cosmos for exploration.
"We
will be exploring whole new territories," Ting says.
"The possibility for discovery is off the charts."
Ting
often compares AMS with high-powered particle accelerators
at facilities such as CERN in Geneva, Switzerland. Rather
than detecting high-speed cosmic rays from across the galaxy,
these underground accelerators make their own particles locally
using tremendous amounts of electrical power. To study the
particles, CERN and AMS employ the same basic trick: Both
use strong magnetic fields to deflect the particles, and arrays
of silicon plates and other sensors inside the detectors track
the particles' curved paths.
Above:
An aerial view of CERN, the European Organization for Nuclear
Research. The Alpha Magnetic Spectrometer is a sort of "mini-CERN"
in space. Image credit: CERN [larger
image]
Many
terabytes of data pour out of these sensors, and supercomputers
crunch that data to infer each particle's mass, energy, and
electric charge. The supercomputer is part of why AMS must
be mounted onto the ISS rather than being a free-flying satellite.
AMS produces far too much data to beam down to Earth, so it
must carry an onboard supercomputer with 650 CPUs to do the
number crunching in orbit. Partly because of this giant computer,
AMS requires 2.5 kilowatts of power — far more than a normal
satellite's solar panels can provide, but well within the
space station's 100 kilowatt power supply.
"AMS
is basically an all-purpose particle detector moved into space,"
Ting says.
 There
are two important differences between AMS and underground
accelerators, though. First, AMS will detect particles such
as heavy nuclei that have vastly higher energies than particle
accelerators can muster. The most powerful particle accelerator
in the world, the Large Hadron Collider at CERN, can collide
particles with a combined energy of about 7 tera-electronvolts
(TeV, a common way to measure energy in particle physics).
In contrast, cosmic rays can have energies of 100 million
TeV or more. The other important difference is that accelerators
smash particles into each other to learn about the particles
themselves, while AMS will sample high-energy particles from
deep space for the sake of learning more about the cosmos.
Right:
MIT physics professor Samuel Ting, 1976 Nobel Laureate and
leader of the AMS team. [more]
For
example, a longstanding mystery in cosmology is the case of
the missing antimatter. According to physicists' best models,
the Big Bang should have produced just as much antimatter
as matter. So, where did all the antimatter go? It can't be
nearby, because if it were, we would see bright X-ray emissions
where the antimatter came into contact with matter and annihilated.
One
explanation could be that some distant galaxies are made entirely
of antimatter instead of matter. Since antimatter doesn't
look any different than ordinary matter, astronomers would
not be able to tell whether a distant galaxy is made of matter
or antimatter just by looking at it. However, AMS would find
strong evidence of antimatter galaxies if it detected even
a single nucleus of anti-helium or a heavier antimatter element.
Collisions
among cosmic rays near Earth can produce antimatter particles,
but the odds of these collisions producing an intact anti-helium
nucleus are so vanishingly small that finding even one anti-helium
nucleus would strongly suggest that the nucleus had drifted
to Earth from a distant region of the universe dominated by
antimatter.
Above:
An artist's concept of the Alpha Magnetic Spectrometer installed
on the International Space Station. [larger
image]
Other
instruments such as the Italian PAMELA satellite have looked
for anti-helium nuclei, but none have been sensitive enough
to rule out the existence of antimatter galaxies. AMS has
about 200 times the particle-collecting power of anything
that has flown before. If AMS detects no anti-helium nuclei,
Ting says scientists will know that there are no antimatter
galaxies within about 1000 megaparsecs — or roughly to the
edge of the observable universe.
Another
mystery that AMS will help solve is the nature of dark matter.
Scientists know that the vast majority of the universe is
actually made of unseen dark matter rather than ordinary matter.
They just don't know what dark matter is. A leading theory
is that dark matter is made of a particle called the neutralino.
Collisions between neutralinos should produce a large number
of high-energy positrons, so AMS could prove whether dark
matter is made of neutralinos by looking for this excess of
energetic positrons.
"For
the first time we could find out what dark matter is made
of," Ting says.
Stay
tuned to Science@NASA for AMS updates.
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Author: Patrick Barry | Editor:
Dr. Tony Phillips | Credit: Science@NASA
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