University of Chicago scientists plan to launch an unmanned balloon
into the highest reaches of the atmosphere over Antarctica in early
December to hunt for tiny particles from interstellar space called
high-energy cosmic rays.

Consisting of atomic nuclei-subatomic scraps of matter-cosmic rays
continually bombard Earth from all directions at nearly the speed of
light. “It’s an almost century-old question: where do cosmic rays
come from?” said Dietrich Muller, University of Chicago Professor in
Physics, who heads the project. “We have learned a lot, but we have
not found everything yet.”

The balloon will carry Muller’s 3-ton instrument to an altitude of
almost 130,000 feet for up to a month to detect and identify the
chemical composition of cosmic rays that have been accelerated to an
energy of a quadrillion (1,000,000,000,000,000) electron volts. This
is 1,000 times more energy than is generated by the most powerful
man-made particle accelerator, the Tevatron at Fermi National
Accelerator Laboratory.

These are the highest energies at which Muller’s team can hope to
observe a cosmic ray particle before it strikes molecules in the
atmosphere and disappears in a cascade of secondary particles. Even
higher-energy cosmic rays do occur, but their rarity makes them
virtually impossible to detect during a weeks-long balloon experiment.

According to prevailing theory, shock waves of exploding stars
(supernovae) from within the galaxy sweep up the particles that
Muller studies and accelerates them to extremely high speeds.
Exploding stars lack the power to account for the highest-energy
cosmic rays, which may come from outside the galaxy.

“If this supernovae theory holds, there should be a cutoff,” Muller
said. Supernovae can probably accelerate particles up to a certain
energy, “then they become inefficient.” The theory predicts that
experimenters should see a higher proportion of iron nuclei as
particles reach higher and higher energies. “Nobody has ever really
verified this,” he said.

The experiment, funded by the National Aeronautics and Space
Administration, is a follow-up to a study conducted aboard the space
shuttle Challenger in 1985. The data from that experiment, built by
Muller and the late Peter Meyer, remain the most detailed ever
obtained on the composition of cosmic rays at extreme energies.

Muller has good reason to travel to Antarctica to conduct this
long-duration balloon experiment. “The balloon remains at a stable
altitude without requiring ballast to be dropped if there is
continual sunshine, which only is the case in Arctic or Antarctic
summer,” Muller said.

Muller had originally planned to launch his experiment from
Fairbanks, Alaska, and fly around the Arctic Circle, but
international red tape got in the way. “This launch was cancelled
year after year because NASA and its Russian counterpart could not
reach an agreement of overflight over Russian territory,” he said.
Only this year did it become possible to launch a balloon with a
3-ton payload from Antarctica’s McMurdo Station.

“We often sit there for weeks and wait for the stratospheric wind
pattern to be just right,” said Muller, a veteran of approximately
two dozen balloon launches. “You also cannot launch if you have
strong winds on the surface.”

The huge helium balloon that will carry Muller’s experiment to the
top of the atmosphere has enough plastic to shroud the Sears Tower,
which stands 1,450 feet tall. Launching such a large balloon often
leads to a race against the clock. “The meteorologists must guarantee
that for the next two hours the wind is not going to shift. It can be
a bit nerve-wracking,” Muller said.

Once launched, the balloon will be at the mercy of the prevailing
winds as it makes one and possibly two journeys around the South Pole
collecting data. The experiment will end when scientists send a radio
signal that will trigger a mechanism to cut the balloon from the
payload, which is equipped with a parachute.