The CREAM Instrument and Ballooning in Antarctica

Why do I bother spending a couple weeks in Antarctica?  And why in Antarctica?  The answers to these questions lie in the nature of the CREAM experiment.  The CREAM (Cosmic Ray Energetics and Mass) instrument is designed to measure the relative abundance of elements in the cosmic rays at high energies.  
What is a cosmic ray?  The name makes them sound like something out of a science fiction movie.  Really, though, the name makes sense in the historical context in which the name was born.  “Cosmic” just means from outside the Earth, and “ray” is what early scientists called anything that was an energetic fundamental particle.  Hence “cathode rays,” “delta rays,” “x-rays,” and “gamma rays.”  We now understand that most cosmic rays are just the nuclei of atoms (atomic elements stripped of electrons) which were accelerated to near light speed in supernova shocks throughout our galaxy.  The relative abundance of the elements in the cosmic rays reflects nearly the same abundances in the Solar System, with a few differences that are understood.  By the way, the Universe is 90% hydrogen and 10% helium.  Everything else is present in minute quantities.  What we experience here on Earth, with high abundances of iron, nickel, silicon, oxygen, carbon, and other useful elements, is very different from the average spot in space.
What is remarkable about cosmic rays is that as interstellar travelers they give us an opportunity to look at the elemental composition of the rest of the galaxy.  Of course, nothing is that easy, and working backwards from what we detect here at Earth to what was present in the interstellar medium when the supernova shock hit is quite challenging.  A lot can happen to a cosmic ray between acceleration and detection.  Most importantly, the cosmic ray can interact with atomic hydrogen gas and spallate, which means it can fragment into smaller pieces.  Thus the cosmic ray elemental abundances have some elements which are relatively more abundant than Solar System values because the increased abundance was created between acceleration and detection at Earth.
The CREAM instrument is measuring the charge (hence identifying the element) and the energy (think of it as the velocity) of each through-going particle.  An overview of the detector and how it works, as well as additional science information, can be found at http://cosmicray.umd.edu/cream.  The region of energy that CREAM is exploring is interesting because it represents a change from galactic supernova as the source of acceleration to something else – we are not sure what.  The shape of a plot of number of particles detected at a certain energy vs. energy is in agreement with predictions by the supernova remnant theory up until a certain energy.  Then the plot changes shape.  CREAM is trying to understand whether there is also a change in the relative abundance of what gets tossed into space by this new mechanism.  If it is, say, heavier in iron (or conversely hydrogen) than the Solar System abundances (and low energy cosmic rays) that could tell us that the source material is different from just plain old interstellar medium, and could be something exotic like material around a black hole or neutron star.
Why Antarctica?  Cosmic rays at the high energies where CREAM is looking are very rare.  Only a few of the highest energy particles strike our detector every day, so we need to look for many days in order to collect enough of them to be sure of their relative abundances.  The NASA balloon program is a cheaper alternative to satellites.  The National Scientific Balloon Facility, operated out of Palestine, TX (small town near Dallas), offers large high altitude balloons to scientific collaborations as alternatives to satellite launches.  The balloons float at 120,000 feet, above 99.5% of the mass of the atmosphere – essentially outer space.  Remember that a jet flies at about 35,000 feet.
Remember “Better, faster, cheaper”?  Well, ballooning is at least faster and cheaper, and in some ways better.  A typical balloon payload can go from proposal to launch in less than five years, while a satellite instrument is often decades in the making.  And the balloon can be launched for well under $1M, while a satellite is many hundreds of millions of dollars.  The disadvantage is the amount of time a balloon is up in the air is much less than a satellite.  This is where Antarctica comes in.  
While a balloon can be launched from anywhere, it is important that it be done so over uninhabited areas, since its direction can not be controlled.  The winds at 120,000 feet blow east-west or west-east, and switch directions twice a year.  A balloon sent up during “turnaround,” which lasts about a week, will come down near where it was sent up.  But a balloon launched at another time can cruise from 20 to 120 knots, and quickly moves the middle of nowhere to some inhabited area. It is very unpleasant to have a 5000 pound instrument crash into your living room.  So launch sites include northern Canada, Ft. Sumner, NM, Sweden (for an Atlantic Ocean flyover), and Antarctica.  Antarctica has the added advantage that the winds are circular, around the Pole (hence the ozone hole, since the air does not mix with the rest of the atmosphere during the Antarctic summer), so something sent up from McMurdo will return about 10 days later, and then 10 days after that, etc…. Float times are limited by the balloon technology more than safety concerns.  Last year the CREAM instrument flew for 42 days, a record.
Want to follow CREAM as it goes around and around the Pole?  Go to the tracking web site run by Wallops: http://www.wff.nasa.gov/BPO/creamweb/edrs/pos_alt.htm
Here are some pictures of the CREAM instrument this year:
It all started earlier this spring in my lab, fabricating the detector for which I am responsible.
After my part was integrated into the instrument, CREAM journeyed to Wallops Flight Facility near Chicoteague VA for communications systems testing.
The instrument in the hangar here at McMurdo, ready for launch.  Note the NKU sticker.
You have already seen this picture of the instrument hanging from the crane, ready to roll out to the launch pad.