GLAST will have two primary instruments:

The Large Area Telescope (LAT) is an imaging gamma-ray telescope that is vastly more capable that any previous instrument.

The GLAST Burst Monitor is a whole sky monitor for short duration but high intensity bursts.

However, unlike gamma rays, cosmic rays do not travel through space in straight lines. This means that using cosmic rays to look at the universe is like looking through frosted glass. Thus, GLAST will use sophisticated software and sensors are designed to identify and ignore the cosmic rays. Since cosmic rays will outnumber the desired gamma rays by as much as 100,000 to 1 (depending on where in the sky GLAST is looking), this job must be done nearly perfectly -- otherwise, most of the supposed gamma ray detections would really be misidentified cosmic rays.

Tests have been done at the Stanford Linear Accelerator Center (SLAC) to simulate cosmic ray hits on the GLAST detectors, but with so much riding on their performance, the detectors ought to be tested on the actual cosmic rays they'll be seeing in orbit. This can't be done at ground level, since cosmic rays are blocked or altered as they pass through the Earth's atmosphere. At the very least, the detectors would have to be taken into the upper reaches of the stratosphere, where cosmic rays can still be seen in their unfiltered state...

So on August 4, 2001, a balloon was launched from Palestine, Texas, carrying a prototype GLAST detection tower. (The spacecraft will carry 16 such towers.) The balloon carried the tower to an altitude of about 20 miles, where in the near vacuum of the upper stratosphere, the detectors could have their first look at primary cosmic rays from space, alongside the gamma rays they're designed to study. And our Team was onhand to observe the fun. For their observations and stories, check out the RET 2001 Travel Log...