The Hitchhiker In Discovery's Baggage Compartment -- USC Experiment Studies Ultraviolet Solar Radiation

The spectacular view from the shuttle's open cargo bay doors, with the blue-white Earth shimmering in the background, is often seen in television coverage of shuttle flights. This time out, frequently seen in camera scans of the cargo bay -- emblazoned in bright red across a large white cylinder -- is a big, fat USC logo.

The cylinder contains the Solar Extreme Ultraviolet Hitchhiker-3 (SEH-3) instrument package, wholly designed and assembled at USC.

"We are measuring the sun's extreme ultraviolet (EUV) radiation in order to better understand the photochemistry produced by the light absorbed in the atmospheres of planets, comets and anything else illuminated by the sun," says Donald R. McMullin, project manager at USC's Space Sciences Center.

EUV radiation paints every object in the solar system, McMullin explains, but much of that radiation is then reflected back into space. What is not reflected is absorbed. Indeed, every atom, molecule or ion absorbs and emits its own precise, characteristic wavelengths of EUV radiation. Knowledge of what radiation has been absorbed and what has been reflected therefore provides a "fingerprint" that enables scientists to identify the atomic constituents of an observed object.

"In the electromagnetic spectrum, EUV radiation is invisible to the naked eye and must be observed from above the Earth's atmosphere with sensitive instruments, just like the solar x-rays in the adjacent spectral region," says Darrell L. Judge, Ph.D., professor of physics and astronomy in the USC College of Letters, Arts and Sciences and director of the USC Space Sciences Center. He is the center's principal investigator for the SEH mission and has been studying solar EUV radiation for more than two decades.

Such experiments must be conducted from space because the Earth's atmosphere completely blocks EUV radiation. But the unattenuated solar light observed above the Earth's atmosphere degrades the materials normally used in solar observing instruments, thus presenting a serious design challenge.

"We met that challenge by eliminating all optical components in most of our instruments," Dr. Judge reports. "The instruments were designed and assembled at the Space Sciences Center, and every piece of metal in them machined in our machine shop."

During the current shuttle trip, McMullin and a USC team operated the SEH-3 package from NASA's Goddard Space Flight Center.

"To carry out the solar observations, the shuttle bay has to be oriented so that our solar instruments are pointed toward the sun with an accuracy of a few arc minutes. This is remarkably accurate for a shuttle-type vehicle," McMullin notes. The pointing is usually accomplished by the shuttle pilot, but NASA's ground control center can steer the vehicle if required.

The shuttle also carries a complementary package from the University of Arizona to measure the EUV light reflected by the planet Jupiter. The University of Arizona and USC instruments together are measuring what goes into the Jovian atmosphere and what comes out.