The Cosmic Hot Interstellar Plasma Spectrometer satellite, or CHIPSat, will embark on a year-long mission to study the bubble of hot gas that fills the space between nearby stars. This gas-filled region, called the Local Bubble, is thought to be the glowing aftermath of long-ago supernova explosions.

At a cost of $14.5 million, CHIPSat was built in three years by engineers and students, led by research astronomer Mark Hurwitz, at UC Berkeley's Space Sciences Laboratory. With off-the-shelf parts tested as the CHIPS instrument was built, Hurwitz's team was able to keep within budget and turn out a satellite that could operate far past its scheduled lifetime.

"This is very much a low-cost mission," said Hurwitz, who noted that NASA is providing an additional $2 million for post-launch data analysis and mission operations. "The reliability will be the best we can make it for the cost."

The data returned by the instrument will help scientists understand our local region of the galaxy, as well as the origin and evolution of the hot gas bubbles that riddle the galaxy.

"Like a neon lamp, which gives off certain spectral features that our eye perceives as red, this much hotter gas in the interstellar medium will give off special features that are deep in the ultraviolet wavelengths," Hurwitz said. "We are looking for that faint glow that should be coming from more or less every point on the sky, that could be attributable to this hot bubble of interstellar plasma.

"By seeing how bright the glow is and whether the spectrum really matches what the models predict, we will be able to help understand how long these hot bubbles live, because we will know more about how quickly they cool."

According to theoretical astrophysicist Chris McKee, professor and chair of physics at UC Berkeley, the Local Bubble is about 300 light years across, one of many regions of hot, low-density gas in the galaxy. Though astronomers think these regions have been swept of gas by the blasts of exploding supernovas and heated by the shock waves from those explosions, no one is certain, since the supernova remnants have faded into obscurity.

X-ray observations of the Local Bubble, and later extreme ultraviolet observations by the UC Berkeley Extreme Ultraviolet Explorer satellite, indicated the presence of gas at a temperature near a million degrees - about the temperature of the sun's atmosphere - but the data were sparse. The CHIPS spectrometer will look at diffuse extreme ultraviolet emissions from large areas of the sky, measuring very precisely the intensity of specific wavelengths in the gas that would indicate the temperature of the gas atoms and their state of ionization, that is, whether they've gotten hot enough to lose some of their electrons.

"CHIPS is going to have much higher resolution than previous instruments and be able to measure the strength of individual lines," McKee said. "That would then tell us an awful lot more about the physical condition in the hot gas."

The data could tell astronomers how long ago the local supernova explosions occurred - perhaps as long as 10 million years ago - and how fast hot gas cools in the interstellar medium. It also could provide information on the cooler cloud of gas hovering within several light years of the sun.

"The sun is currently inside a local interstellar cloud, a wisp of warm gas at about 7,000 degrees that is surrounded by the hot local bubble," said UC Berkeley research physicist John Vallerga, who is looking forward to analyzing CHIPS data. "It's not clear how this wisp can exist in the hot local bubble, but with CHIPSat, we can attempt to understand whether the warm local cloud in which our sun resides is evaporating into the million degree gas."

CHIPS was one of two low-cost UNiversity-class EXplorer (UNEX) missions chosen by NASA in 1999 after a peer-reviewed competition. One fell by the wayside, and the other, CHIPS, was bounced off several satellites before it found a spot aboard a Boeing Delta II rocket whose primary payload is ICESat - the Ice, Cloud, and land Elevation Satellite. The rocket is scheduled for launch from Vandenberg Air Force Base, Calif., on Thursday, Dec. 19, in the late afternoon. Since it is piggybacking on ICESat, the larger satellite will be launched first, then CHIPSat will be kicked into a nearly polar orbit 380 miles (600 kilometers) above Earth.

"We are going to turn the satellite on slowly and carefully, so we won't be operating the science instrument until about a month after launch," Hurwitz said. "By six months, we should have some interesting data."

The approximately 190-pound (85-kilogram) micro-spacecraft is about the size of a large suitcase, with most of the outer surface covered by gold solar panels. The CHIPSat spacecraft was built by SpaceDev, Inc., of Poway, Calif., while the CHIPS instrument itself was built by engineers and students at UC Berkeley's Space Sciences Laboratory.