The ICESat spacecraft sits atop CHIPSat, which is included within the Delta 2 rocket's dual payload launching structure. Photo: Boeing

The Ice, Cloud and Land Elevation Satellite - ICESat - is designed to find out whether the polar ice sheets are expanding or melting, either scenario a tell-tale sign of environmental change on a vast scale.

The other satellite, known as the Cosmic Hot Interstellar Plasma Spectrometer, or CHIPS, will study how the debris from exploding suns cools and ultimately becomes the raw material for new stars in what amounts to a galactic recycling program.

Both satellites are scheduled for launch Saturday atop a Boeing Delta 2 rocket from Vandenberg Air Force Base, Calif.

Of the two, ICESat is the star of the show, costing $282 million including data analysis and the price of the Delta 2 rocket. The suitcase-size CHIPSat cost just $16 million, including $2 million for data analysis, and is hitching a ride into orbit with ICESat.

Once operational in a 373-mile-high orbit around Earth's poles, ICESat will use a sophisticated laser altimeter to collect precise elevation data that will help researchers determine the ice-sheet mass balance, that is, how much new ice forms every year compared to how much is lost.

An artist's concept of ICESat in Earth orbit. Credit: Ball

"The ice sheets that cover Greenland and Antarctica today are over two miles thick in places," said Jay Zwally, ICESat project scientist. "But very simply, we do not know if they're growing or shrinking.

"Almost every newspaper article we read says that when the climate warms, we're going to see the melting of the polar ice sheets and the flooding of coastal areas. The truth is, we really don't know."

ICESat is equipped with an instrument known as the Geoscience Laser Altimeter System, or GLAS for short, that shares design elements with a laser altimeter currently in orbit around Mars.

By precisely timing how long it takes a laser beam to fire, hit the surface and bounce back to a 31-inch telescope, scientists can determine the elevation of the ice sheets below to an accuracy of six inches. Firing 40 times a second, GLAS will collect readings at roughly 1-mile intervals.

Zwally said about three-tenths of an inch of water from the entire surface of the ocean ends up in the planet's ice sheets as snowfall.

"That's about three inches every 10 years," he said. "If there was only water going in and nothing coming out, sea level would drop three inches every 10 years.

"But approximately the same amount is coming back out in the form of icebergs and melting at the edges," he said at a recent news conference. "We don't know which is greater. The difference is what we call the mass balance."

Many researchers expect the ice sheets to melt more around the edges and to grow more in the center. Recent measurements in Greenland using aircraft-borne instruments show the edges are, in fact, melting more than expected.

But there is little or no data about what might be going on in the central regions of the ice sheets.

"That's what ICESat will do," Zwally said. "ICESat will provide (precise measurements) over the whole ice sheets from season to season and from year to year."

It is a complex problem and one that until now has been fraught with uncertainty. A United Nations panel on climate change, for example, has predicted that over the next 100 years, the sea level could rise anywhere from three to 33 inches.

Over time, ICESat measurements should allow scientists to chart changes in ice sheet elevation that are equivalent to only a few tenths of a millimeter of sea level change.

"For the future, we expect increases in melting and increases in snowfall," Zwally said. "It's going to be a race between the two of these. We're going to get more melting at the edges, more snowfall at the center and just what the contribution will be is uncertain. It could be either plus or minus."

Of more purely intellectual interest, the 132-pound CHIPS satellite will shed light on how the hot debris left over from exploding suns cools off and mixes with cold, rarified clouds of gas and dust already present in the vast reaches between the stars.

It is that material that ultimately clumps together and somehow coalesces to form new generations of stars.

An artist's concept of CHIPSat. Credit: NASA

"The key question is how long these hot bubbles of gas exist," said Mark Hurwitz, CHIPS principal investigator at the University of California at Berkeley. "Another way to ask that question is how quickly do they cool?"

The question is easier asked than answered.

As it turns out, the hot gas marking the aftermath of a supernova cools and radiates energy in the extreme ultraviolet region of the spectrum, invisible to human eyes but detectable by instruments tuned to receive it.

The problem for astronomers is that such radiation cannot travel far through the interstellar medium and is difficult to observe at the extreme distances that characterize interstellar space.

As a result, the theoretical models astronomers use to explain how the galaxy's recycling system works are based on indirect evidence.

"It's a little bit like observing the tail and trying to use a model to infer the size of the dog," Hurwitz said. "Obviously, you'd like to be able at some point to see the dog and make sure your model is right. For these processes, extreme ultraviolet radiation is the dog."

Fortunately for astronomers, nature has provided a nearby supernova remnant, a hot, rarified bubble 300 light years across that is close enough to be observed at extreme ultraviolet wavelengths.

In fact, the bubble in question is centered roughly on our sun, implying the supernova that created it two to 10 million years ago probably was close enough to have affected Earth's biosphere.

While CHIPS is not concerned with the supernova's effects on Earth, radiation from a nearby blast would have seriously degraded the planet's protective ozone layer, according to Stephen Thorsett, a supernova expert at the University of California-Santa Cruz.

"It is likely that it would have temporarily increased cancer rates, and might (depending on the distance) have had a pretty significant effect on mortality rates," he said by email.

"But we really don't know enough about either the physical details or the biological responses to know if it would be a dramatic enough event to find its effects in, say, sedimentary or ice core records."

In any case, Hurwitz said, such a close-range supernova would have had an apparent brightness of 30 times that of the full moon.

But in the millions of years since the blast responsible for the local bubble occurred, the clouds have expanded and dissipated to the point where the energy is spread out across the entire sky.

In fact, the gas making up the local bubble is so thin today a coffee mug dipped into it would contain, at best, a single hydrogen atom. That same mug, dipped into the interstellar medium just beyond the boundary of the local bubble, would contain about 500 atoms.

How the two mix together is at the heart of the CHIPS mission.

"So today, if you took the brightness of the full moon, spread that over the entire sky and then divided it by 10 million, that is approximately the intensity that CHIPS will be trying to detect," Hurwitz said.