Weightlessness is one of the most intriguing aspects of spaceflight. Sure, the view from low-Earth orbit is nice and the prospect of someday visiting other worlds is exciting.

	NASA
	Senior CMU biology major James Torchia and junior chemistry major Sujata Emani on March 11 aboard NASA's KC-135A reduced-gravity research aircraft.

But no gravity, or zero-g, just looks like fun.

It also is one of the biggest challenges of extended spaceflight, such as the manned missions to Mars envisioned by President Bush. Low gravity conditions not only cause astronauts to lose bone mass, but also seem to suppress their disease-fighting immune systems.

To better understand this phenomenon, four Carnegie Mellon University students this spring went aloft in NASA's KC-135A reduced-gravity research aircraft to conduct experiments on how zero-g affects the structure of cells.

Though their experiments focused on the machinery that cells use to move, the experimenters couldn't help but notice the effect zero-g had on their own movements.

"It certainly was an experience we will never forget," said Sujata Emani, a chemistry major from Cleveland. "The kid side of me wants to say it was totally cool."

Emani and her cohorts, biology majors James Torchia, Caroline Chen and Candice Spier, each took opportunities to tumble effortlessly and float in the aircraft's 60-foot-long cargo hold, but most of the hours they spent aloft were spent performing experiments.

The four-engine KC-135A, which is similar to a Boeing 707 airliner, creates brief periods of low- or zero-gravity by flying a series of parabolas ---- steep climbs followed by steep dives, which leave the occupants weightless for 30 seconds or so each time the plane reaches the top of the parabola and begins its dive.

"It's much gentler than a roller coaster ride," said Spier, of Springfield, Ohio, a skydiver and pilot-in-training who nevertheless experienced the brief discomfort that has given the plane the evocative nickname of Vomit Comet.

The students, advised by Adam Lindstedt, associate professor of biological sciences, were selected by NASA for the flights as part of the agency's Reduced Gravity Student Flight Opportunities program. They made two flights, two students at a time, from Johnson Space Center in Houston in mid-March.

Concerns about how earthlings would fare in space without the constant pull of gravity are as old as the space program itself. NASA scientists have long understood that sustained spaceflight results in a loss of bone mass, similar to the osteoporosis suffered by many older adults.

One theory was that this occurred because bones don't carry the same loads in space; the hope was that regular weight-bearing exercises in space might reduce the rate of bone loss.

But a study published this spring in the online version of the Journal of Bone and Mineral Research shows that the conditioning exercises now used aboard the International Space Station aren't enough.

Researchers from the University of California, San Francisco, and Baylor College of Medicine in Houston used three-dimensional CT scans to study the bones of 14 American and Russian crewmembers from the space station. They found that the crewmembers lost bone from the porous interior of the hip at a rate of 2.2 percent to 2.7 percent for each month in space and bone from the dense outer shell of the hip at a rate of 1.6 percent to 1.7 percent a month.

	NASA
	Caroline Chen, a biology major from Long Island, N.Y., conducts a cell experiment while strapped to the floor of the KC-135A.

Those rates are comparable to the losses noted a decade earlier by crews of the Russian Mir space station, noted Thomas Lang, a UCSF radiologist.

"The lack of clear progress in the interval between Mir and Station missions indicates a need for continued efforts to improve musculoskeletal conditioning regimens during longer space missions, such as those proposed for the moon and Mars," Lang said.

But it may not be a simple matter of conditioning, said Torchia, a senior from Mt. Lebanon. In addition to loss of bone density, extended spaceflight has been found to suppress immune cells, particularly T cells. The lack of gravity appears to have effects on the cellular level, not just the systemic level addressed by conditioning.

Studies in the 1990s showed zero-g caused irregularities in the cytoskeleton, the system of tiny filaments that supports the cell membrane and, because the filaments are constantly rearranged, enables cells to move. The cytoskeleton, thought to be the gravity-sensing component of the cell, also has a role in signalling between cells. Under weightless conditions, however, these filaments become disordered.

One thing that has not previously been studied under zero-gravity is the effect on cell motion. By rapidly building filaments of the protein actin, cells move by extending one of their sides.

In their experiments, the students loaded neutrophils, a type of white blood cell, into syringes. During periods of weightlessness, as well as periods of increased gravity, during the KC-135A flights, they then added formyl-Methionine-Leucine-Phenylalanine, or fMLP, a substance that stimulates the formation of actin filaments.

Finally, at intervals of 5, 10 and 15 seconds, they added formaldehyde to halt the movement and fix the cells for later study.

They prepared 30 of these experiments for the flights. During the flights, one student would manipulate the syringes to add fMLP and then formaldehyde, while a second student would time the experiments and keep records.

Back on the ground, the students are using a technique called flow cytometry to measure the amount of actin that was activated in the cells and then are optically studying the cells for differences in the organization of the filaments.

Though there was some fear that the cells were damaged during transport back to Pittsburgh after the flight, Torchia said last week that "it looks like we will get some results."