"It was amazing, the best experience ever," said Chai, a junior majoring in bioengineering at the University of California, Berkeley. "And I didn't even throw up."

Chai, 20, and three other UC Berkeley students participated in a special NASA program that gives college students access to one of its unique assets, a research plane - often referred to as the "vomit comet" - that simulates the weightlessness of space.

This Saturday (May 5), the four-person team will demonstrate an experiment they conducted on the NASA plane at the Lawrence Hall of Science from noon to 2 p.m.

The NASA Reduced Gravity Student Flight Opportunities Program has been in existence since 1995 out of NASA 's Johnson Spaceflight Center in Houston, administered by the Texas Space Grant Consortium. Chai and her co-scientists, the first UC Berkeley students to apply for the program, proposed to test an improvement to the treadmills astronauts pound regularly in space to prevent bone loss and potential injury. Without such high-impact exercise, astronauts can lose as much as 2-3 percent of their bone mass per week.

With the help of NASA scientists and a borrowed 500-pound treadmill, the team spent two weeks, Feb. 8-17, at Ellington Field near Houston setting up their experiment for two flights of the same Boeing KC-135A used to train astronauts. On Feb. 13, during their first series of 30 zero-G dives, each 30 seconds long, Chai and recent UC Berkeley integrative biology graduate Lanny Rudner, 22, were the guinea pigs, running at 6.5 miles per hour while strapped into the treadmill. A day later, Chris Hamerski and Bev Guo, both 22, flew another series of tests.

Just for fun, each group reserved a few parabolic dives for floating and tumbling around the plane, an experience Chai compared to the Drop Zone ride at Great America.

"I'm still on a high from the trip," she said.

As for the experiment, preliminary results indicate it was a success.

"This was a very challenging project to pull off, but they did it, thanks to an enormous amount of help from NASA people," said Rodger Kram, a former UC Berkeley assistant professor of integrative biology who first suggested the idea to the team and served as their faculty advisor. "This project not only may benefit NASA and astronauts, but it also gave the students a great experience. They were totally charged up."

Chai and Rudner first got excited about zero-G experiments while taking a course called "Mars by 2012," taught each year at UC Berkeley by former NASA engineer Larry Kuznetz. After presenting a class paper about simulating reduced gravity on Earth, they cast about for ways to further their interest until Kram suggested they propose an experiment for the NASA program.

The two teamed up with Guo, now a senior majoring in integrative biology, and Hamerski, who graduated last year in molecular and cell biology, to write and submit a proposal. It was accepted last summer, but postponed because of airplane maintenance until this spring, when the foursome flew with 47 other teams from around the country.

The idea behind treadmills in space is to mimic the pounding and leg stresses astronauts would experience while walking or running on Earth, in hopes of preventing the bone deterioration seen on long space flights.

The problem, said Kram, now in the Department of Kinesiology and Applied Physiology at the University of Colorado, Boulder, is that treadmills in use today are not very effective, primarily because they rely upon awkward and uncomfortable rubber bands to pull astronauts down onto the treadmill as they run. The elastic straps hurt so much that astronauts run in a crouched position with bent knees. While this is less painful, "they're not banging their bones the way they need to," Kram said.

Based on laboratory experiments with simulated reduced gravity, Kram and his UC Berkeley colleagues recently showed that runners could increase peak impact forces while using only a moderate downward pull from the shoulder straps if, at the same time, another rubber band pulled them forward.

"The idea is, if you could combine a moderate downward pulling force with this forward pulling force, they'd get the same kind of impacts that you'd need to stimulate the bones," he said. It's like running downhill, added Chai.

Hamerski, who will enter medical school this fall at UC San Francisco, had performed essentially the same experiment in simulated reduced gravity for his senior thesis in Kram's Earth-bound Locomotion Laboratory.

"In the Locomotion Lab, we used a modified mountain climbing harness to pull upward with rubber bands to simulate reduced weight," Hamerski said. "The KC-135 gave us the chance to try this in zero-G."

In an article in the May 2001 issue of the Journal of Biomechanics, UC Berkeley graduate student Young-Hui Chang, Hamerski and Kram provide convincing evidence that this forward-pulling harness works. While pulling down with only 38 percent of a runner's weight - the force of gravity on Mars - and pulling forward with about 1/5 the runners' weight, they found they could increase the impact force to more than a runner would experience under normal gravity.

"We've shown that you can increase the functional loading a runner experiences and mimic what they experience on Earth," said Chang, now a post-doctoral fellow at Emory University in Atlanta, who got the idea of pulling forward while studying the metabolic costs of alternately braking and speeding up when running. "Now it's up to NASA to give it a try and see if impact forces comparable to those on Earth actually prevent bone loss."

The airborne experiment by Chai, Rudner, Hamerski and Guo was the first microgravity test of this hypothesis, and it confirmed what Chang and Kram had predicted.

"We got to one-G impacts with only a half-G downward force and 20 percent reduced G horizontal force," Chai said. "That's in the comfort zone, and a lot easier than running when you're pulled down by a force of one G."

Staff members with the NASA program seemed intrigued by the students' experiment, and Kram thinks that NASA scientists would be interested in looking more closely at the technique.

"This project is going somewhere. It's not very fancy, but it seems to be one of the more promising solutions to bone loss during long-term space flight," he said.

As for the students, they're captivated by space. Rudner has been accepted at Baylor College of Medicine, where he hopes to become a flight surgeon, and Hamerski said he would find it "awesome" to some day practice medicine in space.

"These students are the dreamers who want to be astronauts," Kram said.

Chai, who plans to pursue an MD/PhD, is hooked too. "I want to find another project and fly again," she said.