Zero-g is a myth. Even in orbit, spacecraft experience tiny accelerations called "microgravity" that scientists monitor using a device named SAMS.

In the near-vacuum of open space, of course, "nobody can hear you scream," as the sci-fi thriller Alien famously put it. But onboard the space shuttle and the International Space Station (ISS), life is filled with activity and sounds.

Some come from the pervasive plumbing, wiring, and mechanical hardware that fills every corner and cranny. If you were aboard, you'd hear air-circulation fans whirring, electric motors humming, pumps switching on and off--constant reminders that you're living in the bowels of a giant machine, switched "on."

Above: Life in space is busy ... and noisy. Pictured here crewmates on the International Space Station perform maintenance on the Treadmill Vibration Isolation System. [more at http://www.spaceflight.nasa.gov/gallery/images/station/crew-5/html/iss005e17402.html]

Others come from the people themselves: an astronaut chugs-away on a spring-mounted exercise bike to prevent muscle atrophy; crew members move around, talk, bump into things, work with clanking metal tools; tinny-sounding music plays from little speakers to keep the crew's spirits up amid their gray, metallic surroundings.

All these little sounds lend a kind of machine "personality" to the spacecraft--like the distinctive feel of your own car. To astronauts the hubbub is familiar, comforting, better by far than dead silence. But to scientists with experiments onboard, the sounds are a sign of something possibly unwelcome. Every clank, hum and buzz corresponds to a tiny jolting acceleration.

They are the "g's in the machine."

Experiments are done in space (more often than not) to escape acceleration. On Earth objects are accelerated downward by gravity at a rate of 9.8 m/s² or 1-g ("one gee"). That constant pull is responsible for effects like convection and sedimentation that can complicate chemistry and physics experiments--effects that scientists would like to leave behind on Earth. Yet after all the trouble of going to space to get away from big g, the space shuttle and the ISS are filled with little g's of their own.

Is that a problem?

"Not usually," answers John Charles, the chief scientist for shuttle mission STS-107. Most of these vibrations are vanishingly small--less than one-millionth the acceleration of gravity here on the ground. (Hence the term "microgravity;" the prefix "micro" means one-millionth.) But sometimes, he says, occasional jolts and ill-timed vibrations can upset the most delicate experiments.

Above: The space shuttle's orbital maneuvering system (OMS) engine fires as the shuttle cruises "upside down" in low Earth orbit.

For example, "combustion experiments really don't like thruster firings." Charles explains: Flames in space do something odd. Instead of forming the familiar teardrop shape of candle flames on Earth, they contract into little balls, which float around and burn using almost no fuel. Scientists suspect these flame balls hold the secrets to leaner burning auto engines. The problem is, flame balls are delicate. A gentle bump is enough to knock one out.

The mission that Charles leads, STS-107--a 16-day flight of the shuttle Columbia, has 80+ science experiments on board. Three of them involve flames and combustion.

One called SOFBALL (short for "Structure of Flame Balls at Low Lewis number") will ignite some of these flame balls in a special chamber where scientists can experiment with them and measure their properties. The shuttle's thrusters will be turned off to avoid sending the floating balls careening into the walls of their chamber. But what if a flame ball winks out anyway? Did scientists just learn something new about flame balls? Or was it one of those g's in the machine?

Right: "Flame balls" like these will float inside a chamber during the SOFBALL-2 experiment aboard the STS-107 shuttle research mission. [more at http://science.nasa.gov/headlines/y2002/21aug_flameballs.htm]

"This is why we have SAMS--the Space Acceleration Measurement System," continues Charles. SAMS is a sensitive accelerometer that monitors vibrations and other small accelerations. "SAMS picks up everything," he says. People coughing. Things bouncing off walls. A knob gently twisted. "The device is so sensitive," he notes, "that thruster firings can overwhelm it."

SAMS was developed for space research missions by a group of engineers and scientists at the Glenn Research Center. "When NASA started doing microgravity experiments onboard the shuttle, we realized that we would have to measure the vibratory g-levels," says Thomas Kacpura of ZIN Technologies, Inc., a contractor who works on SAMS. "Otherwise how would you know if a blip in your data was real or not?"

SAMS sensors have flown before on 22 shuttle missions, on the Mir space station, and one is permanently mounted in the Destiny lab module of the International Space Station. "It's indispensable for space research," adds Charles.

Below: The SAMS Free Flyer, like the one onboard the space shuttle Columbia (STS-107). [more at http://microgravity.grc.nasa.gov/MSD/MSD_htmls/samsff.html]

On the shuttle Columbia (STS-107), SAMS is located near the SOFBALL experiment, which is inside the SPACEHAB module in the middle of the shuttle's cargo bay. Data from SAMS are transmitted directly to Earth where researchers can monitor the microgravity environment in near-real time and make decisions accordingly. If the shuttle is still vibrating after a thruster firing, for instance, they might wait a while before igniting their flame balls. SOFBALL isn't the only experiment that will benefit because SAMS can detect vibrations throughout the ship. Its record of micro-accelerations will be like a communal well--all are free to draw from the well as needed.

So astronauts can go ahead and ride their exercise bikes, cough, turn knobs ... even scream if they feel like it. SAMS won't neutralize those vibrations, but it lets researchers keep track of them. And that sounds like good science.

Editor's Note: Not all of the g's in the machine make noise. Tune in next week for a follow-up story, "Gravity's Last Laugh," which will explain some other slow and silent accelerations onboard the shuttle that researchers are monitoring using a device called OARE.

NASA's Office of Biological & Physical Research (http://spaceresearch.nasa.gov/) supports the ongoing STS-107 research mission (http://spaceresearch.nasa.gov/sts-107/index.html) .

This article is about the Space Acceleration Measurement System (http://microgravity.grc.nasa.gov/MSD/MSD_htmls/samsff.html) --or "SAMS" for short. SAMS on the International Space Station (http://pims.grc.nasa.gov/) transmits data to Earth in real time. Take a look! (http://pims.grc.nasa.gov/html/121_f02/PIMS_ISS_color.html)

Microgravity links: The ISS microgravity environment (http://stationpayloads.jsc.nasa.gov/D-aboutiss/D6.html#microgravity) (Johnson Space Center); Microgravity Research Program Office (http://microgravity.nasa.gov/) (Marshall Space Flight Center); National Center for Microgravity Research on Fluids and Combustion (http://www.ncmr.org/) ; Understanding Microgravity Discipline Brochure (http://mgnews.msfc.nasa.gov/db/understanding_ug/understanding_ug.html) (Marshall Space Flight Center)