When you see astronauts floating around the International Space Station and hear they are in microgravity, what does that really mean? We know that the astronauts appear weightless, but the space station and all of its contents, including the astronauts, actually still are under the influence of Earth's gravitational pull. This is because the station remains in orbit around our planet. Both station and astronauts are actually falling around Earth.

A great way to visualize how orbits work is to play this Shoot a Cannonball into Orbit game, demonstrating the physics of acceleration and gravitational force. Both spacecraft and crew are going at the correct speed horizontally to maintain their altitude above Earth, and together experience a continual state of free fall. This near weightlessness is commonly referred to as "microgravity." Gravity on Earth is abbreviated as 1g. The prefix "micro-" refers to one-millionth, so that microgravity implies 1/1,000,000 of Earth's gravity.

For various reasons, this ratio is not constant; however, the deviation is slight enough that, for simplicity's sake, we describe it as a constant. For example, at the space station's current altitude, about 240 miles above us, there still is a very thin atmosphere that imparts aerodynamic drag, and the drag changes slowly as the space station orbits Earth. This drag counteracts the station's free fall to a very small degree; but, it is still there and plays a part in scientific investigations.

Microgravity researchers take advantage of the fact that their experiments, along with everything else in the space station, are free falling. While in orbit on the space station, these investigations do not experience the effects of gravity, such as buoyancy, convection or sedimentation. These effects, which tend to cause fluids to move and mix in our 1g environment here on Earth, are greatly reduced in orbit.

The precise value and fluctuations of the microgravity environment are important in interpreting the data from station investigations. There are accelerometer systems in orbit to measure the microgravity environment. Two of those systems are sponsored by NASA's Glenn Research Center in Cleveland: the Space Acceleration Measurement System (SAMS) and the Microgravity Acceleration Measurement System (MAMS).

MAMS measures low frequency, low magnitude vibrations or accelerations below 0.01 hertz. This is one vibration every 100 seconds, which is very slow. Typically, large massive structures vibrate slowly. These accelerations include the effects of aerodynamic drag that are typically smaller than one micro-g. The nature of these accelerations is such that measurements can be made at one location and applied to any other location on the space station.

SAMS, on the other hand, deploys multiple accelerometers throughout the space station to measure higher frequency accelerations, between 0.01 to 300 hertz, which typically range from 10 to several thousand micro-g. These vibrations require measurements close to the point of origin and tend to come from equipment like fans and pumps or from crew activity such as exercise. In addition, SAMS measures transient vibrations, which are relatively brief and fairly strong.

These accelerations can be caused by crew movement pushing off bulkheads and landing, from vehicle thrusters to maintain attitude or reboost altitude, vehicle dockings and machinery start up. Such activities can produce peak measurements of more than ten-thousand micro-g's.

"SAMS also plays an important role in monitoring the space station's structural integrity," said Ken Hrovat at Glenn. "SAMS' measurements are analyzed to determine the exact nature of flexing and bending of important space station structures, its 'backbone,' to assess vehicle longevity." Recent analysis suggests that the space station will be sturdy and safe enough as a microgravity research platform until about the year 2028.

As a result of the measurements collected by SAMS and MAMS, researchers are able to monitor continuously the true nature of and small changes in the microgravity environment on the space station. Researchers at Glenn receive the data from these instruments as it streams down from station, displaying in near real-time on the Web. An archive of the data provides this information to sustaining engineering, scientific investigators and the microgravity community at-large.

'How do we know it's really microgravity?' Thanks to SAMS and MAMS we sense its effects and we measure it!