As the Shuttle orbits Earth at altitudes of 240 to 400 kilometers (150 to 250 miles), it is immersed in ionospheric plasma.
While in this environment, the Shuttle/Spacelab can be used to deploy small satellites and retrieve them, expose detectors directly to natural plasma, disturb the plasma with beams of energetic particles, and operate in coordination with ground-based facilities and other satellites.(image right from science.howstuffworks.com)
During a Shuttle/Spacelab mission, the ionosphere becomes a laboratory for studying processes that occur near Earth and throughout the universe, and the vehicle itself becomes an instrument for experiments. The space plasma environment is studied by three techniques: active experiments, in-situ probes, and remote sensing.
Active experiments introduce agents (particles, waves, chemicals) into the ionosphere to trace, modify, or stimulate the environment. The Shuttle itself stimulates the environment as it passes through the plasma, creating a wake and other disturbances.
Spacelab is ideally suited for active experiments. Instead of waiting for nature to perform, scientists can create artificial auroras, particle beams, plasma waves, and wakes.
Ordinarily unseen magnetic field lines and wind patterns may become visible in clouds of color produced by chemical releases, enabling us to watch and photograph the form and motion of space plasmas. (image right depicts the gas cloud given off by the orbiter, producing reactions that modify the density of nearby plasma, click image for detail)
In active experiments, investigators introduce a known stimulus and measure the environment's response to test hypotheses about the natural processes of particle acceleration, wave and wind movement, chemical releases, and energy release. (
The Space Experiments with Particle Accelerators (SEPAC) flown on the Spacelab 1 mission used the Shuttle as a platform for active space plasma research. The investigation used a particle accelerator that could emit electron beams from 1,000 to 7,500 volts and up to 1.6 amps and a magnetoplasma dynamic arc jet which emitted pulses of argon ions. Several passive probes were carried to observe the shape of the beam and to measure wave and particle interactions.(image right of SEPAC instrument compliment, from JAXA space education center)
An electric field sheath develops around the vehicle and, like a boat, the Shuttle creates a wake in the plasma. The wake is depleted of plasma as the Shuttle collides with and displaces the gas, and various instabilities occur as the wake region is refilled with plasma. (image upper left: The top panel of these two spectrograms shows the angle of diffracted particles as they fill the wake left by the Shuttle. The bottom panel shows the distribution of ion energy over time)
One Spacelab 2 investigation took advantage of chemicals that the Shuttle routinely releases when thrusters are fired to maintain or change altitude: exhaust consisting mainly of water vapor, carbon dioxide, and hydrogen. The effects of these releases are temporary and are not detrimental to the environment, but they do cause some interesting physical and electrical changes in the ionosphere.The exhaust triggers chemical reactions that cause electrons to combine with ions in the upper atmosphere, leaving temporarily depleted plasma areas or "holes."
Observations also were made of atmospheric airglow created as molecules react with sunlight and of the glow associated with the Shuttle. It has been suggested that hydroxyl (OH) is a candidate species for producing the troublesome Shuttle glow which may interfere with some astronomical observations. (image right: plasma glow that surrounds the Shuttle as it travels through space)
The glow has been studied on other missions by scientists from different disciplines who have proposed various theories concerning the glow.