STS-75 press kit

Edited by Richard W. Orloff, 01/2001/Page 16
Earth's Charged-Particle Environment and the Tethered Satellite System
TSS-1R will make use of Earth's magnetic field and electrically charged atmosphere for a variety of
experiments. Just as a bar magnet produces invisible lines of force known as "field lines," so does Earth.
The Sun is a ball of electrically charged, or ionized, gas known as plasma. Plasma from the Sun, the solar
wind, continually rushes past Earth; most is deflected around the planet, but some penetrates Earth's upper
atmosphere, creating electric fields. Lightning is a commonly seen form of plasma. More than 99 percent
of matter in the universe exists in the plasma state.
Speeding through the magnetized ionospheric plasma at almost five miles per second, the Tethered
Satellite should create a variety of very interesting plasma-electrodynamic phenomena. These are expected
to provide unique experimental opportunities, including the ability to collect an electrical charge and drive
a large-current system, generate high voltages (around 5,000 volts) across the tether, control the satellite's
electrical potential and its plasma sheath (the layer of charged particles created around the satellite), and
generate low-frequency electrostatic and electromagnetic waves. While ground-based scientists are limited
to small- scale experiments, Earth's ionosphere offers TSS-1R scientists a vast laboratory for space plasma
experiments that cannot be conducted any other way.
The tether system consists of a five-foot (1.6-meter) diameter battery-powered satellite secured by a strong,
electrically conducting cord, or tether, to the satellite support structure attached to the Shuttle orbiter. Data-
gathering instruments are mounted in the Shuttle's cargo bay and middeck area, and on the satellite. During
the second day on orbit, the STS-75 crew will reel the satellite out on its tether -- which looks like a long
white shoelace - to about 12.5 miles (20.7 kilometers) away from the Shuttle, into the ionosphere. TSS-1R
scientific instruments will allow scientists to examine the electrodynamics of the conducting tether system,
as well as clarify the physical processes of the near-Earth space environment and, by extension, throughout
the Solar System.
The conducting tether's generator mode will produce electrical current at a high voltage, using the same
basic principle as a standard electrical generator. A small portion of the mechanical energy of the Shuttle's
more than 17,500-mile-an-hour orbital motion will be converted into electrical energy as the electrically
conducting metal strands in the tether's core pass through Earth's magnetic field lines.
The conductive outer skin of the Tethered Satellite will collect free electrons from the space plasma, and
the resulting voltage will cause the electrons to flow down the conductive tether to the Shuttle. An electron
accelerator, also called an electron gun, will then eject them back into space. Scientists expect the electrons
to travel through the ionosphere to complete the loop required to close the circuit, just as a wire must close
the circuit between the positive and negative poles of a car battery before current will flow. They will use a
series of interdependent experiments -- conducted with electron guns and tether current-control hardware
along with a set of diagnostic instruments -- to assess the nature of the external current loop within the
ionosphere. This also will shed light on the processes by which the circuit is completed at the satellite and
the Shuttle.
Scientific Investigations
Of the TSS-1R mission's 12 scientific investigations, NASA will provide six, ASI will provide five and the
U.S. Air Force Phillips Laboratory will contribute one. Seven experiments include equipment that either
stimulates or monitors the tether system and its environment, two will use ground-based instruments to
measure electromagnetic emissions from the TSS, two will use satellite and orbiter-mounted instruments to
study tether dynamics and one will provide theoretical support in the area of electrodynamics.
Only a complete set of data on plasma and field conditions can give an accurate understanding of the space
environment and its interaction with the tethered system. TSS-1R science investigations are complementary