LG-1997-02-451-HQ
ABOUT THE PICTURE
U.S. astronaut Jeffrey A. Hoffman (left) and astronaut Maurizio
Cheli, of the European Space Agency, set up a Glovebox experiment
during USMP-3.
USMP MISSION SERIES
Conducting experiments in microgravity lets scientists observe
forces that are always present on Earth but are often hidden by
the stronger force of gravity. Some experiments performed in
microgravity on the space shuttle require the hands-on attention
of the astronauts. Others can be monitored from a control center
on Earth. The U.S. Microgravity Payload (USMP) series was planned
by NASA for experiments that could be controlled from the
ground.
Because the astronauts do not need to have access to these
experiments, they are installed in an open carrier that sits in
the shuttle's cargo bay and is exposed to space during orbit. If
the astronauts needed to reach the experiments, they would have to
suit up and take a space walk. But so far in the mission series,
they have not needed to do this because the control of the
experiments from the ground has gone so well.
Soon scientists will not even have to travel to the experiment
control center in Alabama to monitor their experiments. During
USMP 3, NASA set up a remote control site in a scientist's own
lab, so that he could send commands to the experiment computer on
the shuttle from where he worked every day. This practice will be
important for the future International Space Station, where
experiments will run for months at a time. Remote sites will save
scientists time away from home and from their labs.
Although the major experiments of the USMP series are conducted by
remote control, USMP-3 and USMP 4 also include smaller
investigations that the astronauts conduct in the Glovebox (GBX).
A Glovebox is a small sealable box with gloves attached for
handling the experiments inside.
SCIENCE HIGHLIGHTS
MISSION DATE EXPERIMENTS GBX
USMP-I Oct. 1992 2
USMP-2 Mar. 1994 4
USMP-3 Mar. 1996 4 3
USMP 4 Nov. 1997 6 3
COMBUSTION: USMP-3 Glovebox
investigations focused on the process of combustion. They explored
how quickly substances will ignite and flames will spread in
microgravity. They also tested smoke detectors designed for the
International Space Station.
CRITICAL POINT TRANSITIONS: Observing
a substance as it changes from a solid to a liquid or a liquid to
a gas is a familiar experience: we often watch ice melt and water
boil. Critical point transitions are similar but are not so easily
observed as they occur at uncommon temperatures and pressures.
Each USMP mission has included an experiment that studies a
critical point transition. As substances undergo these special
changes, they display unusual properties. Some substances become
superconductors, which can carry electricity with no loss of
energy. Others become superfluids, which can carry heat without
energy loss. Scientists want to learn more about these transitions
because they may lead to inventions like energy cables that
conserve power and more efficient transportation systems.
Observing these unusual transitions is difficult on Earth because
gravity distorts the results. Conducting the USMP experiments has
helped scientists to better understand these extraordinary
transitions.
MATERIALS SCIENCE: When metals are
melted and then cooled, the solids they form are made up of
crystals. Metals form three basic types of crystals: planar
crystals, which are flat; cellular crystals, which look like the
cells of a beehive; and dendrites, which are the most complex and
look like tiny fir trees. During the USMP series, scientists
studied all three crystal formations by melting metals in furnaces
and cooling them at different rates. The structure of the crystals
determines what properties the metal will have: whether it will be
hard, or brittle, or rustresistant. On Earth, gravity causes flows
in the melted metal that affect crystal formation. In
microgravity, these flows are eliminated, and scientists can
observe and control the solidification process better. With the
clues they gather in space, scientists can improve the quality of
metals produced on Earth.
WHAT IS MICROGRAVITY?
The force of Earth's gravity extends far into space. You would
have to travel 6.37 million kilometers (almost 17 times farther
away from the Earth than the Moon) to reach a point where the
strength of Earth's gravity is one-millionth of what it is on
Earth's surface. Why, then, do astronauts and objects float in the
space shuttle as if they were weightless? Weight is the force with
which a body is attracted to the Earth. If an object is falling
due only to the force of gravity, its apparent weight (that which
could be measured while in freefall) is nearly zero. Any object in
a state of freefall experiences microgravity. or near
weightlessness. An orbiting spacecraft is actually falling around
the Earth. The spacecraft's altitude and speed cause its fall to
match the curvature of the Earth, so that it never hits the Earth
but continually orbits the planet. All objects carried by an
orbiting spacecraft are also in a state of freefall.
To learn more, try these internet addresses:
http://liftoff.msfc.nasa.gov/sts-75/usmp-3/usmp-3.html
http://microgravity.msad.hq.nasa.gov/