The Complete Book of Spaceflight: From Apollo 1 to Zero Gravity

170 gravitational constant (G)
Advanced Animal Habitat will house up to six rats or a
dozen mice and be compatible with a compartment
called the Mouse Development Insert that will accommodate
pregnant mice and subsequently their offspring
from birth through weaning. The Plant Research Unit
will support plant specimens up to 38 cm in height (root
+shoot) through all stages of growth and development.
The Insect Habitat will house Drosophila melanagoster and
other insects for multigenerational studies and for radiation
biology, and the Egg Incubator will support the
incubation and development of small reptilian and avian
eggs prior to hatching. All of the Gravitational Biology
Facility habitats will have the experimental capability of
selectable gravity levels. The Aquatic Habitat, the Cell
Culture Unit, the Advanced Animal Habitat, and the
Plant Research Unit will be used on the Centrifuge Facility’s
2.5-m-diameter centrifuge when selectable gravity
levels of up to 2g are needed. The Insect Habitat and Egg
Incubator will be equipped with internal centrifuges,
which will provide selectable gravity levels from zero to
1.5g.
gravitational constant (G)
The constant that appears in Newton’s law of gravitation.
It is the attraction between two bodies of unit mass
separated by a unit distance, and has the value 6.672 ×
10 −11 N.m 2 /kg 2 .
gravitational energy
The energy released by an object falling in a gravitational
field; a form of potential energy.
gravitational field
The region of space around a body in which that body’s
gravitational force can be felt.
gravity
The universal force by which every piece of matter in
space attracts every other piece of matter. Its effects
become obvious only when large masses are involved.
So, for instance, although we feel the strong downward
pull of Earth’s gravity, we feel no (gravitational!) pull at
all toward smaller masses such as coffee tables, vending
machines, or other people. Yet a mutual attraction does
exist between all things that have mass—you pull on
Earth, just as Earth pulls on you. Until the beginning of
the twentieth century, the only universal law of gravitation
was that of Isaac Newton, in which gravity was
regarded as an invisible force that could act across empty
space. The force of gravity between two objects was proportional
to the mass of each and inversely proportional
to the square of their separation distance. Then, in 1913,
Einstein published a revolutionary new theory of gravita-
tion known as the general theory of relativity, in which
gravity emerges as a consequence of the geometry of
space-time. In the “rubber sheet” analogy of space-time,
masses such as stars and planets can be thought of as
lying at the bottom of depressions of their own making.
These gravitational wells are the space-time craters into
which any objects coming too close may fall (for example,
matter plunging into a black hole) or out of which
an object must climb if it is to escape (for example, a
spacecraft leaving Earth for interplanetary space). 209
gravity gradient
The variation in the force of attraction between two bodies
expressed as a function of the separation distance.
gravity gradient stabilization
A useful, passive method to achieve stabilization of
satellites by using the gravity gradient of the primary
body. An orbiting spacecraft will tend to align its long
axis (more precisely, the axis of minimum moment of
inertia) with the local vertical—that is, in a radial direction.
Gravity Probe A
A NASA geodetic satellite designed to test the general
theory of relativity. Gravity Probe A carried a spacequalified
hydrogen maser clock to an altitude of 10,000
km on a two-hour suborbital flight and verified the gravitational
red shift predicted by Einstein’s theory to a precision
of 70 parts per million.
Launch
Date: June 18, 1976
Vehicle: Scout D
Site: Wallops Island
Gravity Probe B
A satellite being developed by NASA and Stanford University
to test two extraordinary, unverified predictions
of Einstein’s general theory of relativity. As the probe
orbits at an altitude of 640 km directly over Earth’s
poles, tiny changes in the direction of spin of four
onboard gyroscopes will be measured very precisely. So
free are the gyroscopes from disturbance that they will
provide an almost perfect inertial reference system. They
will measure how space and time are warped by the presence
of Earth, and, more profoundly, how Earth’s rotation
drags space-time around with it. These effects,
though small for Earth, have far-reaching implications
for the nature of matter and the structure of the universe.
Gravity Probe B is scheduled for launch in April
2003.