In 1989, the Galileo space
orbiter was released from the STS 34 flight of the Atlantis orbiter. Then the
orbiter's inertial upper stage rocket pushed it into a course through the inner
solar system. The craft gained speed from gravity assists in encounters with
Venus and Earth before heading outward to Jupiter. During its six year journey
to Jupiter, Galileo's instruments made interplanetary studies, using its dust
detector, magnetometer, and various plasma and particles detectors. It also
made close-up studies of two asteroids, Gaspra and Ida in the asteroid belt.
The Galileo orbiter's primary mission was to study Jupiter, its satellites, and
its magnetosphere for two years. It released an atmospheric probe into
Jupiter's atmosphere on 7 Dec 1995.
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| Jupiter and its shrunken great red spot. By NASA, ESA, and A. Simon (Goddard Space Flight Center) [Public domain], via Wikimedia Commons |
Jupiter's mass is 2.5 times that
of all the other planets in the Solar System combined—this is so massive that
its barycenter with the Sun lies above the Sun's surface at 1.068 solar radii
from the Sun's center. Jupiter is much larger than Earth and considerably less
dense: its volume is that of about 1,321 Earths, but it is only 318 times as
massive. Jupiter's radius is about 1/10 the radius of the Sun, and its mass is
0.001 times the mass of the Sun, so the densities of the two bodies are
similar. A "Jupiter mass" (MJ or MJup) is often used as a unit to
describe masses of other objects, particularly extrasolar planets and brown
dwarfs. So, for example, the extrasolar planet HD 209458 b has a mass of 0.69
MJ, while Kappa Andromedae b has a mass of 12.8 MJ.
Theoretical models indicate that
if Jupiter had much more mass than it does at present, it would shrink. For
small changes in mass, the radius would not change appreciably, and above about
500 M⊕
(1.6 Jupiter masses) the interior would become so much more compressed under
the increased pressure that its volume would decrease despite the increasing
amount of matter. As a result, Jupiter is thought to have about as large a
diameter as a planet of its composition and evolutionary history can achieve.
The process of further shrinkage with increasing mass would continue until
appreciable stellar ignition is achieved as in high-mass brown dwarfs having
around 50 Jupiter masses.
Although Jupiter would need to be
about 75 times as massive to fuse hydrogen and become a star, the smallest red
dwarf is only about 30 percent larger in radius than Jupiter. Despite this,
Jupiter still radiates more heat than it receives from the Sun; the amount of
heat produced inside it is similar to the total solar radiation it receives. This
additional heat is generated by the Kelvin–Helmholtz mechanism through
contraction. This process causes Jupiter to shrink by about 2 cm each year. When it was first formed, Jupiter was much
hotter and was about twice its current diameter.
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