piter. When its next orbit caused at least twenty-one
fragments to collide with Jupiter in July of 1994, the
HST provided the sharpest images. The HST was
also used to study objects in the outer solar system,
including early evidence for volcanoes on Io and
some of the first Kuiper Belt objects beyond Pluto
and similar to it.
On January 25, 1994, NASA launched the Cle-
mentine spacecraft to orbit the Moon. During 348
lunar orbits, the surface was mapped and analyzed
for three months. Clementine maps revealed the
South Pole-Aitken Basin, the largest known crater in
the solar system, and provided evidence for ice de-
posits that were later confirmed by the Lunar Pros-
pector. In 1997, the American Astronomical Society
(AAS) announced their grazing-collision theory for
the origin of the Moon.
During the last two months of 1996, NASA
launched the Mars Global Surveyor, which began or-
biting on September 12, 1997, and the Mars Path-
finder to demonstrate low-cost air-bag landing tech-
niques for its Sojourner rover, which landed on
July 4, 1997. The combined missions analyzed the at-
mosphere, magnetism, and geology of Mars. On Au-
gust 6, 1996, NASA announced the results of their
analysis of an Antarctic meteorite, identified as com-
ing from Mars, and claimed that it contained fossil
microorganisms.Beyond the Solar System The COBE satellite pro-
vided the most important information about the
universe at large in 1992 when it detected ripples in
the cosmic background radiation that were identi-
fied with the formation of galaxies. By 1995, the HST
began to obtain photos of infant galaxies in the early
universe. In 1996, HST photos showed evidence for
a supermassive black hole at the center of a galaxy,
and by the end of the decade the HST had detected
ten such galactic black holes.
The Compton Gamma Ray Observatory surveyed
gamma-ray bursts (GRBs) throughout the decade
and finally detected the first one with an optical af-
terglow in 1999. Analysis of HST measurements then
showed that its source is beyond our galaxy, confirm-
ing that GRBs were the most energetic events since
the big bang, probably caused by the collapse of mas-
sive stars into black holes.
During 1995, the first three brown dwarfs (sub-
stellar objects) were discovered, most notably at the
Palomar Observatory, where a brown dwarf was ob-
served in orbit around a red-dwarf
star (Gliese 229). During the last
half of the decade, about twenty gi-
ant extrasolar planets were discov-
ered from the wobble of their host
stars, most by Geoffrey Marcy and
associates in California. In 1998,
the HST and the Keck II telescope
obtained images of gaseous disks
around two stars, showing evidence
of possible planet formation.
Perhaps the most important dis-
covery of the decade, announced at
the 1998 AAS meeting, was new evi-
dence from supernova studies that
the expansion of the universe was
speeding up. This evidence of an ac-
celerating universe suggests a new
form of “dark energy” dominating
the mass-energy content of the uni-
verse.Impact New information about as-
teroids and the collision of comet
SL-9 with Jupiter gave a better un-62 Astronomy The Nineties in America
The spacecraft Galileo obtained the first close photograph of an asteroid, Gaspra, on
October 29, 1991.(NASA)