Space exploration
Definition Use of satellites and other spacecraft to
gather scientific information about space and
other planets
During the 1990’s, NASA sent large spacecraft to Venus,
the Sun, and Jupiter and launched three large astronomi-
cal observatories, which reshaped scientists’ ideas about the
solar system and the universe. Human exploration contin-
ued with shuttle flights to the Mir space station, and NASA
embarked on programs to explore space using smaller,
cheaper spacecraft.
In collaboration with Russia, eleven space shuttle
missions flew to the Mir space station between 1995
and 1998. The shuttles delivered new solar-power ar-
rays and performed crew exchanges allowing Ameri-
can astronauts to carry out scientific experiments
on Mir.
The National Aeronautics and Space Administra-
tion’s (NASA) Magellan spacecraft, launched by the
space shuttleAtlantison May 4, 1989, entered orbit
around Venus on August 10, 1990. Because of the
thick clouds, Magellan mapped the surface of Venus
using radar, identifying fields of small lava domes
and larger shield volcanoes. Magellan found few im-
pact craters, indicating that lava flowed over the sur-
face over the past billion years.
Ulysses, a joint effort between NASA and the Eu-
ropean Space Agency (ESA), was launched by the
space shuttleDiscover yon October 6, 1990. It flew
near Jupiter in February, 1992, swinging Ulysses out
of the plane of planetary orbits. Ulysses passed over
the Sun’s south pole in mid-1994 and the north pole
in mid-1995. Ulysses discovered that charged parti-
cles emitted from the Sun’s poles move twice as fast
as those near its equator.
Galileo, launched on October 18, 1989, by
Atlantis, became the first spacecraft to fly near an as-
teroid. On October 29, 1991, the spacecraft passed
within 1,600 kilometers (994 miles) of Gaspra and
took pictures that showed a cratered, irregular body.
On August 28, 1993, Galileo passed within 2,400 ki-
lometers (1,491 miles) of Ida, discovering that it had
a moon, which was named Dactyl. While en route,
Galileo observed the collision of Comet Shoemaker-
Levy 9 with Jupiter in July, 1994. Galileo arrived at Ju-
piter on December 7, 1995. A descent probe mea-
sured the composition of the atmosphere, while the
2.5-ton orbiter observed Jupiter and its moons. Gali-
leo found evidence for a saltwater ocean on Europa
and observed intense volcanic activity on Io.
Space Observatories The Hubble Space Tele-
scope was launched byDiscover yon April 24, 1990.
Because Hubble orbits above Earth’s atmosphere,
its images are not compromised by atmospheric dis-
tortions or light scattered by the atmosphere, and
Hubble can observe regions of the spectrum ab-
sorbed by the atmosphere. During commissioning,
the main mirror was found to have “spherical aber-
ration,” causing light from a point to spread out,
compromising the telescope’s resolution. The prob-
lem was solved during a servicing mission in 1993.
Since then, Hubble has revolutionized ideas about
the universe, contributing to the discovery of dark
energy, a force that causes the universe to expand at
an ever-increasing rate, and the discovery and char-
acterization of planets outside the solar system.
The Compton Gamma Ray Observatory, launched
onAtlantison April 5, 1991, produced the first all-sky
map of high-energy gamma-ray sources and discov-
ered a possible cloud of antimatter above the center
of the galaxy. The Chandra X-Ray Observatory,
launched on the space shuttleColumbiaon July 23,
1999, detected an object, possibly a neutron star or
black hole, at the center of the Cassiopeia A super-
nova remnant and imaged a previously unknown
ring around the central pulsar of the Crab nebula.
“Smaller, Faster, Cheaper” Missions On January
25, 1994, the Clementine spacecraft, a joint mission
by NASA and the Ballistic Missile Defense Organiza-
tion, was launched to test defense technology. Cle-
mentine, the first of a series of small spacecraft de-
signed for low-cost space missions, orbited the
Moon, taking 620,000 high-resolution images and
320,000 infrared thermal images over a two-month
period. Clementine’s results suggested that there
was ice at the bottom of a crater near the Moon’s
south pole.
NASA initiated its Discovery program for small,
inexpensive spacecraft using innovative technology
to fly highly focused missions designed by a small
team of investigators. The first Discovery project, the
Near Earth Asteroid Rendezvous (NEAR) space-
craft, launched on February 17, 1996, flew by the as-
teroid Mathilde on June 27, 1997. Images showed
Mathilde to be 53 kilometers (33 miles) in diameter,
smaller than astronomers had estimated. It was
found to be twice as dark as charcoal, suggesting thatThe Nineties in America Space exploration 793