Artistďs conception of
Pioneer Venus spacecraft.
the beginning of the space age in 1957, the United States and the Soviet
Union have sent dozens of probes to explore Mercury, Venus, and Mars, the
three inner planets. While many of these spacecraft failed to reach their
targets, many others have sent back valuable information.
decades, scientists and lay people have speculated on the possibility of
life on Mars. Therefore, it is not surprising that the so-called “Red
Planet” has been the object of most of our attention. Since 1960, humans
have tried to send probes to Mars 33 times. Of these, only ten have fully
succeeded in their objectives. Nine of these attempts have been American.
Soviet Union first launched two Mars probes in 1960, both of which failed
due to defective launch vehicles. Three modified probes, one of which was
actually intended to land on Mars, were dispatched in 1962. The first,
known officially as Mars 1, launched on November 1, 1962, became
the first spacecraft sent by any nation to fly past Mars. Ground
controllers eventually lost contact three months before the probe silently
flew by Mars in June 1963 at a distance of 197,000 kilometres (122,410
first true successes came when the National Aeronautics and Space
Administration (NASA) launched Mariner 3 and Mariner 4 to
fly past Mars in November 1964. While Mariner 3 failed, Mariner
4 was a spectacular success, the first great achievement of deep space
exploration. The spacecraft flew by Mars in July 1965 and sent back 22
photographs of the surface of Mars, showing the planet to be an ancient
moon-like body with an abundance of craters. NASA followed this success
with Mariner 6 and Mariner 7, both of which flew by Mars in
July and August 1969, returning more information about its surface.
Following these flyby missions, NASA's Mariner 9 became the first
spacecraft to go into
orbit around Mars in November 1971. During its yearlong mission,
Mariner 9 mapped 85 percent of the planet's surface in unprecedented
detail. It identified 20 volcanoes including Olympus Mons, a giant feature
that dwarfed anything similar on Earth.
Fresh crater in centre of older
crater basin on Mercury, taken by Mariner 10.
Mariner 9's success set the stage for perhaps the most ambitious
missions ever launched to Mars -the Viking 1 and 2 missions.
These two spacecraft, each comprising a lander and an orbiter, were
designed to spend months studying the surface of Mars from orbit and down
on the ground. The two landers successfully set down on Mars in July and
September 1976, respectively. Their first clear photographs of the Martian
surface showed a cratered terrain resembling that of the Moon. Both
landers also conducted experiments to detect signs of life, but the
results were inconclusive. The two orbiters returned more than 50,000
photographs of the surface over the course of several years, mapping 97
percent of the surface.
Soviets were much less fortunate in their Mars missions. Between 1969 and
1973, the Soviets tried nine times to send spacecraft to Mars. Although
several of them reached the planet, only Mars 5 in 1973-74,
successfully succeeded in orbiting and photographing the planet. More than
a decade later, in 1988-89, the Soviets fared no better when they launched
the Fobos twin probes to survey both Mars and its oddly shaped moon
Phobos. Both spacecraft failed to achieve their primary goals. A final
mission in 1996, the ambitious Mars 8 project, comprising an
orbiter, two surface penetrators, and two independent stations, also
meanwhile has had an uneven success rate in more recent years. Mars
Observer, a 2.5-ton spacecraft designed to map the surface of Mars,
failed in 1992 to enter orbit around Mars. Mars Global Surveyor,
launched in 1996, succeeded in entering orbit around Mars in September
1997. It has continued to return high-quality data and photographs of the
red planet. The spacecraft has tracked the evolution of dust storms and
most important, found convincing evidence for the presence of liquid water
on or near the surface. Mars Pathfinder, also launched in 1996, was
another stunning success. The spacecraft successfully landed on Mars on
July 4, 1997, and released a 10.5-kilogram (23-pound) rover named
Sojourner that trekked around the landing area collecting information and
taking spectacular photographs.
Volcanoes Ceraunius Tholus and
Uranius Tholus, taken by Mars Global Surveyor Mars orbiter camera, April
NASA's more recent missions, Mars Climate Orbiter (launched in
1998) and the Mars Polar Lander (launched in 1999), failed to
achieve either of their objectives. Fortunately, a third spacecraft, the
2001 Mars Odyssey probe, successfully entered orbit around Mars in
October 2001 to begin three years of mineralogical analysis from orbit.
Japanese spacecraft, Nozomi, is also on its way to Mars, and is
expected arrive in December 2003.
Soviets have fared much better in their missions to Venus. Of the 35
probes launched, 22 have succeeded in their objectives. Of these, 17 have
been Soviet. They were the first to attempt to send probes to Venus when
they launched two spacecraft to Venus in 1961, one of which actually flew
past the mysterious planet, although by then its radio system had failed.
first successful mission to Venus-in fact, the first successful planetary
mission-was Mariner 2. The American spacecraft flew past Venus in
December 1962 at a distance of 21,600 miles (34,762 kilometres) and
returned data about its atmosphere.
Soviets scored a big success when in March 1966, when Venera 3
reached the surface of Venus, becoming the first space probe to ever
impact another planetary body. Through the late 1960s, the Soviets
continued to send probes to Venus to obtain data from the inhospitable
surface, but none succeeded until Venera 7 returned the first
information from the surface in December 1970. For 23 minutes, the lander
returned data about conditions on the ground before succumbing to the
extreme heat and pressure. It was the first time that any probe had
returned information from the surface of another planet.
Soviet probes launched in 1975, Venera 9 and Venera 10,
returned the first photographs from the surface of Venus. These images
showed flat rocks spread around the landing area. Two new probes,
Venera 11 and Venera 12, landed on the planet in 1978 (although
they were unable to return photographs). In 1982, another pair, Venera
13 and Venera 14, returned the first color photographs of the
surface. Probes such as Venera 15 and Venera 16 also mapped
the Venusian surface from orbit using high-powered radars. Perhaps the
most ambitious Soviet mission to Venus was the successful Vega 1
and Vega 2, launched in 1984. The two spaceships were each made up
of landers, atmospheric balloons, and flyby probes for encounters with
comet. The French-made balloons, each weighing about 46 pounds (21
kilograms), transmitted important data about the atmosphere as they
drifted slowly through the Venusian skies.
also mounted several ambitious missions to Venus. These have included the
Pioneer Venus missions, comprising an orbiter with a powerful radar, and a
spacecraft with three small atmospheric entry probes. The two spacecraft
were launched in 1978. The smaller probes scattered through the atmosphere
and collected data as they flew down to the surface. Data indicated that
the Venusian atmosphere is relatively clear below about 19 miles (30
Magellan combined radar and
altimetry image of three volcanoes.
then, NASA has conducted only one mission to Venus, the Magellan
orbiter flight. Launched in 1989, Magellan successfully entered
orbit around Venus in August 1990. By the time its mission ended in 1994,
Magellan had successfully mapped 98 percent of the surface of the
planet using sophisticated radars that could peer through the thick
atmosphere. The spacecraft discovered that at least 85 percent of the
surface of the planet is covered with volcanic flows.
spacecraft has been sent to the small planet closest to the Sun, Mercury.
In late 1973, NASA launched Mariner 10, which on its way to
Mercury, passed by Venus and used that planet as a “gravity assist” to
send it toward Mercury. Through 1974, Mariner 10 flew by Mercury
three times, the closest at a range of only 203 miles (327 kilometres).
Each time, the spacecraft returned the first photographs of the surface of
the planet. The photos showed a terrain similar to the Moon. Maximum
daytime temperatures were on the order of 369° F (187° C) although
temperatures in the shade went as low as -297° F (–183° C). Contact with
Mariner 10 was lost in March 1975.