Artistďs conception of Pioneer Venus spacecraft.

Since 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.

For 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.

The 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 miles).

The 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.

The 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 failed.

NASA, 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 18, 2002.

Two of 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.

A Japanese spacecraft, Nozomi, is also on its way to Mars, and is expected arrive in December 2003.

The 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.

The 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.

The 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.

Two 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 Halley's 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.

NASA has 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 kilometres).

 
Magellan combined radar and altimetry image of three volcanoes.

Since 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.

Only one 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.