The planet Mars has been on the space exploration agenda ever since the space race of the 1960s, when the United States and Russia were vying for a number of cosmic firsts — first artificial satellite, first man in orbit, first man on the moon, the list goes on.

The race to Mars started in 1960 when the Soviet Union launched the first of a series of planetary probes. One after another, the probes were failures, but the drive to succeed survived.

Recently there have been both successes and failures.

The long-running Mars rovers Spirit and Opportunity, for example, are still roaming the planet more than two years after their mission was scheduled to finish. But NASA's Mars Polar Lander simply vanished just before entering Mars's atmosphere in 1999 and in November 2006 the Global Surveyor spacecraft orbiting the planet went silent and is assumed lost.

But NASA hasn't given up on their ambitions for the Red Planet. The Reconnaissance Orbiter, launched in 2005, is now starting to send back the most detailed images from Mars ever seen. NASA will launch the Phoenix polar lander in August 2007 and a more advanced rover called the Mars Science Laboratory in 2009.

The planet could become crowded, too, if other nations follow through on their ambitious plans. The European Space Agency plans to send its own Mars rover in 2011 while Russia hopes to land probes on Mar's moon Phobos in 2009 and the planet itself in 2012. China and Japan have also set long-range goals that include Mars exploration.

The study of the planet is the first step in preparation for the next giant leap for mankind — a manned mission to Mars.

Here are some significant achievements in the exploration of Mars:

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Reconnaissance Orbiter

NASA launch (Courtesy NASA TV)

Launched: Aug. 12, 2005
Country: United States
Mission: The Mars Reconnaissance Orbiter (MRO) carries six instruments, including the most powerful telescope ever flown to another planet. It has begun to study the Martian atmosphere and surface, searching for evidence of water, past and present, just below the surface. The MRO is expected to beam more data back to Earth than all previous Mars missions combined. The MRO entered Mars orbit on March 10, 2006. If the mission continues to be successful, its observations will be complete in December 2008. After that the orbiter will serve as a communications relay between future spacecraft and scientists back on Earth.

Spirit and Opportunity

Spirit was launched on June 10, 2003

Launched: June 10 and July 7, 2003
Country: United States
Mission: Two identical exploration rovers designed to cover about 100 metres each Martian day. Spirit landed on Mars on Jan. 3, 2004, in the Gusev Crater, a wide basin that may once have held a lake. Opportunity arrived on Jan. 24, 2004, and explored in the Meridiani Planum. Both have found evidence of liquid water on Mars. The mission was expected to last until late April 2004, but the rovers have proven surprisingly long-lived. In 2005, NASA extended the rovers' mission on Mars for another 18 months and three years after their arrival the two rovers were still operating.

Mars Express

Launched: June 2, 2003
Country: European Space Agency
Mission: Consists of two parts: the Mars Express Orbiter and Beagle 2, a lander. Beagle 2 was released from the orbiter on Dec. 19, 2003, and was expected to land Dec. 25. But the ESA lost contact with the 68-kilogram lander. Beagle 2 was equipped with a sampling arm and a burrowing "mole" that were intended to take samples from below the surface. The orbiter continues to search for signals from Beagle 2 and will map the surface and atmosphere of Mars.

Odyssey

Odyssey (Courtesy of NASA)

Launched: April 7, 2001
Country: United States
Mission: To orbit Mars for three years, gathering information that will help determine whether the environment on Mars was ever conducive to life, and studying potential radiation hazards for possible future manned missions to Mars. Odyssey will also study the climate and geology of the planet. In addition, it will be used as a communications relay until October 2005.

Mars Climate Orbiter and Mars Polar Lander

Launched: Dec. 11, 1998, and Jan. 3, 1999
Country: United States
Mission: A pair of missions under the Mars Surveyor program, both of which were lost. The orbiter was to have observed Martian weather patterns. It burned up in the atmosphere after a navigation error. The lander was to have landed near the edge of the Martian polar ice cap and sent a probe deep into the soil. NASA lost contact with the spacecraft shortly after it started its descent to the surface.

Pathfinder

Sojourner (Courtesy of NASA)

Launched: Dec. 4, 1996
Country: United States
Mission: The mission involved two components, a stationary lander that used inflated airbags to soften its descent, and the Sojourner land rover, a six-wheeled, remote-controlled vehicle that moved along the Martian terrain. The mission lasted three times longer than expected, ending on March 10, 1998. More than 17,000 images of the surface were sent back. The mission also included analyzing rock and soil samples and measuring atmospheric pressure, temperature and wind. Data from Pathfinder suggested that the planet may have been "awash in water" three billion to 4.5 billion years ago, according to NASA.

Global Surveyor

Launched: Nov. 7, 1996
Country: United States
Mission: To capture high-resolution images of the surface and study the topography and atmosphere of the planet. The surveyor arrived on Sept. 11, 1997, and began taking images of the planet. In 1999, scientists used Surveyor's data to assemble the most detailed map of the Martian surface produced to date, revealing the planet's volcanoes, mountains, valleys and impact basins

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The map revealed dozens of intriguing features on the planet's surface, some of them unique in the solar system. A southern hemisphere site known as the Hellas impact basin, thought to be the result of an asteroid impact, is shown to be 10 kilometres deep and 2,000 kilometres across. It's surrounded by rings of debris from the impact, stretching some 4,000 kilometres from the centre of the basin. "To our knowledge, it's the deepest crater anywhere in the solar system," says David Smith of NASA's Goddard Space Flight Center in Greenbelt, Md.

The map also highlighted the contrast between the planet's hemispheres. The north is very low and smooth, while the south is rough, mountainous and heavily cratered. The northern hemisphere, on average, is five kilometres lower than the southern hemisphere, which would have affected the flow of water thought to have been present on the Martian surface billions of years ago. Any water that was present, Smith says, would have tended to flow northward, perhaps creating a vast ocean in the planet's northern hemisphere. Smith cautions that there is no direct evidence for this but says, "We can't rule out the possibility that there was a large amount of water there."

On Nov. 2, 2006, NASA lost contact with the Surveyor, nearly 10 years after it launched.

The probe's contributions to science didn't end there, however. A month after it was lost scientists looking back at old data got one last discovery: images that showed recent changes in surface features that might have been made from the flow of liquid water.

Observer

Launched: Sept. 25, 1992
Country: United States Mission: A Mars orbiter that was to have studied the planet's geology and climate. NASA lost contact with the spacecraft three days before it was to have entered orbit. The mission cost nearly $1 billion US.

Phobos 1 and 2

Launched: July 7 and July 12, 1988
Country: Soviet Union
Mission: A pair of spacecraft that were to have sent landers to the Martian moon Phobos. Both were lost before they completed their missions.

Viking 1

Launched: Aug. 20, 1975
Country: United States
Mission: To obtain high-resolution images of the surface, characterize the structure and composition of the atmosphere and surface, and search for evidence of life. The Viking 1 spacecraft became the first probe to land on Mars.

Mariner 9

Launched: May 30, 1971
Country: United States
Mission: Mariner 9 became the first spacecraft to orbit another planet, beating the Soviet Union's Mars 2 and 3 when it arrived at Mars on Nov. 14, 1971. The mission resulted in a global mapping of the surface of the planet. Images included the first detailed views of the Martian volcanoes, Valles Marineris, the polar caps and the planet's moons. Though the spacecraft was turned off just under a year after it entered orbit, Mariner 9 will continue circling the planet until its orbit decays sometime after 2021.

Mars 2 and 3

Launched: May 19 and 28, 1971
Country: Soviet Union
Mission: The missions consisted of two identical spacecraft, each with an orbiter module and a lander module. The two landers failed to live up to expectations — the Mars 2 lander crashed and the Mars 3 lander stopped working 20 seconds after a soft landing. However, the two orbiters were a huge success. During an eight-month period, they completed about 380 orbits and sent back 60 pictures. Information collected during the missions was used to create surface relief maps, and scientists learned more about the landscape, surface temperatures and atmosphere of Mars.

Mariner 6 and 7

Launched: Feb. 25 and March 27, 1969
Country: United States
Mission: The two spacecraft studied the surface and atmosphere of Mars during close flybys. They sent back about 200 pictures that showed the surface of Mars to be very different from that of the moon. Scientists also learned that the southern polar cap was composed predominantly of carbon dioxide.

Mariner 4

Launched: Nov. 28, 1964
Country: United States
Mission: The first successful flyby of Mars. It also sent back the first pictures of the Martian surface, along with other data.

Mars 1

Launched: Nov. 1, 1962
Country: Soviet Union
Mission: To send an automatic interplanetary station to fly by Mars and collect images and data. Though the mission wasn't a complete success — the spacecraft's orientation and communication systems eventually failed and it never reached the target of about 11,000 kilometres from Mars — Mars 1 collected a large amount of interplanetary data and reached a distance of about 193,000 kilometres from Mars, the closest to date.

Marsnik and Sputnik probes

Launched: various dates, 1960-1962
Country: Soviet Union
Mission: These were the first attempts to send probes to Mars. However, the missions were failures — the probes either failed during launch or broke apart leaving Earth's orbit.