NASA's twin Mars rovers, Spirit and Opportunity, have been on the surface of Mars for more than eight years.
NASA's twin Mars rovers, Spirit and Opportunity, have been on the surface of Mars for more than eight years.
Credit: NASA/JPL

The promised warranty on the Spirit rover was 90 Martian days (or "sols"). It ended up lasting more than 2,200 sols and gave a window into Mars' early and wet history.

But there was one obstacle Spirit couldn't overcome: an unexpected sand trap. NASA spent weeks trying to help the robot on to safer ground, with little headway. Trapped, the rover eventually stopped transmitting information back to Earth.

The rover left behind a trove of scientific information about Mars history. It also paved the way for a more robust rover to follow: Curiosity, which is still exploring the Red Planet today.

Following the water

Spirit is one of a suite of spacecraft in NASA's second wave of Mars exploration. The agency sent several missions to Mars in the 1960s and 1970s, but paused after the Viking 1 and Viking 2 landers in the 1970s did not bring back definitive evidence of current or past life.

According to the agency, interest in Mars began to turn again in the 1980s and 1990s, as research on Earth uncovered microbes not only surviving, but thriving in extreme environments such as underwater volcanic vents. Further, the Viking pictures showed possible evidence that the area had water in the past.

NASA sent Mars Global Surveyor to the planet in 1996 to map out possible water sites, and also placed the Mars Pathfinder and Sojourner rover mission to the surface in 1997 with great scientific and public success.

This spurred enough interest for more rovers. NASA ultimately launched two rovers towards Mars: Spirit, and Opportunity. More formally known as the Mars Exploration Rovers, the machines received their names from 9-year-old Sofi Collis following a naming contest.

Spirit and Opportunity together cost $800 million and carried a large collection of scientific equipment. They had panoramic cameras to scope out their surroundings, and a small spectrometer that could seek out signs of heat on the surface. Each rover also had an arm, which carried tools such as a microscopic imager to get a close-up look at rocks on the surface.

Going for Gusev

It took two years of wrangling for scientists and engineers to agree on landing sites for Opportunity and Spirit. "The places most appealing to scientists (the side of a cliff, for example, on which the planet's history is recorded in layers of sedimentary rock) are often the most frightening to engineers charged with the robot's safety," NASA wrote of the process.

Opportunity targeted Meridiani Planum based on a layer of hematite that MGS spotted from above. Hematite is an iron oxide that often forms in water. (And as it turned out, Opportunity found lots of hematite on the surface.)

Photo of sunset on Mars taken by NASA's Spirit rover in 2005.
Photo of sunset on Mars taken by NASA's Spirit rover in 2005.
Credit: NASA/JPL/Texas A&M/Cornell

Spirit's destination was Gusev Crater, which stretches bigger than the state of Connecticut. From MGS pictures, scientists were pretty sure the crater held water in the ancient past. Gusev itself was dug out by an asteroid or comet impacting the planet as early as 4 billion years ago.

Spirit was the first of the rovers to leave Earth. It blasted off from Cape Canaveral on June 10, 2003, and was on its way to Mars within hours. Seven months later, on Jan. 3, 2004, Spirit made its final descent to Mars.

After snapping open a parachute, the rover careened to the surface in a cocoon of airbags, rolling safely to a stop on the surface. Right on target in Gusev, too. Opportunity also landed safely and at the right spot, on Jan. 25.

Reboots, water and wheels

Spirit was still sitting in its landing shell when it spotted the first possible signs of water in the distance: carbonate, which often forms in wet environments. "We came looking for carbonates. We have them. We're going to chase them," said Phil Christensen, one of the Spirit scientists, in a press release.

But within a week, Spirit was in trouble. It stopped sending data from the surface on Jan. 21, 2004. Within two days, NASA later determined the rover's computer was perpetually rebooting due to a software error; it restarted more than 60 times in three days.

The agency stabilized the rover in February. Then, in March, Spirit hit a jackpot: it found a volcanic rock that had hints of a watery past in its composition.

Three months later, NASA was surprised when Spirit stumbled across hematite, which is a mineral that can form in water. Opportunity also found hematite at its landing site halfway across Mars.

By late 2005, Spirit had driven up a nearby landmark, Husband Hill, to take a look at the landscape around it. It was the first time a rover climbed a hill on another planet.

The area was a testament to Mars' early violent history, NASA said. "We've got this dramatic topography covered with sand and loose boulders, then, every so often, a little window into the bedrock underneath," stated science instruments principal investigator Steve Squyres at the time.

One of Spirit's wheels quit in March 2006 as the rover was racing to a slope to get enough sunlight to last the winter. NASA dragged the wheel behind the rover, slowly moving Spirit an hour a day as the sun's strength allowed. It safely arrived at its destination in April.

The location proved to be a good spot to stop, as the rover found "water-altered minerals" nearby when it resumed operations in late 2006.

Latter years on Mars

Spirit journeyed 4.8 miles (7.7 kilometers) during its years on Mars, more than a dozen times the distance that NASA planned to travel. Spirit soldiered on despite a spite of mechanical and Martian difficulties.

Funny enough, the tricky wheel ended up being useful to the mission; in March 2007, NASA announced the rover churned up some soil that had sulfur and water traces in it.

As the year passed, it uncovered the site of a possible volcanic outburst, and survived an extensive dust storm. Another storm in late 2008 put Spirit's power down to concerning levels, but the rover pulled through.

Martian winds cleared some of the dust away in February 2009. In April, Spirit began to have rebooting trouble from its computer again, with periods of what NASA described as "amnesia." The rover began driving again as NASA worked to fix the problem, but then ran into a worse problem: sinking sand. The rover unexpectedly broke through a crust on April 23 into softer sand, and couldn't get out again.

NASA spent months running simulations and sending commands to the stranded rover, but also performed some science while standing in place. The agency was delighted to see sand with basalt, sulfate and silica in it, all revealed to the rover as it tried to get out of its trap. One press release called the location, Troy, "one of the most interesting places Spirit has been."

On Dec. 31, 2009, NASA warned there may not be enough power to last the winter. Spirit's last communication came on March 22, 2010, and it remained silent as NASA spent months hailing it.

"Engineers' assessments in recent months have shown a very low probability for recovering communications with Spirit," NASA wrote in a press release on May 24, 2011. Besides, the space assets being used to look for Spirit would soon be needed for Curiosity.

NASA concluded its efforts to reach Spirit that month. However, the rover's place in history is secured from its six years of discovery on the Red Planet.

— Elizabeth Howell, SPACE.com Contributor