In 2003, NASA launched twin Mars Exploration Rovers (MER) robot geologists called Spirit and Opportunity to explore selected spots on the planet Mars. They landed on January 3rd and 24th, 2004, on opposite sides of Mars and immediately set to work studying their surroundings. Spirit landed at Gusev and Opportunity settled down at Meridiani Planum. Gusev was once a lake that formed in an impact crater, while the Meridiani site showed evidence of once having liquid water.
The goals of the MER mission were to search out rocks and soil that may have been in contact with water, and study their chemical makeup. Each rover was equipped with a panoramic camera (Pancam), a miniature thermal emission spectrometer (to identify rocks and soils chemically), a Mössbauer spectrometer (to study the mineral content of rocks on Mars, that is, to do spectroscopy on them), an alpha particle x-ray spectrometer to do close-up analysis of elements in Mars rocks and soil, magnets to collect magnetic dust particles for the spectrometers to study, a microscopic imager to provide up-close images of rocks and soils, and a rock abrasion tool (nicknamed the RAT) to clear off rocky surfaces so other instruments could study them.
The rovers were designed to travel across a variety of rocky and sandy terrains on Mars at a top speed of two inches per second. In actual practice, they have moved much more slowly. Both were outfitted with solar arrays to provide power for the onboard batteries. Over time, those solar arrays became covered with dust. The Spirit rover, which was the first to image small dust storms called “dust devils”, also benefited from these little whirlwinds because they cleaned the dust off its solar panels as they passed.
Each rover outperformed their original 90-day mission plans. The Spirit Rover traversed across nearly five miles of Martian terrain before shutting down for good in 2010. It became stuck, and was unable to free itself. The mission planners redefined its mission from roving to become a stationary research platform, to study climate and other events at the rover’s site. In March, 2010, the rover likely entered a low-power hibernation state, and never woke up. Mission controllers suspect that its batteries were too low to keep the mission clock running.
Spirit is still perched at a spot called “Troy”. Its landing site was called Columbia Memorial Station, after the astronauts who died in the Columbia shuttle disaster a set of nearby hills was named for them. During its mission, one of Spirit’s most memorable images was made when it reached the edge of Bonneville Crater. The rover drove along the southern rim of the crater before continuing on toward the Columbia Hills, three kilometers away. After several years of exploration, Spirit reached its final resting place. Mission controllers have not heard from it since its final transmission on March 22, 2010. It had just completed six years and three months of exploration.
As of 2014, the Mars Exploration Rover Opportunity mission continues. Like its sister rover, Opportunity was scheduled for 90 days, but last lasted nearly a decade. It has traveled nearly 25 miles across the Martian surface in that time. Opportunity has visited Endurance Crater, Erebus Crater, and Victoria Crater, where it spent nearly a year exploring the rock ledges and sandy pit of the crater. Along the way Opportunity has studied many different types of soils and rocks that have come into contact with water. data it has provided has allowed planetary scientists to understand the water history of the Red Planet in greater detail. The rover continues to study the Martian surface at Meridiani Planum, and as of April 2014, it is exploring Murray Ridge along the west rim of Endeavour Crater.
Each of the two Mars Exploration Rovers have sent back many panoramic and scientific images of the Mars surface, as well as close-up shots of rocks, including a meteorite. The images and data sets they provided will be of tremendous interest to future Mars explorers when they land to study the Red Planet in person.