NASA's twin robot geologists, the Mars Exploration Rovers, launched toward Mars in 2003 in search of answers about the history of water on Mars.

The Mars Exploration Rover mission is part of NASA's Mars exploration program, a long-term effort of robotic exploration of the red planet. The program takes advantage of each launch opportunity to go to Mars, which comes around every 26 months as the planets move around the sun.

The first rover, Spirit, was launched on June 10th. The seconed rover, Opportunity, launched on July 7. The two rovers were delivered in landing craft to separate sites on Mars in January 2004.

Primary among the mission's scientific goals is to search for and characterize a wide range of rocks and soils that hold clues to past water activity on Mars. The spacecraft will be targeted to sites that appear to have been affected by liquid water in the past.

After the airbag-protected landing craft settle onto the surface and open, the rovers will roll out to take panoramic images. These images will give scientists the information they need to select promising geological targets that will tell part of the story of water in Mars' past. Then, the rovers will drive to those locations to perform on-site scientific investigations over the course of their 90-day mission.

The primary science instruments on the rovers are:

• A panoramic camera (Pancam), for determining the mineralogy, texture, and structure of the local terrain.
• A miniature thermal emission spectrometer (Mini-TES), for identifying promising rocks and soils for closer examination, and to determine the processes that formed Martian rocks. The instrument will also look skyward to provide temperature profiles of the Martian atmosphere.
• A Mössbauer Spectrometer (MB), for close-up investigations of the mineralogy of iron-bearing rocks and soils.
• An alpha particle X-Ray spectrometer (APXS), for close-up analysis of the abundances of elements that make up rocks and soils.
• Magnets, for collecting magnetic dust particles. The Mössbauer Spectrometer and the Alpha Particle X-ray Spectrometer will analyze the particles collected, and help determine the ratio of magnetic particles to non-magnetic particles and composition of magnetic minerals in airborne dust and rocks that have been ground by the Rock Abrasion Tool.
• A microscopic imager (MI), for obtaining close-up, high-resolution images of rocks and soils.
• A rock abrasion tool (RAT), for removing dusty and weathered rock surfaces and exposing fresh material for examination by instruments onboard.

The rover is designed to drive up to 40 meters (about 44 yards) in a single day, for a total of up to one kilometer total (about three-quarters of a mile).

Moving from place to place, the rovers will perform on-site geological investigations. Each rover is sort of the mechanical equivalent of a geologist walking the surface of Mars.

The mast-mounted cameras are mounted 1.5 meters (5 feet) high and will provide 360-degree, stereoscopic, humanlike views of the terrain.

The robotic arm will be capable of movement in much the same way as a human arm with an elbow and wrist, and will place instruments directly up against rock and soil targets of interest. In the mechanical "fist" of the arm is a microscopic camera that will serve the same purpose as a geologist's handheld magnifying lens.

The rock abrasion tool works like a geologist's rock hammer to expose the insides of rocks.

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