MER A and B differ significantly from the pioneering rover, Sojourner, which began exploring the Mars surface on July 4, 1997, during the Mars Pathfinder expedition. Sojourner carried a single scientific instrument and only roamed about 100 yards (91 meters) from the mother ship.
MER is a 400-pound (180-kilogram), 5-foot-square (1.5-meter-square) all-terrain vehicle with a platform rising about 3.5 feet (1.0 meters) above the ground. Each rover carries nine cameras, five scientific instruments, and a robotic arm with geological instruments.
Every Martian day (37 minutes longer than Earth's day), MER will travel about 44 yards (40 meters). The rover has the potential to cover about three-quarters of a mile (1.2 kilometers) during its estimated three-month life span on the Red Planet.
"It is going to drive around and do a lot autonomous things on Mars and at least twice a day it'll call home, tell us what it has done, what its seen, and we'll give it commands about what it's going to do next," says Randy Lindemann, lead engineer of the rover design team at JPL.
The rovers' tools can penetrate Martian rocks to get a closer look.
"Geologists on the Earth want to get inside the rocks because often times the outsides are coated or altered by interacting with the atmosphere and water," says Joy Crisp, a geologist and MER project scientist at JPL. "[If] you want to find out how the rock formed you crack them open with the rock hammer."
MER is equipped with a Rock Abrasion Tool, nicknamed RAT, that drills a two-inch (five-centimeter) diameter hole in a specific rock sample and allows the scientists to peer insidefrom more than 120 million miles (200 million kilometers) away.
A microscopic imager at the end of the robotic arm acts like a geologist's hand lens, revealing the texture and particle structure of minerals. Three types of spectrometers can also analyze the chemical composition of a specimen.
"That may tell us that a rock was erupted by a volcano or was laid down by water or some other process," Crisp says.
First the rovers must safely touch down on Martian soil. They bounce on the surface in an air bag cocoon, and then emerge from the landerone of the trickiest stages of the entire operation.
Large rocks or a steep slope could tip over the rover, derailing each U.S. $400 million mission.
That's why the scientists at JPL are rehearsing Martian maneuvers in the sandbox, which may come in handy again in January. Should the rover get stuck, the JPL scientists can troubleshoot a solution there.
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