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How Spirit keeps Mars within reach

From The Mars Spirit rover has a complex, robotic arm that  is helping NASA study the Red Planet.
A picture taken by one of the Spirit rover's navigation cameras shows the robotic arm stretched out in front of the robot, bristling with scientific instruments and a rock-grinding tool.
A picture taken by one of the Spirit rover's navigation cameras shows the robotic arm stretched out in front of the robot, bristling with scientific instruments and a rock-grinding tool.NASA/JPL/USGS
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Just as geologists on Earth carry their science tools with them in the field, so too does the Mars rover Spirit, but the probe doesn't stop at just a hand lens and hammer. At the end of its robotic arm sit a cluster of instruments that combine traditional field tools with the equipment typically found in geological laboratories.

Spirit's robotic arm, known formally as the Instrument Deployment Device (IDD), is a mechanized mass of scientific instruments that reaches just over three feet (almost one meter) out in front of the rover. The whole assembly is about the size of an adult human arm and works like one too, complete with shoulder, elbow and wrist joints that allow the five degrees of freedom that we use everyday.

"This little robot arm is incredibly complex," said Eric Baumgartner, lead engineer for the IDD at NASA's Jet Propulsion Laboratory, which developed the arm and the rover it's attached to. "I'm not sure that people have a full appreciation of it."

The IDD is one of the most dexterous robot arm NASA has ever sent off-planet and is as complicated as any industrial robot, but at only a fraction of the weight, Baumgartner told The arm weighs about 13 pounds (six kilograms), one-third of which stems from the science instruments at the end. When not in use the entire contraption folds into a special slot under the front of the rover.

"[The arm] has been likened to a bowling ball at the end of a fishing rod," said Scott Stanley, director of advanced programs for Alliance Spacesystems, Inc., which built the robot arm for JPL. But the arm structure is strong, made of hollow titanium, that keeps the scientific package steady and accurate as it shifts from one instrument to another, he added.

So far, the arm has worked flawlessly. Spirit mission controllers pressed it into service last week, using the microscopic imager (MI), as well as its Mössbauer and Alpha-Particle X-ray spectrometers to study a bare patch of Martian soil and Tuesday on the rock dubbed Adirondack.

The microscopic imager and spectrometers are three of four instruments that make up part of the rover's Athena science package developed by researchers at Cornell University in Ithaca, New York. The fourth instrument on Spirit's arm is a rock abrasion tool or RAT to drill into rock surfaces. The rover's panoramic camera and miniature thermal emission spectrometer are also part of the Athena package but located elsewhere on the rover.

A combination microscope and high-resolution digital camera, the MI is designed to give Earth geologists a close-up glimpse of Martian rocks. The imager was based on the standard geologist hand lens, an optical tool most geologists carry with them when they go out into the field.

"It has essentially the same view that a geologist would have looking through a hands lens," explained the Ken Herkenhoff, part of the astrogeology team of U.S. Geological Survey and payload element leader for the MI. But the MI is producing the highest-resolution images ever taken of the Martian surface, he added.

The microscopic imager has a viewing area of just three centimeters in diameter and a resolution 20 times higher than Spirit's panoramic camera - which itself is the most powerful color camera system ever sent to another planet. The sensitivity - the imager can show features the width of a human hair - is crucial because it allows scientists to study the size and shape of grains in sedimentary rocks, which may help determine whether water ever flowed on Mars.

In addition to studying rocks, the imager can be used to study sand blown about my Martian wind and determine if Mars' great dunes are formed by sand grains or dust aggregates.

"We would love to find fossils, but frankly my hopes of finding them are pretty slim," Herkenhoff said. "Fossils are hard enough to find here on Earth."

On the robot arm with the microscopic imager is the rock abrasion tool (RAT), a grinder that shears away the surface layers of a rock sample to buff away accumulated dust or weathering effects. The tool performs the same function as a rock hammer on Earth, allowing geologists to compare the inner and outer structures of a rock.

With three electric motors running on 11 watts of power, RAT can drill a two-inch wide (about 50 millimeters) hole about two-tenths of an inch (5 millimeters) into Martian rock. Its two grinding wheels spin fast, at about 3,000 revolutions per minute, in a slow precession about one another. Together they can cut through hard volcanic rock in about two hours.

"It's a very slow dig," explained Mame McCutchin, spokeswoman for Honeybee Robotics Ltd. in New York City. The company built the RAT for JPL. "It's really more of a polisher."

Rat's grinding teeth are made of up 7,600 tiny pieces of diamond grit, held together by resin and layered on top of each other so that as one layer wears away, another is ready to take its place.

"We tried using regular diamonds [for teeth]," McCutchin said. "But they are not forever, they wear down." The diamond grit-resin combination has a longer lifespan, she added, and RAT has its own brush to wipes away any residue from a rocky meal.

In addition to putting soil and RAT-chewed rock samples under the microscope, Spirit will also examine the chemical and mineral make-up of the Martian landscape as well with two spectrometer sensors attached to its arm.

The Alpha-Particle X-ray Spectrometer (APXS) is an instrument that bombards a target with alpha particles, pairs of protons and neutrons, and X-rays, then gathers up the radiation scattered back from the target. The returned emissions give researchers an idea of the elemental chemical composition of any given rock.

Like the AXPS, the Mössbauer Spectrometer uses radiation to study the Martian environment over a long period of time, in this case 12 hours. The instrument uses gamma radiation to probe for iron-bearing minerals within Mars rocks or soil and researchers plan to make observations at different times of day to determine how the Mössbauer spectrum changes with temperature.

"These instruments are the same kind of thing you would see in a geology lab [on Earth]," Herkenhoff said of the spectrometers. Herkenhoff admits that human geologists can carry out the same processes that Spirit's arm instruments can much faster, but the robotic appendage makes up for that by keeping its spectrometers on hand in the field.

The key to Spirit's mission is its ability to rove wild around Mars.

"It gives us an incredible amount of flexibility not to be just a lander stuck in its tracks," said Baumgartner. "That we can take this whole scientific package with us around Mars is such a tremendous joy for me."