[meteorite-list] NASA's RATs Go Roving on Mars

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:27:51 2004
Message-ID: <200311191647.IAA26637_at_zagami.jpl.nasa.gov>

http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=680&mode=thread&order=0&thold=0

NASA's RATs Go Roving on Mars

Summary: Instruments on the Athena Mars Exploration Rovers, Spirit and
Opportunity, will measure the composition of Martian rocks, searching for
evidence of past water. But how will they "see" the real rock
beneath all the dust? The Rock Abrasion Tool (RAT) comes to the rescue,
cutting away the surface to reveal fresh rock.


NASA's RATs Go Roving on Mars
By Stephen Hart
Astrobiology Magazine
November 19, 2003

Several Athena instruments will measure or photograph the surfaces of Martian rocks
to determine different aspects of their compositions. All of these tools closely approach
the surface of a rock and measure only the surface; they cannot "see" inside the rock.

But Martian rocks lie under a blanket of fine dust, and beneath the dust the rocks may
include a weathered layer, like the layer of rust on an abandoned car. This weathered
surface can differ from the rock underneath just as rust differs from the steel under it.

To reveal the real Martian rock beneath the dross, the Athena rovers will carry an
instrument called the Rock Abrasion Tool, or RAT. The RAT scrapes away a few
millimeters of a rock surface. The freshly exposed circle of surface is then ready for
instruments such as the Microscopic Imager, which takes close-up photographs and
the three spectrometers, which measure the chemical composition of the rock surface.

"Cutting into the rock beyond the weathered surface rind will reveal a flat cross
section of fresh rock," says Stephen Gorevan, Co-Investigator for the Athena Science
Payload. Gorevan is chairman of Honeybee Robotics, in New York, which
designed and built the RAT.

The RAT works somewhat like a surface grinder, the sort a machinist might use to
clean or shape a metal surface. But, Gorevan says, "the RAT must cut strong rock
with much less force than a typical person would exert with a hand held grinder."
"The RAT needed to be specially designed to execute low-force cutting."

RAT Teeth

The RAT rasps rock with two spinning diamond-studded epoxy plastic cutting tips
mounted on a wheel, which is turned by a motor. (A second wheel holds a set of
spinning brushes to wipe away dust and grit.) As the wheel holding the cutting tips
rotates, the tips trace out a pair of spirals, eventually sweeping out a 45-millimeter
(1.8-inch) circle. A separate motor moves the tips toward the rock surface. Because of
its low-speed cutting action it takes two to four hours to cut the 45-by-5-millimeter
(1.8-by-0.2-inch) depression the RAT will not heat the rock enough to cause any
change in its chemical composition.

Robo RAT

Once the rover's robotic arm, the Instrument Deployment Device, presses the RAT
against a rock, the RAT takes over.

"The RAT is truly a robotic device," Gorevan says. "Encoders attached to each of
the three RAT motors provide us with precise position information. The encoders
are discs attached to the motor shaft; a particular shaft position gives a particular
encoder signature that is readable by the sensors accompanying the encoder." Data
from the encoders feeds directly to the Rover's central computer, which contains
software to interpret the data.

"We can count revolutions and know exactly where the motor shaft rotations have
taken us. The position feedback primarily tells us how deeply we have cut into the
rock. The torque feedback (provided by monitoring how much current the grinder
motor is drawing) is used by the grinding algorithm to cut through the rock
according to how strong the rock is. One of the great challenges we face is not
having any information at all as to how strong any Martian rock is. No previous
mission provided any rock penetration."

What's Next?

"Testing, testing and more testing," Gorevan says. "This is what consumes us now and
will be part of what we do even after we land; indeed even after the surface mission is
over."

The team plans to test the RAT in wind tunnels at the NASA Ames Research Center.
"Here we will simulate high Martian winds at Mars [atmospheric] pressure to see if
the ground-up material transported by the thin Martian atmosphere poses any hazard
to the Hazcams, which are located close to the RAT."

But the most important tests will challenge the RAT with rocks of varying types on
Earth.

Each rock type has different hardness and brittleness. And the IDD might press the
RAT against rocks in different ways in different situations, pressing harder in some
situations and less firmly in others. The RAT readouts will provide a numeric signature
of how the RAT behaved while cutting each type of rock. The team can then match known
rock types with these signatures.

"One can imagine searching our rock library [on Earth] for rocks that match the
feedback signature we saw on the mission," Gorevan says. This process takes the RAT
to a new level. It becomes more than a service robot for other MER instruments;
it can return its own data, data that the RAT team can use to analyze the material
of the rock.
Received on Wed 19 Nov 2003 11:47:56 AM PST