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Planetary News: Mars Exploration Rovers (2004)

Mars Orbiter Camera's Eagle Eye in the Sky Spots Spirit Rover Tracks at Gusev Crater

By Emily Lakdawalla
27 September 2004

The spacecraft Mars Global Surveyor has been in orbit at Mars for seven years and is still pulling off amazing feats of aerial imaging. The latest unbelieveable image is of the tracks left behind by the Mars Exploration Rover Spirit as she trundled across the surface of Gusev Crater to Bonneville Crater.

Spirit's landing site as seen by MOC on March 30, 2004 (Spirit sol 85)
Click to enlarge >
Spirit's landing site as seen by MOC on March 30, 2004 (Spirit sol 85)
Credit: NASA / JPL / MSSS
Spirit's landing site as seen by MOC on January 19, 2004
Click to enlarge >
Spirit's landing site as seen by MOC on January 19, 2004. No tracks yet!
Credit: NASA / JPL / MSSS

These images show a stunning level of resolution; the rover is only about the same size as a dining room table. How did the Mars Orbiter Camera team capture these images? It required a tricky technique referred to as "cPROTO" imaging.

According to Malin Space Science Systems, which built the Mars Orbiter Camera, "The 'PROTO' part of 'cPROTO' refers to the movements the Mars Global Surveyor spacecraft must make to acquire the image -- it must be 'pitched' in the downtrack direction to obtain the 50 cm/pixel view, and rolled to allow the spacecraft to point at the specific target of interest." That is, while the spacecraft is passing from pole to pole, it begins facing forward along its track, and pitches to keep its camera pointed toward the target, ending by facing backward along its path. In this way, it can spend longer looking at the same patch of ground than it could without pitching. At the same time, the spacecraft must roll steadily in order to compensate for the spacecraft's east-west motion along the ground.

Pitch
Roll
Yaw

So-called "PROTO" imaging is now relatively routine. But in order to eke out the most possible resolution from the Mars Orbiter Camera, Malin Space Science Systems added another component of motion. "The 'c' in 'cPROTO' stands for planetary motion compensation. While Mars Global Surveyor is pitching, rolling, and moving along its orbit, Mars is rotating underneath it (just as Earth is rotating right now, as you read this, such that the Sun, Moon, and stars appear to move in the sky). The pitch and roll of Mars Global Surveyor are timed in such a way as to account for the rotation of Mars, as well as the desired image resolution and target location."

The cPROTO technique is incredibly challenging to pull off. cPROTO images cover a very tiny area on the surface of Mars, 3 kilometers wide by 3 to 4 kilometers high (about 1.9-2.5 miles). Because the spacecraft moves at 3.3 kilometers per second (2.1 miles per second), targeting the right spot on Mars requires split-second timing. The team has tried to image the Spirit landing site with rover tracks three times (twice in March and once in June) but missed the target on two of the three attempts.