Finding Spirit

No ‘X’ Yet to Mark the Spot

see also gallery of Spirit’s Sol 1 images and slideshow

It might initially seem straightforward to locate where a rover is.

Mars Global Surveyor
In basin of Gusev, near smaller craters suspected to be close to Spirit. The streaks are dust devil tracks, as also shown elsewhere in the banner image: left Hartmann painting, middle, on Mars, right, in Arizona.
Image Credit: Mars Global Surveyor, Malin Space Systems

After all, it is now sending back spectacular surface images, with large features like hills on the horizon, all the while beginning to point its high-gain antenna–a rover instrument that looks like a lollipop–with enough accuracy to find the tiny speck in the martian sky called Earth.

But as with most maps, knowing where you are on a globe is easier than knowing a street address in a foreign city. The exploration team does know their rover landed in an impact crater called Gusev. But since Gusev is the size of Connecticut, their next step is to narrow down this large, flat area that they call the landing ellipse.

Solving this puzzle presents a fascinating detective story, and mission scientists have taken to pooling bets on the accuracy of their initial guesses. One striking feature that scientists noted from the last three images snapped just seconds before landing [ from a descent imaging system, or DIME] was a pattern of five craters that matched well with orbital images for Gusev (right). Those three images are an important clue, and the DIME system was a new addition to this mission’s landing sequence.

Mars 2001 Odyssey
Gusev Crater (Released 3 January 2004) Mosaic produced by Noel Gorelick,ASU/THEMIS Team. Left: Latitude -14.8 Longitude 175.2E (184.8W) Image Size (km) 42.8×89.8 Mars 2001 Odyssey, Thermal Emission Imaging System (THEMIS), Mars Orbital Laser Altimeter (MOLA) Team. 2001 Mars Odyssey launched on April 7, 2001, and arrived at Mars on October 24, 2001. It also provides the communications relay for U.S. and international landers, including missions in NASA’s Mars Program, the Mars Exploration Rovers.
Image Credit: NASA/JPL/Arizona State University

One thing seems certain so far: the Mars Exploration Rover named Spirit has bounced to the ground somewhere in this scene [left]. The image is composed of nine thermal imaging system [THEMIS VIS] frames from the orbiting Mars Odyssey spacecraft. The images have been stitched digitally together, producing a complete picture of the region in which the rover landed.

The dark streaks are places where wind has either scoured off the brighter martian dust or has covered it up by sand. Note the interesting lobes of material evident along the right side of the image just below the large crater. These may be due to the emplacement of water rich sediments or possibly lava flows.

Ground Truth

In 2003, a new technique was pioneered by the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) experiment to allow the camera to obtain images with better than 1 meter (~ 3 ft) per pixel resolution. The Mars Global Surveyor (MGS) spacecraft launched in November 1996, and also relays data back to Earth from surface landers.

Mars Global Surveyor
Locating Landers on Mars MGS MOC Release No. MOC2-595, 4 January 2004
Image Credit: Mars Global Surveyor, Malin Space Systems

To get the resolution needed to view a surface object about the size of a large umbrella from space, a remarkably ingenious method is being pioneered on MGS. By pitching the spacecraft at a rate faster than the spacecraft moves in its orbit around Mars, MOC is able to obtain pictures with a down-track resolution of about 50 cm/pixel (~20 inches/pixel), although the cross-track resolution remains ~1.5 m/pixel (5 ft/pixel).

One of the key goals of this image motion compensation (IMC) technique is to be able to image landers, such as the Mars Exploration Rovers, Spirit and Opportunity, on the martian surface.

The two pictures shown right were acquired during the IMC testing in 2003.

The first shows the location of the Mars Pathfinder lander (MPF, 1997) and the nearby boulder, Yogi. The second image shows the location of the Viking 1 (VL-1, 1976-7) lander.

These locations were determined by using sight lines from the landers to near and far objects seen in the pictures acquired by the landers, and then matching these to locations in earlier, 1.5 to 3.0 m/pixel MOC images. Then, the IMC images, shown here, were acquired by MGS so that the actual landers, sitting on the martian surface, might be resolved.

This technique only works well when the location of the lander is already fairly-well established.

It would be extremely difficult to find a lander for which the location is uncertain, such as Viking 2 or Mars Polar Lander (in fact, for Mars Polar Lander, it would take over 60 years to map out the entire landing ellipse in which the spacecraft was lost). The two images shown here are illuminated from the left and show areas only a few hundred meters across.

What’s Next

Mission planners expect to get precise coordinates in the next few days for Spirit.

Meanwhile the successful orbital insertion of the first European orbiter, Mars Express, also promises high resolution overhead images. [Having entered polar orbit on Sunday, the orbiter will pass over the Beagle 2 target site beginning January 7]. Over its multi-year mission life, an onboard camera on Mars Express offers high-resolution stereo views of Mars. Its comprehensive maps will feature 10 meter resolution, but some particularly interesting regions will get a close-up view to 2 meters [about the size of small car, as seen from orbit].


Related Web Pages

NASA
Jet Propulsion Laboratory
Malin Space Systems
Mars Global Surveyor
Arizona State THEMIS
Mars 2001 Odyssey