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How Do Scientists and Engineers Find Spirit?

January 08, 2004

Spirit has been on the surface of Mars for more than 36 hours, and every minute scientists and engineers are getting closer to determining exactly where their precious package has landed. Like a jigsaw puzzle, scientists and engineers must put together at least four complicated pieces before they have the whole picture: orbital data that shows the overall landing-site terrain, descent images taken on the rover's ride down, views of the horizon seen by Spirit's own eyes, and navigation data collected as the spacecraft entered the atmosphere and during the Mars Odyssey Orbiter's overflights of the rover.

Puzzle Piece #1: Orbital Images

In the past year, cameras on the Mars Global Surveyor and Mars Odyssey orbiters have been taking pictures of this "landing strip" every time they passed overhead. Scientists started off with the knowledge that Spirit was somewhere within this long oval-shaped ellipse to which navigators guided the spacecraft.

Puzzle Piece #2: Descent Images

This image, taken by the Descent Image Motion Estimation System (DIMES) camera located on the bottom of the Mars Exploration Rover Spirit's lander, shows a view of Gusev Crater as the lander descends to Mars.
This image, taken by the Descent Image Motion Estimation System (DIMES) camera located on the bottom of the Mars Exploration Rover Spirit's lander, shows a view of Gusev Crater as the lander descends to Mars.

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While the spacecraft was hurtling toward the martian surface, it was able to take images a little less than a mile above Spirit's landing site. Scientists compared these images with those taken by a camera on the Mars Global Surveyor spacecraft. Using the crater features in the descent images as a guide, scientists were able to find the approximate landing location within the larger orbital picture. That helped them narrow down the landing location to a 1.3 kilometer (0.8 mile)in diameter area.

Puzzle Piece #3: Surface Images

Tim Parker, geologist
Tim Parker, geologist

Add some images from the surface of Mars, and geologists like Tim Parker can eventually tell you where Spirit is hiding. "The real hunt began once we received the first images from the surface," explained Parker. He is using hills on the horizon seen by the rover to build a vector plot. Parker's vector plot has a series of lines that project from the lander to compass directions on the horizon. Each line is from a single point of view to landforms that have been identified on the horizon. The map is then placed over the orbital image (Puzzle Piece 1!) and moved into various positions until the lines match up to the features in the orbital image. This "match" will increase the accuracy of the rover's estimated location. "We'll soon know within 300 meters...or about a third of a mile...where the rover is," says Parker. "We're really waiting for additional higher resolution images and range data to come back from the rover in the next few days so that we can identify the rover's position more accurately."

Puzzle Piece #4: Navigation Data

The same navigators who worked months to bring Spirit to Mars are assisting scientists in locating Spirit's exact position. They're contributing their detailed knowledge and experience of radio signal tracking. It's a little like listening to the sound of a train whistle - it changes as it approaches, passes, and continues on. Navigators compared the radio frequency as it left Spirit with the frequency as it was received by the Deep Space Network stations here on Earth about ten minutes later.

Ralph Roncoli, Spirit navigation team member
Ralph Roncoli, Spirit navigation team member

Staring on Spirit's third day on Mars, navigators began using another way to find Spirit's landing site through the Mars Odyssey orbiter. "We know the orbit of Odyssey around Mars very precisely, so by having Odyssey listen to the change in the frequency of the radio signal from Spirit as it flies over the rover, we can pinpoint where the signal came from in relation to where Odyssey was at that moment," explained Ralph Roncoli, Spirit navigation team member. "We collect this navigation data from Odyssey during the first week on Mars, and we hope to nail the location very soon," said Roncoli.

So where exactly is Spirit? Stay tuned as the puzzle pieces come together....
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