ITHACA, N.Y. -- For just under two minutes, shortly before 3:14p.m. Eastern Time on Friday, Dec. 3, a camera directed toward the southpolar region of Mars will capture and store a series of about 20 imagesunique in the annals of planetary exploration: the surface of a planet(other than the moon) as seen from altitudes ranging from about 4 miles toonly about 30 feet.
The camera, known as the Mars Descent Imager, or MARDI, will bepositioned between the legs of the Mars Polar Lander, with the exhaust ofthe hydrazine engines in view. It will begin clicking its shutter afterthe lander vehicle's heat shield has been jettisoned -- about 6.5kilometers (about 4 miles) above the surface -- and while the craft isstill swinging on its parachute harness. The last few images -- perhapseight -- will be captured after the parachute has been jettisoned at aboutthe 1 kilometer (.62 mile) altitude and as the craft makes a controlleddescent, slowed by retro rockets, to the frigid northern edge of theMartian south pole's layered terrain.
"MARDI's images will make all of us much more comfortable in makinginterpretations of the lander's pictures because they will give us acontext," says Peter Thomas, a senior researcher with Cornell University'sastronomy department. "For the first time we will have a complete scale ofpictures of Mars, from less than a millimeter all the way up to orbiterpictures." The camera has a 70-degree field of view, and the estimateddifference in resolution between the first and the last black-and-whiteimages will be a factor of about 800.
Thomas is one of three Cornell researchers on the MARDI team, ledby Michael Malin, president of Malin Space Science Systems, San Diego. Alsoparticipating in the development of the imaging system, and present atNASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., to interpretthe images after they are received from the Mars Polar Lander, are Cornellastronomy professors Joseph Veverka, who also is chair of the Cornellastronomy department, and Steven Squyres. Also on the team are M.A.Caplinger of Malin Space Science and M.H. Carr of the U.S. GeologicalSurvey in Flagstaff, Ariz. MARDI was developed under a $3.5 million JPLcontract.
At present, the highest-resolution images of the Martian surface,taken from orbit, are made up of pixels (or picture elements) each covering1 1/2 yards of terrain. That is about to change dramatically to imageswith each pixel covering a fraction of an inch of the surface.
The descent camera pictures will be used to interpret groundfeatures and will aid in the mission's main purpose, studying the layers ofice and dust covering the polar region. These images will be captured witha "nesting" technique, meaning that each successive image will be nestedwithin the previous picture. As the spacecraft loses altitude, eachsuccessive image will cover a smaller area within the previous largerimage. The camera has no ability to aim, but simply points where thespacecraft points. "The first image will be several kilometers on oneside, but the camera has a fairly wide angle so that even with thespacecraft swinging on a parachute, the images should remain nested withinone another," says Thomas.
The nesting technique, he notes, will enable researchers to find aground feature, such as a boulder, in the image taken closest to theground, then work back to the largest picture. The spacecraft's electronicmemory retains each image, plus details of when the image was taken, whichdirection the spacecraft was pointing at the time and its altitude. Inthis way, says Thomas, "you can take pictures and reconstruct them fromthat geometry."
The number of images returned to JPL will be limited by the storagecapacity of the spacecraft's memory. For this reason, the on-boardcomputer has been programmed both to reject some images taken by the cameraand to write over others. The computer will be instructing the camera tocapture images in different image formats (in terms of pixels) based bothon altitude and the number of images already taken. If the computerdetermines that the altitude has not changed sufficiently, it will not savethe image.
"If the memory's storage is full and the camera is still takingimages, the computer is programmed to throw out some lower-resolutionpictures," says Thomas. "We want to maintain nesting and protect thehigher resolution images as we get really close to the surface. Thoseimages closer to the surface are of platinum value."
The "overwhelming purpose" of the descent camera's images, saysThomas, is to tie what will be seen with the lander's camera on thesurface of Mars with images taken a few feet from the surface. "We've seenthe whole of Mars in 100-meter resolution, but only 1 percent of thesurface in three-meter resolution. These images will be filling the gap."
The web version of this release, including accompanying photos, may befound at http://www.news.cornell.edu/releases/Dec99/Mars.camera.deb.html
The above story is based on materials provided by Cornell University. Note: Materials may be edited for content and length.
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