The Mars Polar Lander descent camera will capture Martian surface as never seen before
News Service
Cornell University
Contact: David Brand
Office: (607) 255-3651
E-Mail: deb27@cornell.edu
FOR RELEASE: Dec. 1, 1999
On Friday, Polar Lander descent camera will capture Martian surface as never seen before: from only a few feet up
ITHACA, N.Y. — For just under two minutes, shortly before 3:14 p.m. Eastern
Time on Friday, Dec. 3, a camera directed toward the south polar region of
Mars will capture and store a series of about 20 images unique in the annals
of planetary exploration: the surface of a planet (other than the moon) as
seen from altitudes ranging from about 4 miles to only about 30 feet.
The camera, known as the Mars Descent Imager, or MARDI, will be positioned
between the legs of the Mars Polar Lander, with the exhaust of the hydrazine
engines in view. It will begin clicking its shutter after the lander
vehicle’s heat shield has been jettisoned — about 6.5 kilometers (about 4
miles) above the surface — and while the craft is still swinging on its
parachute harness. The last few images — perhaps eight — will be captured
after the parachute has been jettisoned at about the 1 kilometer (.62 mile)
altitude and as the craft makes a controlled descent, slowed by retro
rockets, to the frigid northern edge of the Martian south pole’s layered
terrain.
“MARDI’s images will make all of us much more comfortable in making
interpretations of the lander’s pictures because they will give us a
context,” says Peter Thomas, a senior researcher with Cornell University’s
astronomy department. “For the first time we will have a complete scale
of pictures of Mars, from less than a millimeter all the way up to orbiter
pictures.” The camera has a 70-degree field of view, and the estimated
difference in resolution between the first and the last black-and-white
images will be a factor of about 800.
Thomas is one of three Cornell researchers on the MARDI team, led by
Michael Malin, president of Malin Space Science Systems, San Diego. Also
participating in the development of the imaging system, and present at
NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif., to interpret the
images after they are received from the Mars Polar Lander, are Cornell
astronomy professors Joseph Veverka, who also is chair of the Cornell
astronomy department, and Steven Squyres. Also on the team are M.A.
Caplinger of Malin Space Science and M.H. Carr of the U.S. Geological Survey
in Flagstaff, Ariz. MARDI was developed under a $3.5 million JPL contract.
At present, the highest-resolution images of the Martian surface, taken from
orbit, are made up of pixels (or picture elements) each covering 1 1/2 yards
of terrain. That is about to change dramatically to images with each pixel
covering a fraction of an inch of the surface.
The descent camera pictures will be used to interpret ground features and
will aid in the mission’s main purpose, studying the layers of ice and dust
covering the polar region. These images will be captured with a “nesting”
technique, meaning that each successive image will be nested within the
previous picture. As the spacecraft loses altitude, each successive image
will cover a smaller area within the previous larger image. The camera has
no ability to aim, but simply points where the spacecraft points. “The first
image will be several kilometers on one side, but the camera has a fairly
wide angle so that even with the spacecraft swinging on a parachute, the
images should remain nested within one another,” says Thomas.
The nesting technique, he notes, will enable researchers to find a ground
feature, such as a boulder, in the image taken closest to the ground, then
work back to the largest picture. The spacecraft’s electronic memory
retains each image, plus details of when the image was taken, which
direction the spacecraft was pointing at the time and its altitude. In this
way, says Thomas, “you can take pictures and reconstruct them from that
geometry.”
The number of images returned to JPL will be limited by the storage
capacity of the spacecraft’s memory. For this reason, the on-board
computer has been programmed both to reject some images taken by the
camera and to write over others. The computer will be instructing the
camera to capture images in different image formats (in terms of pixels)
based both on altitude and the number of images already taken. If the
computer determines that the altitude has not changed sufficiently, it
will not save the image.
“If the memory’s storage is full and the camera is still taking images, the
computer is programmed to throw out some lower-resolution pictures,”
says Thomas. “We want to maintain nesting and protect the higher
resolution images as we get really close to the surface. Those images
closer to the surface are of platinum value.”
The “overwhelming purpose” of the descent camera’s images, says Thomas,
is to tie what will be seen with the lander’s camera on the surface of Mars
with images taken a few feet from the surface. “We’ve seen the whole
of Mars in 100-meter resolution, but only 1 percent of the surface in
three-meter resolution. These images will be filling the gap.”
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[Image caption:
http://www.news.cornell.edu/releases/Dec99/Mars.camera.deb.html]
Aerial photographs of Antarctic dry valleys illustrate the “nesting”
technique to be used by the Mars Descent Imager. In the first photo, the
locations of the series of nested descent images is shown, each a factor
of 2:1 smaller than the previous image. The second photo shows the
largest nested image, which is 500 meters across and has a resolution
of 5 meters per pixel. The third photo shows the final nested image,
which is 160 meters across and has a resolution of 16 centimeters per
pixel. The boulder, top right, in this image measures about 6 meters
across. PHOTO CREDIT: Malin Space Science Systems
