Illustration of Mars Polar Lander

October 22, 1999
Web posted at: 5:47 a.m. EDT (0947 GMT)

From CNN Space Correspondent Miles O'Brien:

PASADENA (CNN) With exactly six weeks to go before their diminutive sensor, shovel and shutter-packed science probe is due to arrive on the surface of Mars, scientists in charge of the Mars Polar Lander mission are huddling to decide precisely where to land.

High-resolution images captured by Mars Orbiter Camera on board the Mars Global Surveyor spacecraft have prompted some worry about the terrain at the primary site near the Martian South Pole.

"At first we thought this was really sort of rolling areas, with pretty gentle slopes," says Mars Polar Lander Project Scientist Richard Zurek. "Now it is clear at least parts of the area that we may end up on are a little rougher than what we expected."

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Zurek and his team chose a primary and back-up landing site based on images captured by a Viking orbiter some 20 years ago. The camera on that spacecraft was only able to see images the size of a football field or larger. And at that resolution, the landing zone looks like a perfect welcome mat for the 12 foot-wide Lander that stands only 3 and a half feet tall.

But the Malin Space Science Systems camera on the Mars Global Surveyor launched in 1996 - can record features as small as 12 feet in size.

Illustration of Mars Climate Orbiter

And what a difference the close ups make: What seemed featureless and smooth to Viking's eyes - is actually some turf with a bad case of acne. The slopes and mesas do not appear to be very large - Zurek says they may rise about 6 feet over a few hundred yards. That doesn't sound like much, but remember, this is a small vehicle.

"At the scale of the Lander even something like a table could represent an obstacle," says Zurek. "Something that is a meter up here and down here can give you problems when you are landing on 3 legs we have no hazard avoidance in that regard."

The Mars Polar Lander is equipped with a radar altimeter designed to let the craft know its altitude as it plummets toward Mars. The information is used to control the pulsing action of a dozen rocket thrusters that will slow the craft down as it nears the surface. It is not capable of guiding the Lander away from a hazardous touchdown location.

Landing on a steep slope could, of course, be catastrophic. It also could leave the orbiter in tact, but listing and thus in a bad attitude for its six solar panels to capture the sun's energy and convert it into power.

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Ironically, the same features that pose the greatest hazards to the Lander are also extremely enticing to scientists.

The Polar Lander is headed to the pole because, beneath the surface, there are alternating light and dark bands that appear to be deposits of ice and dust.

Scientists believe the layers can give them key clues on the climate history of Mars much like growth rings on a tree. Should Mars Polar Lander nestle up near a steep hill, or cliff, it could offer an unexpected glimpse of some exposed layering.

"It gives us, in effect, access to the subsurface even without digging into that soil," says Zurek.

But weighing the potential bonanzas against the busts is what this business is all about. Since it has only been one month since a very notable bust (the loss of the Mars Climate Orbiter) the Jet Propulsion Lab/UCLA team is probably a bit more jittery than they might be otherwise.

So they are looking hard at the images of the backup landing site about 120 miles away -which is as large a course correction as can be made at this stage in its 470 million-mile journey. But as they pore over images of both locations, they realize there are some limits to what they can and should do.

"We picked two sites that were as different as you could get and still land on this layered terrain that scientifically is the core of our mission," says Zurek, "but what we are finding is if you find it over here - you find it over there it is quite likely it is typical of this kind of terrain."

So in the end, the two sites may be a geologic toss-up and a tie in this game favors the primary landing site. Sometime next week, the scientists will announce their choice, cross their fingers and then, on October 30, send the commands that will fire the thrusters to home in on the desired landing site on December 3.