Recent failures of spacecraft bound for Mars have emphasized the difficulty of getting safely to the Red Planet. Since September's loss and presumed crash of the Mars Climate Orbiter, officials at the Jet Propulsion Laboratory have scrutinized every detail of the Polar Lander's flight path, in order to ensure that the craft is on the correct course to its destination.

But the heavy focus on the trip to Mars, and NASA's perfect three-for-three record for martian landings, may disguise the fact that a landing on a planet is even more dangerous and difficult than entering orbit -- and just as difficult as getting there.

Mission operators at NASA were fortunate that the Mars Pathfinder and two Viking landers made picture-perfect touchdowns, but the polar landing site is the uneven and may hold unknown obstacles to a soft landing.

At the end of its 11-month voyage through space, the Mars Polar Lander will use its rocket engines to slow its descent and softly set down on Mars' mysterious polar layered terrain.

Unlike the dusty rock-covered ground that the Viking and Pathfinder landers observed at lower latitudes, the surface around the martian poles appears to be alternating layers of ice and dirt -- layers scientists think might help answer questions about the planet's annual climate changes.

Although the topography of the polar region is extremely uneven and varied, mission planners tried to select the smoothest site they could for the lander's touchdown.

Using images from the Mars Orbiting Camera aboard the Mars Global Surveyor, officials at the Jet Propulsion Laboratory believed they had found a suitable area when they announced the landing site in August. Later images from the orbiting camera revealed the site to be rougher than originally hoped, but not so much to cause planners to go to their backup site.

It is critical that the lander touches down on relatively flat ground. If the three-legged spacecraft lands halfway into a ditch, or over a drop-off, it could crash. If it lands safely, but is tilted too far to the south, east or west, the craft could have trouble aligning its solar panels to produce the power it will need to conduct the mission. The December 3 landing near the south pole will occur during the martian southern spring when the sun moves low in the northern sky, and does not offer the power it would if it was passing overhead.

While anxious about the risks, mission scientists are eager for the opportunities presented by the polar layered terrain. It offers humans the chance to see and study a surface unlike anything else yet known.