Ending an 11-month cruise, NASA's Mars Polar Lander late today was hurtling toward its target landing site in the late spring of the Martian south pole.

By about 3 p.m. (EST) Friday, if all goes well, the Polar Lander will have become the fourth U.S. craft ever to touch down on the surface of the Red Planet and the first to land in the polar region of any extraterrestrial world. Its two passengers, basketball-sized projectiles named Scott and Amundsen after two of Earth's polar explorers, will have become the first penetrators ever shot into an alien body.

The mission is the second wave of a long-term assault on the planet aimed at learning more about its geology, climate and potential for supporting life, including possibly future visits by humans. The Polar Lander, equipped with a robotic digging arm, will focus on analyzing soil samples for signs of water.

The first signals of arrival could arrive within an hour of the landing, depending on the spacecraft's health, with an image following soon after. The incoming trove will be piped directly onto the Internet with minimal delay, scientists said.

But first, the Polar Lander has to negotiate a complex entry into the Martian atmosphere, hurtling in at more than 15,400 mph and experiencing heat from friction up to 3,000 degrees Fahrenheit. It must then pop a parachute, and power on retro-rockets to brake its fall. It needs a bit of luck to help it land "on its feet," avoiding dangerously steep slopes or precipices, big rocks and other perils too small for ground controllers on Earth to have detected. And because it is aimed for an extreme latitude far from the equator, the geometries of the arrival 150 million miles from Earth make it even trickier than most, according to flight operations manager Sam Thurman.

"It's a very complex process [involving] literally years of effort by hundreds of people" all aimed at that final 5.5 minutes, Thurman said.

This mission has been subjected to unusually exhaustive scrutiny in recent weeks by investigators determined to make sure the Polar Lander does not suffer the fate of a sister ship, the Mars Climate Orbiter, which was destroyed in the Martian atmosphere in September because of a navigation error.

The orbiter was, among other things, to have served as a high-speed communications relay for the lander and without it the return of data and pictures will be slower.

As a package, the lost orbiter and the lander together cost $328 million, including their launches. The concerns are further amplified by NASA's inability, in two attempts, to position an instrument in Martian orbit that could have provided crucial information on the dramatic variations in the density and altitude of the thin Martian atmosphere, engineers said. The instrument was flown aboard the billion-dollar Mars Orbiter, which failed catastrophically as it arrived at Mars in 1993. A duplicate was flown aboard the ill-fated Mars Climate Orbiter.

Another craft, the Mars Global Surveyor, is functioning well in orbit and helping fill the gaps left by the failures.

It has transmitted a stream of high-resolution images of the varying terrain that have helped the team zero in on the desired landing target area, revealing surprising features such as gently swelling terrain, rough ridges giving way to patterns of gullies "as broad as streets" that were possibly formed as ice evaporated into gas, steep escarpments and even undulating sand dunes.

All in all, however, "This is a reasonably safe site. In fact, it's one of the smoothest places we've seen on Mars," said Michael Malin, of Malin Space Science Systems Inc., whose cameras are aboard the Polar Lander and the orbiting Surveyor.

On Nov. 8, the seasonal frost (mostly frozen carbon dioxide, or dry ice) retreated beyond the landing area, leaving what scientists hope will be dirt with a tale to tell. "What we learn should provide not only local but global insights," said David Paige of UCLA, a lead Polar Lander scientist. Scientists are fascinated by the strangely layered polar terrain, whose alternating bands of color, they think, may contain different mixtures of dust and ice reflecting important climate patterns.

Scientist Bruce Murray, a veteran of the first Mars missions, said this is the first time researchers may be able to directly sample ice and terrain carved by ice. "This is the Martian Antarctic . . . this area, even more so than on Earth, controls the climate on Mars."

The First Day

The Mars Polar Lander is scheduled to land at 3 p.m. (EST) today. Here is the lander's mission itinerary if all goes well:

10 minutes before touchdown, Lander separates from cruising unit and spacecraft starts navigating itself.

4 minutes, 33 seconds before touchdown, spacecraft hits the Martian atmosphere at 15,400 mph.

2 minutes before touchdown, parachute deploys 4 1/2 miles above the surface.

1 minute, 50 seconds before touchdown, a camera at the bottom of the lander begins taking pictures of the landing.

1 minute, 40 seconds before touchdown, the lander's three legs are deployed and radar activated to guide the spacecraft toward the surface.

About 3 p.m.: Touchdown.

3:37 p.m.: The first communication since atmospheric entry expected. Lander will transmit information on the status of the spacecraft, meteorological data and, possibly, images.

SOURCE: Associated Press