Two U.S. spacecraft are speeding toward Mars for a January landing. Their goal is to determine if the planet ever had the conditions to sustain life. A European spaceship is also on its way for a December touchdown to do similar research. But their safe arrival is not assured. After six-and-a-half month journeys, each craft must go through several highly technical and precise maneuvers to deploy a lander in just the right spot. The record is not good.

If the past is a guide, then the chance of three consecutive safe landings on Mars is poor.

"We call Mars the death planet," notes Colleen Hartman, who directs solar system exploration at the U.S. space agency, NASA.

"So many spacecraft have gone to Mars and somewhere along that long journey, they've failed. Worldwide, there have been 33 launches to Mars, and two-thirds of those have failed," she said.

The most recent losses were in 1999. A U.S. orbiter bypassed Mars because confusion over the use of two different measuring systems, metric and English, caused navigation errors. A NASA lander crashed into the planet because of errors in a computer program that determined its altitude.

NASA's chief of solar system science, Ed Weiler, says only three of nine U.S. and Russian landing attempts have succeeded, the U.S. Pathfinder mission in 1997 and a pair of U.S. Viking flights two decades earlier.

"One for three is a great batting average in baseball, but when these things cost this much, it's not that great for space," he said.

Now that they are off the ground, the U.S. and European spacecraft are crossing 500 million kilometers of space to reach Mars. Between now and January, NASA engineers will command the U.S. vehicles to make several course corrections and change their orientation periodically to ensure their solar energy panels face the sun.

But as Mr. Weiler points out, the voyage is the easy part. The hard part comes at arrival.

"Landing on Mars is tough. I mean, too many things can go wrong, too many moving parts!," he said.

The spacecraft are to arrive at Mars at a speed of more than 19,000 kilometers per hour. NASA engineer Arthur Amador says an intricate series of steps has to occur to deploy their landers and bring them to a halt on the ground within only six minutes.

"That type of deceleration and all of the many critical events that need to occur precisely during those six minutes is one of the things that really adds the most difficulty and risk to landing on Mars," he explained.

First, the landers must separate from their speeding spacecraft above the Martian atmosphere and enter it at exactly an eleven-and-a-half degree angle. This low angle limits heating caused by friction with the atmosphere.

"If it's not exactly right, then we could overheat the vehicle and that would be a problem," emphasized Mr. Amador .

The atmospheric friction will help slow the landers, but parachutes must deploy at 8.5 kilometers above the surface to slow the descent still more. Twenty seconds later, the bottom halves of their heat shields are jettisoned and the top halves unreel a 20-meter tether to lower the landers further.

Seconds before touchdown, gas generators inflate tough airbags five times stronger than steel. Then retrojets fire to stop descent 15 meters above ground. The tethers are cut and the landers fall to the ground on their protective cushions, bouncing and rolling to a halt.

Even if all goes as planned, Ed Weiler stresses the unexpected could interfere. "We can do absolutely everything right and we'd get a gust of wind that exceeds the limits, which ruins the mission," he said. "All it takes is a boulder the wrong size in the wrong place. So that's why I'm glad we have two rovers."

A second one increases the odds that some data is gathered in case the other landing fails.

As intricate as a Mars touchdown is, NASA has succeeded with the same parachute, rocket, and airbag drop before, during the 1997 Pathfinder landing. Arthur Amador says that after the 1999 losses, the space agency added more backup systems and put the craft through more testing and reviews.

"We believe that given our design and our validation program and the margins that we've put into our mission plan, we ought to be okay," he said.

If so, the landers will each release a six-wheeled rover to travel up to one kilometer over three months. They will examine rocks and sample the atmosphere to ascertain the history of Mars' climate and determine if life could have survived there.