It will complete the 300 million mile (500 million km) journey with a blistering six-minute plunge through the Martian atmosphere, during which the Mars Exploration Rover (MER) will decelerate from a space cruising speed of 12,000 miles (20,000 km) per hour to zero at the 0435 GMT.

"It will be six minutes from hell," said NASA administrator for space science Ed Weiler, describing the descent of the first of two robots -- MER-A -- to be followed by MER-B on January 25.

The robots were flying to Mars aboard two space probes, Spirit and Opportunity, launched respectively on June 10 and July 7. They are to land on opposite sides of the planet and spend three months searching for signs of life.

As of late Saturday MER teams at NASA's Jet Propulsion Laboratory here were good to go.

"Right now, the spacecraft's health is excellent, we've got the spacecraft all ready," said deputy mission manager Mark Adler.

The calculations needed to bring the MER down in the target ellipse, 60 km (37 miles) long by three km (2 miles) wide, 15 degrees south of the Martian equator, were mind-boggling.

"It's like playing golf ... where you tee off in Paris and the hole is in Tokyo," said navigation team chief Louis D'Amario. "It's the equivalent of locating something the size of a quarter in New York City, seen from Los Angeles."

But, he added, the probability of hitting the ellipse was "99 percent.)

D'Amario said that at 20H00 GMT Saturday, the Spirit probe was 58,000 miles (94,000 km) from the surface of Mars and "the navigation status is truly excellent."

NASA describes the voyage's end game this way.

Fifteen minutes before entering Mars' atmosphere, now traveling at 12,000 miles (20,000 km) per hour, Spirit will separate from its cruising stage rocket.

During the first four minutes of descent, atmospheric friction will heat the lead edge of the probe's thermal shield to around 2,600 degrees F. (1,400 C).

Just under two minutes from touchdown, its parachute will open. Twenty seconds later, the probe will jettison the spent lead edge of the shield, exposing the rover's protective cone, encased in in uninflated air cushions.

"In the final six seconds," said NASA, "the retro rockets on the upper shell will fire and the tether (to the parachute) will be cut about 15 meters (49 feet) above the ground."

During that 15 meter freefall, the air cushions will inflate and if all goes as planned the rover will a soft landing, bouncing about 15 times over a distance of a kilometer (1,000 yards) before coming to a standstill.

At that point, the air cushions deflate, the three segments of the protective cone open and, over the ensuing two hours, the rover will deploy the solar panels that are to power its onboard equipment and protect it from the Martian cold.

This, said Jennifer Trosper, Spirit Mission Manager for Surface Operations, will be a "critical deployment...what the rover needs to do to be in a safe mode. It ends with the deployment of the solar panels. At this point, the rover is in a power safe mode."

The MER mission, at a record cost of 820 million dollars, will involve some 250 NASA specialists and researchers who over three months will micro-manage the two six-wheeled rovers, weighing 180 kgs (400 pounds) each, roughly the size of a subcompact car.

This latest and most sophisticated effort to unlock the secrets of the red planet is beginning just days after the planned December 25 arrival of the ill-fated European robot Beagle 2, which has not been heard from since that date.

If all goes well with the MER-A landing, NASA engineers plan soon to deploy its panoramic camera, capable of transmitting back to earth 360-degree high-resolution color photos of the Martian landscape.

Mars Exploration Rover Spirit Mission Status Pasadena - 4 hours - Jan 3, 2004 Navigators for NASA's Spirit Mars Exploration Rover put the spacecraft so close to a bull's-eye with earlier maneuvers that mission managers chose to skip the final two optional maneuvers for adjusting course before arrival at Mars. With less than four hours of flight time remaining, Spirit was on course to land within a targeted ellipse 62 kilometers long by 3 kilometers wide (39 miles by 2 miles) within Mars' Gusev Crater. A trajectory correction maneuver scheduled for four hours before landing was cancelled. "The navigation status is truly excellent," said Dr. Lou D'Amario, the mission's navigation team chief at NASA's Jet Propulsion Laboratory, Pasadena, Calif. A slight trajectory adjustment on Dec. 26 was the fourth and final for the flight. Preparations in the past two days for arrival at Mars have included an adjustment that will open Spirit's parachute about two seconds earlier than it would have been without the change, in order to compensate for recent weather on Mars. "A dust storm seen on the other side of the planet has caused global heating and thinning of the atmosphere at high altitudes" said JPL's Dr. Mark Adler, Spirit mission manager. Also, engineers sent commands today to alter the timing when several pyro devices (explosive bolts) will be put into an enabled condition prior to firing. Enabling will begin 40 minutes earlier than it would have under previous commands. These pyro devices will be fired to carry out necessary steps of descent and landing, such as deploying the parachute and jettisoning the heat shield. Mars is 170 million kilometers (106 million miles) away from Earth today, a distance that takes nearly 10 minutes for radio signals to cross at the speed of light. Counting that communication delay, Spirit will hit the top of Mars' atmosphere at about 04:29 Jan. 4, Universal Time (8:29 p.m. Jan. 3, Pacific Standard Time), and reach the surface six minutes later.

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