CU-led Spirit rolls onto the Mars surface; Opportunity approaches

This image, taken by the front hazard-identification camera on the Mars rover Spirit, shows the rover's robotic arm. The arm was deployed from its stowed position beneath the "front porch" of the rover body early on the morning of Jan. 16. This is the first use of the arm to deploy the microscopic imager, one of four geological instruments located on the arm. NASA/JPL/US Geological Survey

By David Brand

PASADENA, Calif. -- Twelve days after it bounced down on the Martian surface, the rover Spirit rolled onto the floor of its landing site in Gusev crater in the early hours of Jan. 15 -- early afternoon, Mars time. Afterward, a relieved Charles Elachi, director of the Jet Propulsion Laboratory (JPL), the mission's manager, declared, "We have six wheels in the dirt."

The successful roll-off was a particular triumph for Cornell astronomy professor Steven Squyres, leader of the rover science program, who has been working toward this goal for the past 15 years. "This is a night extraordinarily rich in its significance," he said. The signal to roll down the ramp was sent from mission control at JPL at 12:21 a.m. Pacific time. At 1:53 a.m. JPL received word from the rover that it was on the surface of Mars, putting Squyres and his science team effectively in charge of the mission.

Among the first images received by JPL was a backward look, showing the rover's tire tracks leading away from the lander, now merely space debris. The tracks show that the wheels did not sink too deep for driving and that the soil has very small particles that provide a finely detailed imprint of the wheels, said Cornell's Robert Sullivan, senior research associate in space sciences and a planetary geology member of the rover science team, at a JPL press briefing on Jan. 16. "Rover tracks are great. For one thing, they mean we're on the surface of Mars. We look at them for engineering reasons and for science reasons," he said.

On Jan. 20 a highly detailed close-up image was sent back after the rover drove about six feet to its first target, a football-size rock dubbed Adirondack.

Meanwhile, Spirit's twin rover, Opportunity, closed in on Mars, in preparation for a scheduled Jan. 25 (12:05 a.m. Eastern time) landing in Meridiani Planum on the other side of the planet. As of 8 a.m. Jan. 18, Eastern time, Opportunity had traveled 444 million kilometers (276 million miles) since its July 7 launch and had 12.5 million kilometers (7.8 million miles) left to go.

On Jan. 16 the Opportunity spacecraft was ordered to fire its thrusters to adjust its trajectory, or flight path, for the first time in four months in order to put it on course for the target. Squyres also heads the science team for Opportunity.

Before the thruster burn -- one 20-second burn and two 5-second pulses -- Opportunity was headed for a landing about 384 kilometers (239 miles) west and south of the intended landing site. Opportunity's schedule still includes two more possible trajectory correction maneuvers.

Spirit's roll-off from the lander platform came after a slow and cautious two-day, 115-degree swivel-in-place on its triangular landing platform, putting the rover in a north-northwest-facing direction. The golf cart-sized vehicle slowly crept down one of the lander's stretch-fabric ramps, moving 3 meters (10 feet) in 78 seconds. The back of the rover ended up about 80 centimeters (2.6 feet) away from the foot of the ramp. A hazardous part of the short journey was a 4- to 6-inch drop off the edge of the ramp onto the landing site, newly named Columbia Memorial Station by NASA.

The long, tense wait between sending the roll-off command and receiving confirmation that the journey had been completed was due both to the 10 minutes it takes for data to travel one way between the two planets and to the fact that once off the rover, the vehicle had to reorient its direct-to-Earth, high-gain antenna. The antenna had to locate the sun in its field of view to get north so that it could then focus on the Earth. Meanwhile, the rover's software was compressing images of the roll-off from the rover's hazard-identification and navigational cameras -- images that would slowly dribble back to Earth.

In a talk to mission managers Jan. 15, Squyres said that the initial exploration plan is to characterize the geological diversity of the area with the rover's suite of instruments so that scientists can "understand typical stuff" on the surface in order to provide a baseline. The rover's first targets, he said, will be rocks, then soil.

The rock Adirondack, in particular, attracted his attention, he said. It is so-named because it is pyramid-shaped with smooth facets, about 35 centimeters by 20 centimeters (14 by 8 inches). On Jan. 20 the rover began examining the composition of the rock with three scientific instruments, the microscopic imager, the Mössbauer spectrometer and the alpha particle X-ray spectrometer.

Longer term, said Squyres, the plan is to send the rover 200 meters (220 yards) to a crater, also shown in the panoramic images, that has a diameter less than 200 meters, a rim 4 to 5 meters (12 to 15 feet) high and which probably "has been excavated down tens of meters." Squyres earlier called the crater "an extremely attractive target" because it would "provide a window to subsurface Mars." Since it's believed that Gusev crater once contained a lake, finding what lies beneath the surface is considered vital. Indeed, the goal of both rovers is to examine the terrain for clues about the past existence of water, making the environments possibly suitable for sustaining life.

After investigating Gusev crater, Squyres said, Spirit will "head for the hills," shown in panoramic pictures about 1 to 2 miles to the east. He noted, "The image all of us want is halfway up the hills looking back where we came from."

The day after the roll off, Spirit exercised its robotic arm -- about the same size as a human arm, with comparable shoulder, elbow and wrist joints -- and examined a patch of fine-grained Martian soil with a microscope at the end of the arm, taking the first microscopic image of the surface of another planet. The rover's microscopic imager is akin to a field geologist's magnifying glass for examining structural details of rocks and soils. The microscope can show features as small as the width of a human hair.

Before driving to Adirondack, Spirit rotated its turret of tools to use two spectrometer instruments on the same patch of soil examined by the microscope. The Mössbauer spectrometer identifies types of iron-bearing minerals. The alpha particle X-ray spectrometer identifies the elements in rocks and soils.

Today (Jan. 22), Spirit completes its 19th Martian day, or "sol" on the red planet. Each sol lasts 39 minutes and 35 seconds longer than an Earth day.