Engineers now think they'll be ready to roll the Spirit rover off its lander and onto the martian surface early Thursday (Eastern Time), a day later than had been hoped, to accommodate rehearsals and a two-day procedure to rotate the rover into position for its long-awaited egress.

This mosaic image taken by the navigation camera on Spirit produces an overhead view of the rover. The front of Spirit is facing south, or downward, in this view. The yellow arrow illustrates the northwest direction the rover will take during the drive off the lander. Credit: NASA/JPL

Overnight Saturday, two small explosive devices detonated, cutting cables holding Spirit's two center wheels in place. The rover's robot arm also was released from its launch locks and stowed for roll off. The only physical connection remaining between the rover and its lander is a final umbilical that routes power and data to and from the lander.

That cable will be cut overnight Monday, using the last of more than 120 explosive devices that began detonating Jan. 3 in a precisely scripted sequence beginning with the lander's separation from its cruise stage just before atmospheric entry.

"We are now fully stood up and all of our six wheels are deployed in their normal configuration and they've all been released from the lander," said mission manager Arthur Amador. "So the only thing that we remain in contact with lander is through our umbical."

Otherwise, "the vehicle status remains pretty darn perfect," he said. "Everybody's extremely pleased with the health and safety status of all the subsystems."

Engineers initially hoped to have Spirit roll straight off its lander, almost due south as it sits on the floor of Gusev Crater. But bunched-up airbag material poses a potential threat to the rover's solar arrays and engineers instead have decided to roll off to the northwest. That will require Spirit to rotate in place about 120 degrees to its right.

A model of the Spirit rover rolls off the lander during a test at the Jet Propulsion Laboratory testbed. Credit: NASA/JPL

First, however, the rover will be commanded to back up eight inches. Then, the four wheels on the outer corners of Spirit's rocker-bogey suspension will be commanded to turn sharply inward. That will cause the rover to rotate about its central axis, literally turning on a dime. Once properly lined up, the rover will roll off the lander's egress aids and onto the surface.

But in keeping with their "brave, but not stupid" approach to Spirit's activation, the JPL engineering team has decided to carry out that rotation in two stages over two days.

To make absolutely sure they understand the rotation and roll-off procedures, engineers plan to spend today testing various commands and techniques using a rover/lander mockup at the Jet Propulsion Laboratory. If no problems surface, the final umbilical will be cut overnight Monday and the rover will be commanded to turn in place about 45 degrees. At that point, pictures and telemetry will be radioed back to Earth for analysis to make sure the procedure is working properly.

The following night - overnight Tuesday U.S. time - Spirit will complete its rotation in two steps. First, it will rotate another 50 degrees, then stop and send back pictures. If all is well, commands will be sent to complete the rotation. That will set the stage for roll off early Thursday U.S. time.

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