PASADENA, CALIF. -- NASA scientists said yesterday they had decided to keep the Spirit rover on its lander for an extra day, putting off its rollout onto the martian landscape until at least late Wednesday. NASA adjusted the robot's schedule based on analysis of photos and data it sent back, they said, and added that another day-long delay is possible. That's not unusual given the complex nature of the mission, they said.

"We adjust (and plan) every day to manage risks and resources," Arthur Amador, a mission manager, said at a news conference.

Cables holding the middle set of the rover's wheels had been cut, leaving it attached to the lander by a single cord that is to be severed late Monday. Spirit's robotic arm was unstowed and swung up in front of the rover in its driving position.

The rover's status was "pretty darn perfect," with plenty of power to perform its chores, excellent communication and a nice, flat position for its exit, Amador said.

Once it rolls onto the surface, Spirit will park for a day or two to give scientists a chance to study the chemistry and mineralogy of the area.

The rover will leave its lander via the second-best route, heading west or northwest, to avoid air bags that blocked the ramp NASA had preferred. Repeated attempts to retract sections of the deflated bags have failed.

The $820 million US Mars Exploration Rover project includes a second, identical rover named Opportunity that is expected to land on the opposite side of the planet Jan. 24.

Each mission should last three months.