NASA's Mars rover Opportunity is scheduled to begin a descent down a rock-paved slope into the Red Planet's massive Victoria Crater.
This latest trek carries real risk for the long-lived robotic explorer, but NASA and the Mars rover science team expect it to provide valuable science.
The rover arrived at the crater nine months ago. Since then it has been driving around the edge and peering in. In part, it is looking for a safe way to descend — all, of course, at the direction of scientists back here on earth.
"Entering this crater does come with some unknowns," said Alan Stern, a Mars rover scientist. "We can't be certain about the terrains and the footing until we go there."
The crater may have formed billions of years ago. All along its walls are layers of rock going back in time. Steven Squires, the lead scientist on the mission, said the crater really caught their eye.
The crater has a large ring all the way around the edge, like a bathtub ring, he said. There is a layer of bright material right at the very top.
Victoria crater is a few hundred feet deep. From orbit it looks like a cupcake wrapper that's been shoved into the ground. John Callas, the project manager, said they have found a way inside that is not too steep.
"You could walk down it, if you watched your footing," Callas said. "Much like going up to the rim of a meteor crater in Arizona."
But the rover is slightly arthritic, Callas said. The steering for one of its six wheels jammed a while back. Fortunately it jammed with the wheel straight ahead.
"Driving down is not a problem, it's coming back out again," Callas said. "We have driven up 32 degrees. The steepest we should see here is 20 degrees. We feel confident can get out safely, provided the rover remains healthy."
In about a week, the plan is to have the rover, dip its toes in the crater then come right back out, as a test.
The original mission for both Spirit and Opportunity called for three months of operation. But the rovers have been on Mars for three-and-half years now. Callas said another couple of years would be fine with him.
He said when the vehicles have become too degraded or lost too much mobility it will be time for them to "pass on." But right now they are doing great stuff, he said.