The Mars rover Opportunity is going to enter a 130m wide crater even though there is a good possibility it may not be able to get out afterwards.
The rover could enter the crater this week
The US space agency has decided the scientific rewards of investigating the rocks inside Endurance Crater outweigh the risks of the buggy getting trapped.
A test rover on Earth has worked out how Opportunity could best roll up and down the crater's slopes.
The real robot will now be sent to what should be the easiest entry/exit route.
If this is no steeper than what the testing runs at Nasa's Jet Propulsion Laboratory suggest a rover can climb, controllers plan to radio Opportunity the command to go into the crater.
It could happen some time this week. A final decision will be made once Opportunity is in position and engineers have had a chance to assess the slope close up and determine whether the rover's wheels will have sufficient grip.
"This is a crucial and careful decision for the Mars Exploration Rovers' extended mission," said Dr Edward Weiler, Nasa's associate administrator for space science.
"Layered rock exposures inside Endurance Crater may add significantly to the story of a watery past environment that Opportunity has already begun telling us.
"The analysis just completed by the rover team shows [the] likelihood that Opportunity will be able to drive to a diagnostic rock exposure, examine it, and then drive out of the crater.
"However, there's no guarantee of getting out again, so we also considered what science opportunities outside the crater would be forfeited if the rover spends its remaining operational life inside the crater."
Opportunity landed on Mars in January, coming to rest in a small depression known as Eagle Crater. There it found rock textures and evaporites (salts) that indicated the rocks in the area probably once bounded the edge of a lake or sea.
The rock layer in which Opportunity found this evidence can be seen also in its pictures of Endurance Crater, but at Endurance deeper, older rocks are also exposed. The prospect of studying these rocks is the reason Nasa is prepared to risk the future of the mission.
"Answering the question of what came before the evaporites is the most significant scientific issue we can address with Opportunity at this time," said Dr Steve Squyres, the lead scientist on the rover's science instruments.
"We've read the last chapter, the record of the final gasps of an evaporating body of water. What came before? It could have been a deep-water environment. It could have been sand dunes. It could have been a volcano.
Engineers have checked the rover's capability on a test slope
"Whatever we learn about that earlier period will help us interpret the upper layer's evidence for a wet environment and understand how the environment changed."
Nasa's other rover, Spirit, is now getting very close to a range of hills dubbed Columbia Hills. The rover has driven almost 3km from its landing site at Gusev Crater where it touched down five months ago.
It is now about a week away - a distance of a few hundred metres - from arriving at the foot of the elevated ground.
Rocks in Columbia Hills may provide insight both into how hills form on Mars and whether the ancient environment at this part of the planet - on the other side of Mars to Opportunity - was also wet.