When the Mars rovers Opportunity and Spirit reached their distant destination in early 2004, NASA scientists hoped the vehicles would probe the planet’s frigid landscape for 90 days before they pooped out or were undone by the harsh Martian environment.

More than 900 days later, however, both robotic explorers are going strong — and Opportunity is on the cusp of what is likely to be its greatest accomplishment.

After enduring an 18-month trek through rugged terrain, dust devils and daily temperature swings approaching 200 degrees, the rover is scheduled to arrive today within easy lens view of a deep and geologically interesting crater. By tomorrow, if all goes well, the little robot that could will be right at Victoria Crater’s edge and in position to peer inside and send back images like none seen before.

“Exploring Victoria is something we joked and fantasized about but never really thought we could realistically get to it,” the principal investigator for the rovers’ science instruments, Steven Squyres of Cornell University, said. “This is the absolutely highest-priority destination we could have reached.”

The reason is that Victoria is an impact crater about 200 feet deep and half a mile wide, with sheer cliffs and layers upon layers of exposed rock. Before Victoria, the deepest crater the rover visited was Endurance, which is a mere 23 feet deep.

The scientists think Victoria Crater is the kind of geological formation that can tell them a great deal about Mars’s history, and especially about whether and when water may have covered parts of the planet. Water is essential for life as we know it.

“Reaching Victoria Crater is like finding a Martian history book,” John Callas said. Mr. Callas is rover project manager at the Jet Propulsion Laboratory in Pasadena, Calif., where the rovers were designed and built. “There are so many more exposed layers than we’ve seen before on Mars, and geologists are very excited about what we might be soon seeing.”

The rover, which has found some evidence that liquid water once existed on the surface of Mars, is now headed for a small notch at the edge of the crater. From there, Mr. Squyres said, Opportunity will be commanded to take pictures of two promontories that project into the crater. Whichever looks most appealing will become the rover’s destination.

“We want to get Opportunity right to the edge and then take panorama pictures of the whole crater,” Mr. Squyres said. “Then we do our fundamental planning for the attack on the rest of the crater.”

Mars is sometimes as close as 36 million miles from Earth, but right now it is 250 million miles away, making the upcoming push by Opportunity perhaps even more remarkable. Across that distance, scientists led by Mr. Squyres will send radio commands to the rover tonight to roll forward, and then tomorrow night, they hope to send it to the lip of the crater.

“We want to get as close as we possibly can, but we obviously don’t want the rover to go crashing down into the crater,” Mr. Squyres said. “Every inch here matters, so we have to be cautious. But we also have to push forward as far as we possibly can.”