After investigating some flat, light and dark toned rocks around Spirit of St. Louis Crater in April, Opportunity chalked up another milestone achievement for the Mars Exploration Rovers (MER) mission – the 4000th sol or Martian day of surface operations – then did what she does best. By month's end, the robot field geologist was closing in on Lindbergh Mound, the strange rockpile near the small crater's rim.

“There was something about seeing that number with the 4 and all those zeros and realizing, 'Okay, we are today planning Sol 4000,'You kind of go, 'Really? Wow!'" said Steve Squyres, MER principal investigator, of Cornell University, in a moment of reflection. "We just cracked four thousand. I don't know what else to say other than it’s astonishing and gratifying at the same time.”

John Callas, MER project manager at the Jet Propulsion Laboratory (JPL), home to all of NASA's Mars rovers, was on the same page. "Sometimes,” he said, “you have to pinch yourself.”

It has been written many times in these pages, and it begs repeating: this rover was sent on a 90-day expedition, with the mission success mobility objective of driving 600 meters. In March, Opportunity completed 42.195 kilometers or 26.2 miles. It’s the first marathon “run” on another planet. And in April – the 4000th sol. “This rover just keeps giving and giving,” said Planetary Society President Jim Bell, professor of astronomy and planetary scientist at Arizona State University and lead scientist on the MERs' panoramic cameras (Pancams).

So does the MER team. Down to a skeleton crew on a skeleton budget compared to Curiosity, the veteran rover and her ops team are still roving to new places and seeing new things every month, sometimes every week. Most importantly, they are uncovering history, producing solar-system-class science, and, chapter by chapter, writing the story of a time when Mars was more like Earth, a time no other mission is investigating.

Opportunity is blazing some of the first human trails on Mars and opening the windows on an alien planet that now looks familiar. The robot is still returning data almost every day and sending home rich, big picture panoramic postcards of breathtaking landscapes, like the one released this month of Spirit of St. Louis Crater. In these panoramic scenes, the planet humans want so desperately to explore is revealed in all its glory, as if you were standing right there looking out. “It’s been spectacular and continues to be spectacular,” said Bell. And so far, the radiation hasn't impacted the rover's brain.

The veteran rover spent much of April in workaday mode, finishing up the science campaign on the light and dark toned outcrops in the apron area surrounding Spirit of St. Louis Crater, the unusual, oblong-shaped depression that lies on the outer portion of the western rim of Endeavour Crater, and right at the entrance of Marathon Valley. From there, Opportunity drove to the rim of the small crater and 'landed' on a mound of rocks dubbed Lambert Field, as in the St. Louis International Airport. After checking out a rock there, the rover took a toe-dip into the small crater, and then backed out, and drove around to its southern end, to face Lindbergh Mound.