Yesterday marked the 2,000th Martian day, or sol, of what was initially planned as a 90-sol mission on Mars for NASA’s Spirit rover.

Spirit’s twin, Opportunity, will reach the 2,000-sol milestone on Sept. 8. Both rovers have found rocks altered by past action of water on Mars. Both show some signs of aging but remain capable of further scientific investigations.

Since their landing halfway around the planet from each other in January 2004, Spirit has driven 4.8 miles and Opportunity has driven 10.7 miles. Together, they have returned more than 246,000 images. Each Martian sol lasts about 40 minutes longer than an Earth day.

The Opportunity rover has eyed an odd-shaped, dark rock, about 2 feet across on the surface of Mars, which may be a meteorite.

Scientists will be testing the rock with the particle X-ray spectrometer to get composition measurements and to confirm if indeed it is a meteorite.

The team spotted the rock called “Block Island,” on July 18, 2009, in the opposite direction from which it was driving. The rover then backtracked some 250 meters (820 feet) to study it closer.

Scientists will be testing the rock with the alpha particle X-ray spectrometer to get composition measurements and to confirm if indeed it is a meteorite.

(Above) After commanding five of a test rover’s six wheels to drive forward, rover driver Paolo Bellutta (left) measures how much the rover moved sideways, downslope, during the maneuver.

Using a test rover in a sandbox at JPL with special soil simulating Spirit’s predicament on Mars, engineers are assessing possible maneuvers for getting Spirit out and onto firmer ground. The tests began on Monday, July 6, with the simplest maneuver on their list of options: driving forward with all five operable wheels.

In the first set of tests, the wheels turned enough to cover tens of meters, or yards, if there had been no slippage. The test rover moved slightly forward and sideways downslope. Weeks of further testing and analysis of results are expected before engineers identify the best moves to command Spirit to make.

(Above) This painting by Pat Rawlings depicts NASA Astronauts hard at work repairing an unmanned Mars rover.