The U.S. space agency NASA has marked the fifth anniversary of the landing of the Mars rover Spirit which touched down on the red planet on January 3, 2004.

Mars Rover

The rover Opportunity landed 21 days later. The two rovers were designed to last three months in the harsh Martian environment, but they remain operational and are still relaying data they collect to scientists on Earth.

Mission managers say the Mars rovers have traveled more than 21 kilometers across the surface of Mars and sent back a quarter-million images.

NASA says either rover could fail without warning, but scientists are pressing ahead with plans for further exploration.

Scientist Steve Squyres says the rovers are significant not only for the data they collect, but also for being humanity's first overland expedition on another planet.

Some information for this report was provided by AP.