The U.S. space agency has extended the mission of the twin robotic Mars rovers by up to 18 months.

NASA says the solar-powered rovers, named Spirit and Opportunity, continue to function well - long past their planned lifetime.

The twin rovers have explored the red planet's terrain since January 2004. Among other findings, they discovered evidence that the dusty, rocky planet once had water.

This is the third extension of the mission, originally slated to last only three months.

The space agency says Opportunity has covered almost five kilometers of terrain, while Spirit is climbing a rocky Martian slope.

Some information for this report provided by AFP and AP.