NASA’s Mars rovers are celebrating five years of activity on the Red Planet. It has been five years since 3 January, 2004, when the space agency launched on Mars its first robot called Spirit. The second robot named Opportunity followed 21 days later.

NASA officials said back then they hoped the two solar-powered rovers would last at least three months, but nobody was expecting them to still be active after five years in the freezing Martian conditions.

"That's an extraordinary return of investment in these challenging budgetary times," Ed Weiler, associate administrator for NASA's Science Mission Directorate, said last month.

Spirit and Opportunity, the two “incredibly resilient” rovers, made more than 13 miles of tracks on Mars' dusty surface and sent more than 250.000 images to Earth. With their help, NASA experts uncovered evidence that Mars was once a warmer and wetter planet.

Scientists working with the space agency were expecting the rovers to come to a definitive halt once the amount of dust on their electricity-generating solar panels would get too thick. However, the occasional gusts of wind cleaned the panels from time to time.

Currently, Spirit is the most affected by accumulation of dust. The 18-month buildup of dust made its solar panels almost unable to provide the rover with the power needed during the planet’s southern hemisphere winter which just ended.

Nevertheless, NASA officials are aware of the fact that either rover could fail at any time without any warning.

"We realise that a major rover component on either vehicle could fail at any time and end a mission with no advance notice,” said John Callas, project manager for Spirit and Opportunity at Nasa's Jet Propulsion laboratory located in Pasadena, California.

Spirit is “celebrating” the five years of activity through work. The rover is currently exploring a 150km-wide bowl-shaped depression known as Gusev Crater where it has found numerous rocks and soils.

On the other hand and on the other side of the planet, Opportunity is currently exploring a flat region known as Meridiani Planum.

Both robots found conclusive evidence that there was liquid water on Mars’ surface.