Spirit and Opportunity, the two NASA's Mars Exploration Rovers, were designed to resist only for 90 days. Defying all expectations, they are still in relatively good shape and doing science 1000 days after their landing on two opposite sides of Mars.

Paradoxically, the reason of their success is precisely what scientists have thought is going to be their doom: Mars' harsh weather conditions. The rovers were caught by storms but instead of being destroyed, their solar panels were cleaned of dust - which would have cut their power supply.

Spirit has traveled 6.9 kilometers (4.3 miles) so far and Opportunity 9.4 kilometers (5.8 miles). Opportunity has traveled so much comparatively to Spirit because it was aiming for the Victoria Crater.

One of the wheels of Spirit doesn't turn anymore and the NASA controllers have to drive the rover backwards. The rovers also cannot drill into the planet's surface anymore. Spirit has run out of abrasives while the arm of Opportunity's Rock Abrasion Tool has jammed.

Right now, the rovers are standing still trying to save just enough energy to get through Mars' winter for a third time. Mars is currently on the far side of the Sun as seen from Earth ("in conjunction"), so the Jet Propulsion Laboratory can hardly communicate with the rovers.

Spirit and Opportunity are staying put, taking some photos, making various measurements, and beaming them to Earth. Most of the data manages to pass through the solar interference. The planet will move out of conjunction on 10 November.

The image shows an evening twilight captured by Spirit on 19 May 2005 with the Sun setting behind the wall of the Gusev Crater about 80 kilometers (50 miles) away.

Photo credit: NASA / JPL / Texas A&M / Cornell