The US space agency's (Nasa) Mars rovers are celebrating a remarkable five years on the Red Planet.
The first robot, named Spirit, landed on 3 January, 2004, followed by its twin, Opportunity, 21 days later.
It was hoped the robots would work for at least three months; but their longevity in the freezing Martian conditions has surprised everyone.
The rovers' data has revealed much about the history of water at Mars' equator billions of years ago.
"These rovers are incredibly resilient considering the extreme environment the hardware experiences every day," said John Callas, project manager for Spirit and Opportunity at Nasa's Jet Propulsion laboratory in Pasadena, California.
"We realise that a major rover component on either vehicle could fail at any time and end a mission with no advance notice, but on the other hand, we could accomplish the equivalent duration of four more prime missions on each rover in the year ahead."
Together, the rovers have driven more than 20km, and returned more than 36 gigabytes of data. This has included a quarter of a million images.
Spirit is exploring a 150km-wide bowl-shaped depression known as Gusev Crater. It has found an abundance of rocks and soils bearing evidence of extensive exposure to water.
Opportunity is on the other side of the planet, in a flat region known as Meridiani Planum.
Its data has shown conclusively that Mars sustained liquid water on its surface. The sedimentary rocks at its study location were laid down under gently flowing surface water.
The rovers are now showing some serious signs of wear and tear.
Spirit has to drive backwards everywhere it goes because of a jammed wheel; and Opportunity's robotic arm has a glitch in a shoulder joint because of a broken electrical wire.
There have been times also when the vehicles have been dangerously short on power because of the dust covering on their solar panels.
When Spirit and Opportunity do eventually fail, Nasa will have to wait awhile for its next surface mission.
It recently delayed this year's planned launch to 2011 of a much more capable vehicle, known as the Mars Science Laboratory (MSL). The rover project has been beset by technical and budgetary problems.
The decision was taken not long after Europe also put back its rover venture known as ExoMars. Officials cited cost concerns.
It is likely all surface missions in future for Nasa and the European Space Agency will be joint affairs because of the high cost of getting spacecraft down on to the planet.
Nasa lost contact with its static Phoenix lander in November. It was operating in much more difficult conditions at a high-latitude location.