We ended 2009 with one anniversary, and we start 2010 with another.

Six years ago on 4 January (GMT), Nasa's Deep Space Network picked up a signal sent 170 million km across the Solar System to Earth.

The Spirit rover [PDF] had landed on Mars.

Engineers promised an initial operational mission of 90 Martian days; and yet, this plucky six-wheeled "mobile geologist" just kept on rolling... and rolling... and rolling.

The US space agency has warned though, that this remarkable robot's days may now be numbered.

For the past nine months the vehicle has been stuck in a sand trap.

With just four working wheels (five intermittently), Spirit cannot get the traction it needs to free itself.

The concern is that the rover will not be able to position its solar panels to make the most of what will soon be a faint winter sun on the Red Planet. Without sufficient power, the robot will not be able to heat its systems, never mind run its science instruments.

If Spirit is unlucky and gets covered by more of the dust that can accumulate on its panels and block out the light (and it doesn't get the wind that can sometimes clean the cells), the rover could die.

Whatever happens in the next few months, the Spirit Mars Exploration Rover will be remembered as a magnificent success.

It was targeted at the 170km-wide Gusev Crater. Orbital images had suggested this near-equatorial location might once have held a giant lake.

The investigation of that watery history got off to a slow start.

Most people will have forgotten by now that Spirit stopped working 18 Martian days into its mission. It took engineers back on Earth about two weeks to find the fix for a flash memory glitch that made the rover constantly re-boot itself.

Once it did get going, Spirit found the volcanic rocks on the plain where it landed had undergone very limited alteration by exposure to moisture.

Only when it got into the Columbia Hills about 2.5km from its touch-down site did the robot discover some rocks and soils that had experienced extensive exposure to water.

One of its biggest finds was made by accident on the 1,158th Martian day of its mission. Spirit was by then driving backwards because its right-front wheel had jammed.

As it dragged this wheel through the soil, it dug a trough. And scientists studying pictures returned to Earth noticed how on this particular day, a bright material had been uncovered.

Detailed examination found this material to be almost pure silica. The rover team concluded the deposits had formed through the interaction of hot spring water or steam with volcanic rock.

On Earth, such locations tend to teem with bacteria. What's more, these types of hydrothermal environments will also entomb any lifeforms as the deposits are laid down.

No wonder the rover's lead scientist Steve Squyres said that of all the rock and soil samples he'd seen on Mars, this was material he really wished could be brought back to Earth to study in a laboratory.

Keep your fingers crossed for the plucky rover. You can follow its progress as Nasa continues to try to ease it out of the sand trap at the Free Spirit website.

A brief word also while I've got your attention on the future of Mars exploration. Just before the festive break, we got word from the European Space Agency that its member states had formally bought into a joint roadmap with Nasa.

The missions despatched in 2016 and 2018 will now be combined ventures.

The 2016 mission will be an orbiter [PDF] designed to track down the sources of methane and other trace gases recently detected at the Red Planet. It will also include a small, short-lived static lander to give Europe the opportunity to try to land something on Mars.

2018 will see two rovers fly to the planet - one European vehicle and one American [PDF].

It will be intriguing to follow the development of these spacecraft in the coming years. Esa has now updated its website to reflect its new ExoMars programme.

Watch this space.