It has survived four times longer than its designers expected, but now things are really going to get tough for the Sojourner rover on Mars. After four weeks of working non-stop seven days a week, its badly depleted battery will now at times be recharged from solar power, and it will effectively have to shut down overnight.

Though its controlling scientists are delighted with its work, and expect it could continue for a long time, in a few weeks the rover will face its toughest test since arriving on 4 July. In autumn, huge dust storms, with winds that could exceed 160km/h and which can last weeks or months, start.

That will limit the scientific work the rover, about the size of a microwave oven, can do. But scientists at the Nasa US space agency have already said they are satisfied with the mission, which began when the Pathfinder crash-landed into a dried-up river valley near the Martian equator four weeks ago after an eight-month journey.

Matthew Golombek, one of the Pathfinder project scientists, said that so far the mission has achieved all its scheduled scientific and engineering goals. The team still plans for Sojourner to do spectroscopic examinations of a number of other rocks in the immediate vicinity of the lander. But last night the team at mission control in Pasadena were waiting to see whether a two-day rest, the first weekend off Sojourner has had, had succeeded in fully recharging its batteries.

The dust-filled atmosphere and swings in temperature, which ranges from a high of -12C during the day to -76C at night, have made it more difficult to keep the battery charged.

Initially, Nasa had expected the Sojourner's batteries might survive for just a week before the charge-discharge cycle would become unsustainable.

But it has kept going for much longer, performing spectrometer studies to establish the composition of rocks that the scientists have dubbed Shark, Half Dome and Wedge.

The lander has been sending back increasingly high-quality pictures and data of the Martian sunrise and sunset.

The spectacular images have come from a growing amount of data, with roughly 50 megabytes, equivalent to that stored on nearly 40 floppy disks, sent over the millions of miles through space from the nearest planet.

The dawn images revealed pale pink sunrises and clouds floating overhead, with reddish tints caused by Martian dust, composed of oxidized iron - like rust - which is present in the atmosphere. Frozen water-ice clouds are evident in the Martian sky during the early-morning hours, but evaporate once temperatures rise.

"We expect late-night and early- morning clouds, but we expect those clouds will burn off fairly rapidly with sunrise, giving way to a dusty Martian day," said Robert Haberle of Nasa. The sunset images showed a sky darkening to salmon-coloured hues.

Although there has not been much variation in these weather conditions since Pathfinder arrived, they are expected to begin changing in about a month, as the Martian autumn arrives, bringing the dust-storm season.

One image which has intrigued observers is that of a shiny object about 1,200m from the lander. But salivating ufologists were firmly put off by Michael Malin, a participating scientist. He said the object is about the same dimensions as Pathfinder, and is probably its discarded back- shell, which separated just before the spacecraft landed.