If all went well last evening, Mars Polar Lander will now be resting comfortably somewhere in the high latitudes of the southern hemisphere of the red planet. One of its main missions during the next three months will be to find out more about the Martian weather.
The package of meteorological instruments abroad the spacecraft is known, appropriately enough, as MET.
It comprises a collection of sensors mounted on the lander at different heights to provide accurate observations of temperature, atmospheric pressure, humidity and wind from the Martian surface to a height of about two metres.
An instrument called LIDAR, for Light Detection and Ranging, will measure the height of any cloud, while the robotic arm of the spacecraft will provide soil samples for an analysis in situ to see, among other things, how much water, if any, it contains.
The trenches dug in the soil by the robotic arm will also be photographed, allowing the layering of the soil to be examined with a view to obtaining a geological record of past events affecting the Martian climate.
We already know a fair amount about the weather on Mars. Normal atmospheric pressure on Earth, if you remember, is about 1013 hectopascals; on Mars it is only seven.
Also, during the night the "air" temperature near the surface of Mars falls to about 100 Celsius, and on a good day rises to little more than 30 Celsius. This combination of a thin atmosphere and low temperature has a consequence which we on Earth would find intolerable: Mars has no running water.
Its average temperature is well below the ice point, and even if some ice were to be melted, the atmospheric pressure is so low that the liquid water would boil instantly and disappear as water vapour.
But it seems it was not always so. Images of the Martian surface obtained from previous missions to the planet show features that seem certain to have been formed by flowing water - deep channels and canyons, and what appear to be ancient lakeside shorelines. There is no general agreement, however, on what form this water might have taken all those aeons ago.
Some scientists suggest that Mars was once much warmer and wetter than at present, with a thicker atmosphere and an abundance of rivers, lakes and seas. Others are of the view that Mars was always cold but that water trapped as underground ice was periodically released when some form of heating caused the ice to melt.
In either case, what has happened to the water is something of a mystery. Perhaps Mars Polar Lander will shortly throw some light upon it.