At 8pm tonight, at the end of an 11-month journey, a robotic emissary from earth will parachute down on to the frozen tundra near the south pole of Mars.
At about four miles above the surface, a camera placed between its legs will start clicking. It will not take aim, it will just point where the spacecraft points. It will take 20 frames altogether, random tourist snaps of a planet as never seen before.
It will go on taking pictures - the last only 30ft from the surface - long after it has shrugged off its parachute and fired retro-rockets to slow its descent. At about 8.17pm the Mars polar lander will send a radio signal to say it is safely down. The message, even at the speed of light, will take 20 minutes to get back to earth: it will be 18.37 GMT before a team of space scientists and engineers in California heave a sigh of relief.
It will not be the only touchdown. Long before the lander makes its descent, it will release two little probes which have been riding along, piggyback, since the launch in January.
Two aeroshells, each the size of a basketball, will crash through the thin Martian atmosphere at 400mph. As they hit the rock of the Red Planet, they will splinter, revealing two dart-shaped probes that will plunge 6ft into the frozen subsoil.
In the nose of each, will be a little laser to seek evidence of water. At the back, will be an antenna and power pack to radio the results back to the lander, poised perhaps 40 miles away.
Back on the lander, a light detector and ranging radar experiment supplied by the Russians will start "reading" the dust levels and ice clouds in the Martian atmosphere. A stero camera will start making three dimensional and panoramic images of the sur rounding landscape. A 12ft-long robotic arm will dig into the terrain below to examine the soil chemistry.
A microphone will pop up, so that people on earth will begin to hear something once unimaginable: the sound of the wind on a planet 2.2bn miles away.
But the first moments tonight will be tense for planetary scientists: three probes bound for Mars have been lost this decade, the last, bearing a British experiment, only in October. The lander is one of a series of missions intended to solve the mysteries of Martian history: was it once swept by raging rivers? If so, where did they go? What happened to the thicker atmosphere needed for liquid water? Could water still be there, frozen under the soil?
Scientists chose the south polar site because evidence from previous missions suggested that the landscape had been scoured or shaped by the changing Martian climate.The photographs taken on the swinging descent are part of big picture, literally.
"For the first time we will have a complete scale of pictures of Mars, from less than a millimetre all the way up to orbiter pictures," says Peter Thomas, of Cornell university in New York state, one of the scientists behind the Mars descent imager. The mission will last 87 days, by which time the temperatures of the Martian winter night will have fallen to -90C, and the experiment's solar power supply will not be enough to keep it going.