It is not to red Mars, to shepherd rovers across its rock-strewn surface using probes and diggers to prospect for evidence of past, and possibly present, life.
It is not to build a probe to fly alongside a comet as it draws near the Sun, watching it change from a frozen mountain to a seething world of gas geysers and dust fountains trailing gossamer spirals in between the planets.
No, for many scientists neither of these missions is number one.The fact is that you cannot talk about Europa for very long without yearning to know more. It is Europa that many believe offers the best chance to find extraterrestrial life in our solar system.
This moon of Jupiter - the fourth-largest of its 12 satellites - is not a lot smaller than our own moon. It's a smooth, ice-crusted world, covered with frozen ridges and rafts. No known world is smoother. If you could stand on its icy surface, you would see nothing more than 1 kilometre high anywhere on Europa.
But it is not a featureless world. There are bright polar plains ringed by pits and plateaux. There are chaotic regions, and there are regular streaks that traverse vast distances across this tiny world. Some of these lines have an almost artificial look about them - as if they were motorways for some alien transport system.
Beneath the frozen landscape there are titanic energies at work. Tides probably cause the ice surface to rise and fall by 20 metres every three days.
The tidal energy is significant. As the ice and rock flex, their movement pumps energy into the interior of Europa, into the rocks below its icy skin. The interior rocks get hot. Because of this, scientists estimate that the ice just above the rock would melt, forming a liquid ocean under the ice.
So there it is. Four hundred million miles away there could be a water environment rich in chemicals and minerals and kept liquid by hot vents where superheated water gushes through fissures from the hot rocks below. All protected from the vacuum of space by a layer of ice more than 1 kilometre thick. This has been a stable environment for possibly tens of millions of years.
To many scientists this means that the essential ingredients are there for life to have evolved. But it's no use just sending a probe to orbit Europa; in order to see whether there really is life there, you have somehow to get beneath its skin and swim in its sunless seas.
It would perhaps be the most difficult space mission ever envisaged. At the moment it is not much more than a dream sketched out in a hazy, ill-defined plan.
The first stage is to send a space probe to orbit Europa, to determine whether there really is a liquid ocean underneath the moon's ice. Nasa scientists said this week that the plans for the probe's instruments are almost finalised, and an announcement is expected early next year, with a launch date scheduled for 2003.
After that, the real mission begins. This time there will be no attempt to enter orbit around Europa; instead the probe will make a direct descent to its icy surface. To stop its being dashed to pieces on Europa's hard ice, retro-rockets would fire to reduce speed. One minute before impact, huge, inflatable balloons would envelop the probe to cushion its impact, dissipating energy as it bounced across the ice.This system worked on Mars, which is a rough world. Europa is a smooth sphere, so it should work there too.
Imagine, in the not too distant future, the following sequence of events. The balloons deflate in the correct order, leaving the probe standing the right way up on the surface. The balloons release, into Europa's thin, almost non-existent atmosphere, more gas than it has known for billions of years.
Harpoons fired in several directions drag behind them curtains of thin foil, thin, flexible solar panels to add to the energy output of the craft's main nuclear generator.
The cylinder that forms the core of the probe heats up and begins to descend into the ice, boring a hole just 30cm across. Once the probe disappears, the water refreezes almost instantaneously.
Emerging hours later into the sea beneath the ice, the probe starts to fall faster. Tiny sensors detect its acceleration and pull the data cable taut, bringing the probe to a halt.
The first task is to wait, and listen. Sensitive microphones strain for the sounds of this alien ocean, the creaking of the ice above and the "ping" of an acoustic sounder. Then the bullet-shaped probe unfurls an array of sensors. A series of electronic tongues taste the water and provide a rapid chemical analysis.
Two searchlights, one narrow to look as far as possible, the other a wide beam, illuminate a sunless sea, the first bright lights in tens of millions of years, perhaps longer.
A smaller sub-probe is jettisoned and darts away, straight down, trailing a fibre-optic cable behind it. For long minutes it continues to fall. Nothing is seen yet by its television camera, other than a featureless pool of light. Then rocks come into view. Within a millisecond the Europa submersible's on-board computer has worked out how far below it lies the sea floor and how long it will take to get there.
Its descent is leisurely - it has many measurements to make on the way. Periodically it halts to take readings of temperature, pressure and water composition. Samples of the water are scooped into a tiny container, and a powerful microscope looks for any sign of microbial life.
Reaching the sea floor is only half of its journey. It now has to find a hydrothermal vent. Its sensors have detected a chemical gradient and it heads off in the direction of the increased concentration, moving upstream in a river of sulphur compounds swirling at the bottom of this strange, ice-covered ocean in a distant world.
After two hours of travel by the probe, scientists are wondering whether it will ever reach the vent. They are concerned for the power supply. Then they spot the vent as a red leer on the infrared display.
From now on, monitoring the water temperature around the probe is an all-important task. It would be all too easy to swim it into a column of superheated water that would melt the craft in seconds. To prevent this, thermometers on stalks precede the probe.
Its searchlights sweep the sea floor and the path ahead.
Soon the hydrothermal vent itself comes into view, a jumbled mound of rocks about five metres high. Out of the top of the rock pile can be seen a winding column of black water. Back on Earth such vents are nicknamed "black smokers", and are known for being oases of life on the barren sea floor.
The probe halts, its cameras focusing in on the area around the base of the mound. Could this be the moment we catch our first sighting of an alien life form?
Dr David Whitehouse is science editor of BBC onlineReuse content