As Britain tucks into its lunch on Friday, hundreds of scientists, engineers and astronomers on the other side of the planet will be nervously watching the skies. Across California and Hawaii, hundreds of eyes will be trained on the moon, watching for the moment when a hi-tech orbiter – weighing more than 2 tonnes and travelling at 9,000kph (5,600mph) – plunges headlong into the lunar surface. The collision will throw a massive cloud of dust and debris up into space before, just a few minutes later, another, smaller, spacecraft follows suit and plummets to its doom.
For most people, it sounds like the stuff of nightmares. But when the impact takes place, the scientists working on the LCROSS mission will not be weeping but cheering – because this crash is happening on purpose.
Indeed, smashing into the moon's surface is the primary objective of LCROSS (the name stands for Lunar CRater Observation and Sensing Satellite). The basic idea behind the project, which launched into space in June, is to try to find out where water might exist in the darkest recesses of the moon's south pole. The best way to do that, apparently, is to throw a spacecraft at it and then examine the debris thrown up by the impact.
More precisely, LCROSS hopes to plunge itself into an area of the moon that appears to be the most likely candidate for harbouring water, and push up a cloud of dust (known as the ejecta) that will allow scientists to photograph and study it for evidence of H2O.
"When the impact occurs there is a brief flash that lasts about 100 milliseconds, followed by the material that has been excavated by the event is lifted up and will fall back into the moon – back into the crater," says Dr Kim Ennico, a scientist based at the Nasa Ames research centre in California, where most of the work on LCROSS has taken place.
That flash of light won't be visible to the naked eye, but for those west of the Mississippi and lucky enough to have a telescope of at least 25 centimetres, it will be the first time that sunlight has ever been cast on the material scooped out of the shadows that cover the moon's south pole.
"We just get a brief glimpse of it; it will travel, we hope, as high as 10km above the surface," Ennico says. "That ejecta curtain, that plume, is going to last only one to one and a half minutes before it falls down and it's out of view."
It is easy to imagine that crashing a spacecraft into the moon involves little more than flinging it in the right direction – much like, say, throwing a ball or firing a missile. In fact, the opposite is true: when the mission reaches its climax at around 12.30pm UK time on Friday, it will be the culmination of more than two years of precise planning.
Indeed, the entire mission reads like a delicate extraterrestrial ballet. Soon after taking off from Florida, LCROSS used gravity to slingshot itself into a wide orbit around the Earth that would eventually coincide with the moon.
As it closes in on the moment of impact, the craft will divide in two. The Saturn fuel tank will detach from the body of the main LCROSS unit – which, stuffed with cameras and sensing equipment, acts as the mission's brain. The empty fuel tank will then hit the moon at a sharp angle while the shepherding craft spends several minutes filming the first impact, analysing the dust cloud and sending information back to ground control. In addition to those observations, an array of telescopes and cameras on Earth and in space will be watching to grab images.
Then the second vehicle will also smash into the moon's surface – providing another bite of the cherry for those observing from thousands of miles away.
"These and several other telescopes participating in the LCROSS observation campaign will provide observations from different vantage points using different types of measurement techniques," says Jennifer Heldmann, who is leading the observation effort.
This is not the first time that a spacecraft has been used as an ad hoc missile. Four years ago another Nasa mission, Deep Impact, purposely collided with the Tempel 1 comet to try to unlock the "primordial soup" inside it. Earlier this summer, meanwhile, the Japanese Kaguya satellite was purposely crashed into the moon after having spent a year orbiting it.
But LCROSS is different from preceding kamikaze missions: bigger, more complex and – perhaps – more important than similar previous ventures.
After all, finding water on the moon has long been a dream for scientists, who want to understand more about its history – important for its own purposes, but also as a way of shedding more light on our own planet's background.
But LCROSS could also have more practical applications if it succeeds in providing evidence of water deposits in the unseen trenches of the lunar landscape.
The target being tracked by the team at Ames is a 98km-wide hole called Cabeus, which lies just a short distance from the Shackleton crater – the proposed location for Nasa's crewed lunar outpost, which it hopes to have completed by 2024. Discovering a potential water supply has obvious benefits for that scheme, not least providing astronauts with a potential reservoir that they can tap into rather than rely on shipments from Earth.
And while the existence of usable water inside Cabeus is still a hypothesis, the decision to target that crater is based on detailed information. Just a couple of weeks ago, the Indian Chandrayaan satellite appeared to confirm the existence of water in the region – subsequently confirmed by data from the Lunar Reconnaissance Orbiter, the vessel that LCROSS hitched a ride into space alongside.
Finding water on the moon would be a major breakthrough – LCROSS lead Anthony Colaprete has called it the "principal resource" in space exploration.
Jack Burns, a University of Colorado astrophysicist who heads the science committee for Nasa's advisory council, goes even further.
"I rank this as a game changer for lunar science," he said in the wake of Chandrayaan's findings. "In my mind this is possibly the most significant discovery about the moon since the Apollo era."
With all this at stake, the team behind LCROSS hopes it can find water rather than just evidence of it. But even if it does not, the mission has already broken new ground – not simply because of its unique objective, but also in the way it was put together.
Only approved in 2006, the project could herald a new generation of fast and inexpensive space missions. Although the numbers involved in bringing LCROSS to fruition are not small – the project has cost around $79m (£49m) and occupied a team of more than 50 for more than two years – it is a drop in the ocean compared with the $18.69bn the agency has budgeted for 2010 alone.
Such high speed and low cost is rare, and while LCROSS did not have to fund its launch directly (it piggybacked on an empty slot in the LRO mission instead) it defrayed other costs by using off-the-shelf components and constructing the satellite in an innovative way. Ennico, who worked on some of the cameras and instruments on board the craft, says that while the time and cost constraints proved challenging, they also helped shape a new view of how to approach space missions.
"I am used to the old Nasa paradigm of taking lots of years to build instruments – but that's usually when you're going after something you've never done before: you don't know what you're going to expect, it's new science and it's new areas you're going to probe, so you have to build a unique instrument," she says. "We found that there were instruments available that are used in industry – they're used in auto car racing, they're used in the military, they're used in environmental fieldwork ... in a sense, we turned it upside down."
That flexible approach also meant that many of the usual, rigid frameworks that apply to space travel have been dropped for LCROSS. For instance, the team has tweaked its target: just last week, they shifted from a plan to crash into the 48km Cabeus A crater to its larger neighbour, Cabeus proper, after data from LRO suggested that the new target would have a higher concentration of hydrogen.
A faster, cheaper future
"It's definitely a different paradigm for Nasa," Ennico says. "But it is a part of the Nasa of old – in which Nasa was launching a lot of missions very quickly on very tight time schedules."
That is an important message for Nasa, at a time when America, and the world, is again scrutinising space exploration. The shuttle project is about to shut down, and interest in expensive space missions faces opposition from politicians faced with more straightforward economic problems.
The age-old criticisms of the agency – expensive, slow and pointless – remain. Even a recent report from the group reviewing America's plans for human space flight, the Augustine Committee, suggested that Nasa simply doesn't have the money to do what it plans to.
It's nearly 50 years since John Kennedy made one of the great rallying cries for space exploration on the field at Rice University in Houston. Why do humanity's best choose to undertake difficult tasks, such as climbing mountains and crossing oceans?
"We choose to go to the moon," he said, famously. "We choose to go to the moon in this decade, and do the other things, not because they are easy but because they are hard."
And while it is unlikely that Kennedy would ever have got away with suggesting that we choose to go to the moon to crash, LCROSS does offer some hope of a faster, cheaper future for space exploration. So when those telescopes watch the moon on Friday, they won't just be anticipating a plume of dust – they will also be hoping that the hard things are about to get a little easier.