Scientists will attempt to make history this week by landing a robotic probe on the surface of a comet more than 300 million miles away.
European Space Agency (ESA) mission controllers will be holding their breath as the craft, called Philae, makes the seven-hour descent on Wednesday.
The spider-like probe will be released by the Rosetta spacecraft, currently orbiting comet 67P/Churyumov-Gerasimenko - a four-kilometre wide sooty lump of ice and dust - at around 8.35am UK time.
Seven hours later Philae is due to touch down on the comet's rugged surface, anchoring itself in place with two harpoons and whirling ice screws attached to each of its three legs.
If all goes well, it will be the first time a man-made object has ever made a controlled landing on a comet.
Nine years ago the US space agency Nasa's Deep Impact mission slammed a projectile into comet Tempel 1 to study debris from the blast.
In contrast, Philae will make a gentle descent at walking pace.
A radio message confirming the landing is due to reach scientists on Earth at around 4pm, having taken 30 minutes to travel 509 million kilometres (316 million miles) across space.
But there can be no guarantee that the mission, which scientists hope will provide valuable information about the origins of comets and the Solar System, will succeed.
Open University space scientist Professor Ian Wright, lead investigator for Ptolemy, a British-made box of electronics on Philae that will analyse material from the comet, said: "It's a very risky undertaking. The Rosetta mission as a whole has already been successful, with or without the landing, but here is a chance to do something extremely challenging that has never been achieved before.
"We know the odds of success are not 100%. There's not a lot of gravity and you've got to stay connected to the surface, but we still don't really know what it's like. If the surface is too soft you run the risk of sinking down into it, and it it's too hard you run the risk of bouncing off.
"My understanding is that some of the flatter regions that look favourable for landing are probably likely to be softer, which might not be what you want. It's all delightfully unknown.
"There are cliffs, boulders and plenty of surface features which have to be avoided. It's going to be a nerve-racking time for the engineers. I'll be keeping my fingers crossed."
Philae has a box-like outer casing covered in solar cells and is about the size of a dishwasher.
It carries a suite of 10 scientific instrument packages designed to measure virtually every characteristic of the comet, including its chemical composition, temperature and magnetic field.
Sensors on its legs will even test how sound travels through the surface.
A drill will extract samples to a depth of 23 centimetres which will be heated, turned into gas, and analysed in the probe's on-board laboratory.
The robot is also equipped with several cameras that will take panoramic and close-up pictures of the comet.
Ptolemy, an instrument about the size of a shoe box, will play an important role in the sample analysis, obtaining accurate measurements of "isotopic ratios" - the relative levels of different atomic "strains" of elements.
These are tell-tale indicators shedding light on the origin of comets and their links to the creation of planets.
Comets are thought to have carried both organic material and water to Earth, and may have been vital to the birth of life.
Prof Wright said: "The comet is very, very old. Analysing the material in a comet is like looking back in history; it's like a time capsule.
"I like to think of it as frozen primordial soup, and this is the stuff that rained down on the early Earth. The idea that comets may have brought the building blocks of life to Earth is one of the reasons why we want to study them."
He added: "I started work on this (mission) 20-odd years ago, and it feels like getting to the end of a marathon. We're all looking forward to reaching the finish line."
The landing site chosen for Philae, originally known simply as "Site J", has now been named Agilka after an island on the Nile in southern Egypt and was picked in a public competition.
Philae takes its name from another Nile island where an inscribed obelisk was found that helped Egyptologists break the code of the Rosetta Stone. Ancient buildings, including the famous Temple of Isis, were moved from Philae to Agilka when their original home was flooded during the building of the Aswan dams in the last century.
Before releasing the robot 22.5 kilometres (14 miles) from the landing site, Rosetta will effectively "dive-bomb" the comet to make sure Philae stays on target. It will then veer off on a trajectory that will permit permanent radio contact with the probe.
Rosetta, launched 10 years ago, finally caught up with the fast-moving comet in August after an epic four billion mile journey that took it across the asteroid belt.
It will travel with the comet as the object flies past the Sun, approaching as close as 118 million miles.