The Europa Clipper space mission is set to launch on Monday, the first step in determining whether our solar system is home to a second habitable celestial body other than Earth - a discovery that will have astonishing implications.
NASA's powerful Europa Clipper probe is preparing to begin its long journey to Europa, one of Jupiter's moons, which it is expected to reach in April 2030.
NASA has never before observed Europa in such detail, and scientists believe it has an ocean of liquid water beneath its icy surface.
The launch will take place from Cape Canaveral, Florida, using a SpaceX Falcon Heavy rocket.
NASA has said it does not plan to launch before 12:06 local time (16:06 GMT) on Monday.
NASA official Gina DiBraccio said at a press conference that "Europa is one of the most promising places to search for life beyond Earth".
The mission will not look directly for signs of life, but will answer the question of whether Europa is habitable, that is, whether it contains the elements that make life possible.
If so, another mission will have to go there to try to find those signs.
"This is an opportunity for us to explore not a world that may have been habitable billions of years ago, like Mars, but a world that may be habitable today," said the mission's science director, Kurt Niebuhr.
The Europa Clipper is the largest spacecraft NASA has ever designed for planetary exploration, measuring 30 metres wide when its huge solar panels, designed to capture the faint light that reaches Jupiter, are open.
The first close-up images of Europa, known to exist since 1610, were taken by the Voyager spacecraft in 1979, revealing the mysterious red lines on its surface.
The Galileo spacecraft then flew over it in the 1990s, confirming the very likely presence of an ocean.
This time, Europa Clipper is equipped with a large number of very advanced instruments, including cameras, a spectrometer, a radar and a magnetometer.
The mission will determine the structure and composition of Europa's icy surface, the depth of its ocean, and even its salinity, and how the two interact to find out, for example, whether water rises to the surface in some places.
All this to understand whether it contains the three essential ingredients for life: water, energy and some chemical compounds.
Bonnie Buratti, the mission's deputy science director, said that if these ingredients are present on Europa, life in the form of primitive bacteria would be present in its ocean, but at a depth too great for Europa Clipper to detect.
If Europa is not habitable, says Nikki Fox, NASA's associate administrator, "it also opens up a whole set of questions: Why did we think there might be signs of life? And why aren't there?"
The spacecraft will travel 2.9 billion kilometres in five and a half years to reach Jupiter. Once there, the main mission will last four years.
The spacecraft will make 49 close flybys of Europa, up to 25 kilometres above the surface.
It will be exposed to intense radiation, equivalent to several million chest X-rays per flyby.
Some 4,000 people have been working on this mission for almost a decade, at a cost of $5.2 billion.
NASA justifies this investment by the importance of the data it will collect.
If it turns out that our solar system has two habitable worlds (Europa and Earth), "think about what that means when that result is extended to billions of other solar systems in the galaxy," says Niebuhr.
"Even setting aside the question of whether there is life on Europa, the mere possibility that it is habitable opens up a new paradigm for the search for life in the galaxy," he adds.
Europa Clipper will work in parallel with the European Space Agency's JUICE probe, which will study two of Jupiter's moons, Ganymede and Callisto, as well as Europa.
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