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A NASA spacecraft must make a historical survey at the beginning of the year: that of the furthest celestial object ever studied, an icy relic from the beginnings of the solar system called Ultima Thule.
At about 6.4 billion kilometers from us, the New Horizons spacecraft must fly over this space object at 5H33 GMT on January 1, just 3,500 kilometers from its surface. That's three times closer than the distance she approached Pluto in 2015.
But what is this strange celestial object, named after a distant island of medieval literature, whose overflight will have its anthem, composed for the occasion by the Queen's guitarist, Brian May, who also holds a PhD in astrophysics.
"This is really the most primitive object ever encountered by a spacecraft," said Hal Weaver, Johns Hopkins University Applied Physics Lab.
Scientists are not sure of its size, estimating it about 100 times smaller than the Pluto dwarf planet whose diameter is close to 2,400 km.
And Ultima Thule evolves in an area with negative temperatures, which suggests that it is well preserved.
"It's really a remnant of the formation of the solar system," says Mr. Weaver.
– "Attic" of the solar system –
Ultima Thule is located in the Kuiper belt, a vast cosmic record remaining from the time of planet formation that astronomers sometimes call the "attic" of the solar system.
Discovered only in the 1990s, this belt is some 4.8 billion kilometers from the Sun, beyond the orbit of Neptune, the planet that is the furthest away.
It is literally teeming with billions of comets, millions of objects like Ultima Thule – which are called planetesimals, celestial bodies from which planets have been formed – and a handful of dwarf planets the size of a continent, like Pluto ", explains Alan Stern, one of the leaders of New Horizons.
"This is important for us in the science of planets because this region of the solar system, so far from the Sun, preserves the original conditions of 4.5 billion years ago," he adds. "So when we fly over Ultima, we'll be able to see how things were at the beginning."
The New Horizons spacecraft, launched by NASA in 2006, travels the universe at 51,500 kilometers per hour. At this pace, if it hit a debris as small as a grain of rice, it could be destroyed instantly.
While flying over Ultima Thule, if she survives unscathed, she'll take hundreds of photos that should reveal their shape and geology for the first time.
New Horizons sent stunning images of Pluto in 2015 – some of which revealed a heart shape that had never been seen before.
– Accurate navigation –
This time, "we will try to take images at a resolution three times higher than we had for Pluto," says Stern.
But, according to him, flying over "requires an extremely precise navigation, much more precise than the one we experienced before".
The Hubble Space Telescope discovered Ultima Thule in 2014. Scientists realized three years later that it was not spherical but possibly elongated and even that it could be two objects distinct.
And it does not project the repeated light that scientists expect to see of a rotating object, raising many questions.
Could it be surrounded by cosmic dust? Many small moons? Oriented in such a way that its poles face the approaching ship?
The US space agency hopes the flyover will provide answers.
The first images should arrive on the evening of January 1st, and be made public the next day.
Live broadcasting is impossible with such a distance, but NASA provides a transmission over the internet during the flyby, with the song of Brian May in soundtrack.
"Bringing together these two aspects of my life, astronomy and music, was an interesting challenge," former comedian Freddie Mercury explained in a statement.
Alan Stern hopes this mission will not be the last for New Horizons. According to him, the ambition is to hunt other objects of the Kuiper belt and to "fly even further in 2020".
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