Nasa's New Horizons spacecraft is all set for its dramatic encounter with a far-off icy world.
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By Jonathan Amos BBC Science Correspondent
Artwork: New Horizons at Ultima Thule Image copyright NASA/JHU-APL/SWRI Image caption Artwork: At this stage, scientists can only speculate what Ultima Thule looks like
Controllers have sent Nasaʼs New Horizons spacecraft a final set of commands ahead of its historic flyby of a distant icy world on Tuesday.
The probeʼs pass of the 30km-wide object known as Ultima Thule will set a new record for the farthest ever exploration of a Solar System body - at 6.5 billion km from Earth.
The upload included a two-second timing correction that ensures New Horizons knows precisely when and where to point its cameras as it sweeps alongside its target at a breathtaking 14km/s.
"The spacecraft is healthy and weʼre excited!" said mission operations manager Alice Bowman when she briefed reporters at the control centre embedded in the Johns Hopkins University Applied Physics Lab (JHU-APL) in Maryland.
New Horizons should get to about 3,500km from the surface of Ultima, with the moment of closest approach set for 05:33 GMT.
It is programmed to acquire gigabytes of photos and other scientific data in the hours leading up to and just beyond the encounter.
Ultima is in whatʼs termed the Kuiper belt - the band of frozen material that orbits the Sun beyond the eighth of the classical planets, Neptune. And Ultima is further out in space even than the dwarf planet Pluto which New Horizons visited in 2015.
Itʼs estimated there are hundreds of thousands of Kuiper members like Ultima, and their frigid state almost certainly holds clues to the formation conditions of the Solar System 4.6 billion years ago.
Hal Weaver, the JHU-APL project scientist on the mission, said the diminutive body was "probably the most primitive object ever encountered by a spacecraft, the best possible relic of the early Solar System".
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Media captionNew Horizonsʼ chief scientist Alan Stern: "Itʼs pure science and pure exploration"
New Horizons has been taking hundred of pictures of its quarry on approach. And although Ultima has appeared only as a faint dot in these images, the information gleaned from them has been critical in helping the mission team develop the navigation model that will be used to guide the probe through its encounter.
But they have also thrown up something of a puzzle: the light reflected off the object is much flatter than expected for a body that is anticipated to be irregular in shape and rotating.
Several explanations for the lack of variation in the "light curve" have been proposed. One could simply be the geometry of the viewing angle; another is that Ultima is actually two or more objects moving about each other.
This latter possibility has long been a topic of speculation.
Why is New Horizons visiting Ultima Thule?
Nasa wanted to explore something beyond Pluto and this object was reachable.
Remarkably, it was only discovered four years ago by the Hubble telescope.
Initially catalogued as (486958) 2014 MU69, it was given the more catchy nickname of Ultima Thule (Pronounced: Tool-ee) after a public consultation exercise.
Itʼs a Latin phrase that means something like "a place beyond the known world".
Like many Kuiper belt objects of its size, it is likely to be composed of a lot of ice, dust and maybe some larger rock fragments, which came together at the dawn of the Solar System.
Theory suggests such bodies will take on an elongated or lobate form. Think potato or peanut.
Distant telescopic observations suggested its surface is very dark, with a bit of a red tinge. That darkness (it reflects only about 10% of the light falling on its surface) is the result of having been "burnt" through the eons by high-energy radiation - cosmic rays and X-rays.
New Horizons will study Ultimaʼs shape, rotation, composition and environment.
Scientists want to know how these far-off worlds were assembled. One idea is that they grew from the mass accretion of a great many pebble-sized grains.
What can we expect from the flyby?
Donʼt blink, you might miss it. Unlike the encounter with Pluto in July 2015, there wonʼt be increasingly resolved images on approach to admire. Ultima will remain a blob in the viewfinder pretty much until the final hours of the flyby.
However, the much reduced separation between the probe and Ultima (3,500km versus 12,500km at the dwarf planet) means that finer detail in the surface will eventually be observed. Features as small as 33m across should be discernible if the pointing of the cameras is spot on.
Because New Horizons has to swivel to point its instruments, it cannot keep its antenna locked on Earth while also gathering data.
Controllers must therefore wait until later on New Yearʼs Day for the probe to "phone home" a status update and to start to downlink some choice pictures.
The "hey, Iʼm healthy and Iʼve got a treasure trove of data" message should be picked up by Nasaʼs network of big radio dishes at 15:28 GMT.
Image copyright NASA / JHUAPL / SWRI
Image caption The detail in the highest resolution pictures should be better than New Horizons achieved at Pluto
Just how big a challenge is this flyby?
In some ways, this event is more difficult than the pass of Pluto.
The object in the viewfinder is almost a hundred times smaller.
New Horizons will get closer than it did to Pluto, which is good for image detail; but it means that if the pointing is off, the probe could be sending back pictures of empty space.
And this really is a major concern. Because Ultima was only discovered four years ago, its position and movement on the sky are much more uncertain than the coordinates for Pluto.
Hence the importance of the final timing commands uploaded to the spacecraft on Sunday.
And, remember, all this is being done at a distance of 6.62 billion km (4.11 billion miles) from Earth.
At that separation, radio signals take six hours and eight minutes to reach home.
What is more, the data rates are glacial - around 1,000 bits a second.
It will be late on Tuesday before the first of a few choice images in downlinked, and it will be September 2020 until every last scrap of data from the flyby is pulled off New Horizons.
The BBCʼs Sky At Night programme will broadcast a special episode on the flyby on Sunday 13 January on BBC Four at 22:30 GMT. Presenter Chris Lintott will review the event and discuss some of the new science to emerge from the encounter with the New Horizons team.
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos
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