The American space agency’s New Horizons probe is in the midst of an encounter with a giant ball of ice and dust nicknamed Ultima Thule.
The flyby, taking place 6.5 billion km from Earth, is the most distant ever exploration of a Solar System object.
New Horizons should be filling its memory banks right now with a swathe of photos and other scientific data.
Once the probe has gone past Ultima, it will turn to radio home a status report that should arrive at 15:28 GMT.
This initial contact ought to give controllers a good idea of how New Horizons performed as it swept over the 30km-wide world just 3,500km from its surface.
“Go New Horizons!” enthused chief scientist Alan Stern at 05:33 GMT, the moment when the spacecraft would have been at its closest point to Ultima Thule in the flyby sequence.
“Never before has a spacecraft explored something so far away.”
Earlier, he said: “I’d be kidding you though if I didn’t tell you that we’re also on pins and needles to see how this turns out.
“We only get one shot at it. Nothing like this has ever been done before, and with any enterprise like this – there comes risk,” he told reporters.
The risk is that New Horizons runs into fragments of ice or rock in the vicinity of Ultima.
With the spacecraft moving at 14km/s, even particles the size of a grain of rice would shred its interior components.
But assuming all turns out well, New Horizons can begin to downlink the gigabytes of stored data, with the first close-up images set for release on Wednesday.
Ultima is in what’s termed the Kuiper belt – the band of frozen material that orbits the Sun more than 2 billion km further out than the eighth of the classical planets, Neptune; and 1.5 billion km beyond even the dwarf planet Pluto, which New Horizons visited in 2015.
It’s 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.
New Horizons has had its long-range camera trained on the object since August. But only on the eve of closest approach did Ultima really start to make an impression in images.
Mission scientist John Spencer presented a picture acquired on Sunday from a distance of 1.9 million km. It represented at that moment the best ever view of Ultima.
“It’s a blob, only a couple of pixels across,” he said. “But you can see from that blob that it’s an elongated blob; it’s not round. And so we’re already seeing there is some interesting shape to this thing.”
When the pictures taken at closest approach are returned, they should achieve a best resolution of about 33m per pixel – more than sufficient to trace different features on Ultima’s surface.
Mission control for the project is based at the Johns Hopkins University Applied Physics Laboratory in Maryland. It got a sprinkle of celebrity stardust on Monday with the appearance of Queen guitarist Brian May.
The rock legend has written a song for New Horizons, and with a PhD in astrophysics he also plans to work on some of the probe’s images.
“This mission is about human curiosity. The need of mankind to explore and see what makes the Universe tick. My song is an anthem to the human endeavour,” he said.
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.
It’s 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. The latest image from Dr Spencer would seem to bear this out, but time will tell.
Distant telescopic observations have suggested Ultima’s 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 probably the result of having been “burnt” through the eons by high-energy radiation – cosmic rays and X-rays.
New Horizons will study Ultima’s shape, rotation, geology, 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 blizzard of pebble-sized grains.
What can we expect from the flyby?
Don’t blink, you might miss it. As we’ve already seen, this flyby isn’t like the one at Pluto where the images became increasingly – but also gradually – resolved on approach. Ultima will flash by and the best pictures will be obtained in a very narrow window.
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, but this does all depend on accurate pointing.
And, remember, because New Horizons has to swivel to train its instruments on a target, it cannot keep its antenna locked on Earth while also gathering data.
Controllers must therefore wait until later on New Year’s Day for the probe to “phone home” a status update and to start to downlink some choice pictures.
The “hey, I’m healthy and I’ve got a treasure trove of data” message should be sent about four hours after closest approach.
Just how big a challenge is this flyby?
In some ways, this event is more difficult than New Horizons’ pass of Pluto.
For one thing, the object in the viewfinder is almost a hundred times smaller.
New Horizons really must get its pointing right or it could be sending back pictures of empty space.
And then there’s the issue of doing everything at a separation from Earth of 6.62 billion km (4.11 billion miles).
It takes radio signals six hours and eight minutes to traverse that space.
What is more, the distance combined with a 15-watt transmitter on the probe means the data rates are glacial – around 1,000 bits a second.
It will be late on Tuesday before the first selected images are downlinked, and it will be September 2020 until every last scrap of data from the flyby is pulled off New Horizons.
The BBC’s 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.
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