Europe's first mission to Mercury arrives at its destination in the coming hours.
It'll be the briefest of visits, however.
The BepiColombo probe is moving too fast to go into orbit and will fly straight by the planet.
But the diminutive world's gravity will have slowed the craft just a little, and further passes in the coming years will eventually see Bepi take up a stable station around Mercury.
That'll be late 2025; patience is required.
For this first flyby, the timing of closest approach is 23:34 GMT, Friday (00:34 BST, Saturday). The flight path will take the probe to within 200km of the planet's surface.
Bepi will be snapping pictures, but not with its high-resolution science cameras.
These can't actually see anything at the moment because they are tucked inside what is referred to as the spacecraft stack.
Bepi is essentially two spacecraft in one. One part has been developed by the European Space Agency (Esa), the other part by the Japanese space agency (Jaxa). The way these two components have been mated for the journey to Mercury obstructs the apertures of the main cameras.
This means the mission's first images of Mercury must be acquired by a couple of monitoring, or engineering, cameras mounted on the exterior of the craft.
They will return simple black-and-white photos but of sufficient quality to make out some familiar features on the surface.
"I think we'll recognise Kuiper Crater. It's bright and has this big fan of ejecta rays," speculated Dave Rothery, a professor of planetary geosciences at the UK's Open University.
"We'll just have to wait and see. We know what should be in the field of view but given the lighting conditions and what these small cameras are capable of - there's some uncertainty," he told BBC News.
The imaging won't begin until after closest approach because that point comes when Bepi is still on the nightside of Mercury. But as the probe pulls away, it should see a clear view of the curved edge, or limb, of the planet. Esa promises to run all the pictures together to make a little movie, most probably for release on Monday.
Even though the spacecraft's instruments have to wait until the mission is properly in orbit in 2025 and the two halves can separate, there will still be some data-taking.
For the scientists behind the UK's Mercury Imaging X-ray Spectrometer, or MIXS, it's an opportunity to better understand the performance of their instrument.
MIXS's detectors pick up a general background noise of energetic particles known as cosmic rays.
"As we go really close to Mercury and one half of the sky is blocked by the planet, then we should see a dip in some of this noise that we've been getting, and that will help us to pinpoint the fact that it is galactic cosmic rays we've been detecting," explained Dr Suzie Imber from Leicester University.
The exercise will ensure the mission team gets the most out of MIXS when eventually it does start observing the planet later in the decade.
This first flyby puts Bepi in a two-to-three resonance with Mercury. That's to say, as Mercury goes three times around the Sun, Bepi will go around twice.
The next flyby in June next year, will slow this to a three-to-four resonance: Bepi will circle the Sun three times compared with Mercury's four circuits.
Further passes in June 2023, September 2024, December 2024, and January 2025 should see Bepi in a regular orbit to begin full science operations in 2026.
What science will BepiColombo do at Mercury?
The European and Japanese elements of the mission will separate when they get into orbit at Mercury and perform different roles.
Europe's Mercury Planetary Orbiter (MPO) is designed to map Mercury's terrain, generate height profiles, collect data on the planet's surface structure and composition, as well as sensing its interior.
Japan's Mercury Magnetospheric Orbiter (MMO) will make as its priority the study of Mercury's magnetic field. It will investigate the field's behaviour and its interaction with the "solar wind", the billowing mass of particles that stream away from the Sun. This wind interacts with Mercury's super-tenuous atmosphere, whipping atoms into a tail that reaches far into space.
It's hoped the satellites' parallel observations can finally resolve the many puzzles about the hot little world.
One of the key ones concerns the object's oversized iron core, which represents 60% of Mercury's mass. Science cannot yet explain why the planet only has a thin veneer of rocks.
"When we get into orbit, we'll then start studying the magnetic field at Mercury, and the surface of Mercury, which has huge temperatures of 450C, the temperature of a pizza oven, and yet it has water on the surface in some places," said Prof Mark McCaughrean, Esa's senior advisor for science and exploration.
"Mercury has a huge metal core. It's very much denser than it should be for its size. We just don't understand how Mercury got to be the way it is. So, there are huge mysteries about the origin of Mercury and that's what BepiColombo is designed to study," he told BBC News.
Europe's MPO was largely assembled in the UK by Airbus.
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2021-10-01 09:56:38Z
CBMiT2h0dHBzOi8vY2EubmV3cy55YWhvby5jb20vZXVyb3Blcy1iZXBpY29sb21iby1taXNzaW9uLWJlYXJzLWRvd24tMDAyNDI0OTM3Lmh0bWzSAVdodHRwczovL2NhLm5ld3MueWFob28uY29tL2FtcGh0bWwvZXVyb3Blcy1iZXBpY29sb21iby1taXNzaW9uLWJlYXJzLWRvd24tMDAyNDI0OTM3Lmh0bWw
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