The US-led Cassini probe to Saturn will destroy itself in the coming hours.
The $4bn (£3bn) mission is ending 13 years of discoveries at the ringed planet by ditching itself in the atmosphere.
With an expected entry speed of 120,000km/h (76,000mph), the spacecraft will rapidly be torn to pieces.
Scientists, however, hope to gain new information on the chemical composition of Saturn’s gases just before Cassini loses radio contact with Earth.
That is likely to occur just after 04:55 local time here at mission control – the Nasa Jet Propulsion Laboratory (JPL) in Pasadena, California (11:55 GMT; 12:55BST).
It will be a bittersweet moment for the hundreds of mission researchers who have come from all over the world to be in the Los Angeles County town for the occasion.
Former US space agency chief scientist Ellen Stofan is part of the probe’s radar instrument group.
“We’ve done amazing science; it’s an amazing team. And I think we can celebrate that we’ve really eked every little bit of science that we could out of the Cassini spacecraft. But then it’s what’s next?
“We want to go back to Titan, we want to go back to Enceladus; there’s so much we don’t know about the interior of Saturn, so people have talked about Saturn probe missions. There’s a lot more to be done,” she told the BBC.
Cassini: A mission of ‘astonishing discovery’
Cassini has revolutionised our understanding of the sixth planet from the Sun.
It has watched monster storms encircle the globe, and witnessed the delicate interplay of ice particles move through its complex ring system.
And then there are those remarkable moons Titan and Enceladus, which host vast bodies of liquid water beneath their icy shells and where scientists say conditions may be favourable for simple life to exist.
But after taking hundreds of thousands of pictures and other measurements, it is now time for Cassini to retire.
It is down to its last few kilos of fuel and the US space agency does not want an uncontrolled probe wandering aimlessly around the outer Solar System. The ship is to be scuttled.
“This was the best solution,” said Earl Maize, the Nasa Cassini project manager. “We could have parked Cassini way outside the rings, and outside the icy moons. We could even have sent it away from the Saturn system but no-one saw any scientific benefit in that.”
In contrast, the “death plunge”, as it has become known, offers a way to get some unique data.
By tightening the probe’s orbit so that it flies in between the rings and the planet’s atmosphere, which it has been doing since April, researchers have gained new insights on the age of the rings, on the internal structure of Saturn and on the composition of its gas envelope.
The scientists will now take the ultimate step in these investigations by driving Cassini straight into the planet.
Eight instruments will be switched on and reporting conditions for as long as is possible.
This could be 60 seconds; with luck it will be a bit longer. But at some point the probe will not be able to maintain stability as gases rush over its irregular shape.
Cassini will start to tumble, break apart and melt – its materials dispersing to become a near-indistinguishable part of the planet it has worked so hard to describe.
The last elements to succumb will probably be the iridium casings on the spacecraft’s plutonium battery. These casings have very high melting temperatures.
Earth-based telescopes will try to see an entry flash, although the chances seem slim. Unfortunately, the Hubble space telescope will not be in a position to participate.
Saturn’s moons: The strange and the wonderful
Cassini has photographed some of Saturn’s 62 moons: the two-tone Iapetus (1) with its walnut-like equatorial ridge around its equator; Mimas (2), which has a giant crater that instantly makes everyone think of the “Death Star” from the Star Wars movies; Hyperion (3), which displays clusters of bizarre pock marks akin to a sponge or wasps’ nest; Atlas (4), which resembles a flying saucer; the potato-like Prometheus (5); and Pan (6), which has a shape that would not look out of place in a ravioli dish.
Controllers spent Thursday getting the last pictures off Cassini. These included a final look at the rings, at Enceladus and Titan, and the point in the atmosphere being targeted in the plunge.
The cameras will be off for the plunge. At a data rate of less than 30 kilobits per second, there is simply no point in trying to feed images in the end phase of the mission.
The mood here in Pasadena is a subdued one. Normally, the big occasions at JPL will mark “beginnings” – the landing of a rover on Mars, or a satellite’s arrival in orbit at a planet.
Some scientists admit to having already shed tears over the imminent end of the Cassini mission; others are in no doubt that the emotion will overcome them when radio contact is lost.
But all are convinced that with Cassini, they have been part of something very special.
“The discoveries at Enceladus are among the most astonishing ever made for planetary science,” said Nasa project scientist Linda Spilker. “To discover there is an ocean world so tiny with the possibility of life so far from the Sun has opened up our paradigm of where you might look for life, both in our own Solar System and in the exoplanet systems beyond.”
The Cassini-Huygens mission is a joint endeavour of Nasa, and the European and Italian space agencies.
BBC News will have live coverage of the ending of the mission on both TV and radio. Inside Science ran a preview of the climax on Radio 4. A Horizon documentary will also review the mission and the final hours in a special programme to be broadcast on Monday 18 September at 21:00 BST on BBC Two. And you can still watch the Sky At Night programme Cassini: The Gamechanger on the iPlayer.
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