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Cassini's end
An artist’s conception shows the Cassini orbiter zooming through the upper layers of Saturn’s atmosphere, heading for a fiery breakup. (NASA / JPL-Caltech Illustration)

Twenty years after its launch to Saturn, NASA has set the Cassini orbiter on a course for certain destruction on Friday – but there’s a decidedly positive spin to the $3.3 billion mission’s end.

“We’ll be saddened, there’s no doubt about it, at the loss of such an incredible machine,” Cassini program manager Earl Maize said Wednesday during a news briefing at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “But I think all of us are going to have a great sense of pride in .. a little bit corny, perhaps … a ‘mission accomplished.'”

The bus-sized, plutonium-powered spacecraft was launched in 1997 and reached Saturn seven years later. It’s logged 4.9 billion miles, sent back nearly half a million images of the ringed planet and its moons, and transmitted 635 gigabytes worth of scientific data so far.

It’ll continue sending data all the way to the end, when it’s expected to break apart and burn up in the upper levels of Saturn’s atmosphere.

The last batch of imagery, including a picture showing the area of Saturn where the breakup is planned, is being captured today and should be back on Earth tonight.

During the mission’s final minutes, Cassini’s robotic “nose” – known as the Ion and Neutral Mass Spectrometer, or INMS – will be sniffing the molecules above Saturn’s cloud tops.

“One of the most important scientific things that we’re trying to figure out is a concept known as ‘ring rain,’ said INMS science team leader Hunter Waite, a planetary scientist at the Southwest Research Institute.

Waite said scientists have long suggested that water ice particles are falling from Saturn’s rings into the atmosphere. “Ring rain is much more extensive than that. It’s much more complicated. … We’re trying to find out exactly what is coming from the rings and what is due to the atmosphere,” he said. “That final plunge will allow us to do that.”

The probe’s magnetometer and plasma science instruments, as well as the radio science system, will also be collecting data for real-time transmission back to Earth – at a rate low enough to ensure that the data’s received all the way up to the probe’s last gasp.

Cassini’s final signals are due to register at the Jet Propulsion Laboratory at 4:55 a.m. PT Friday, after making the 83-minute transit from Saturn to Earth at the speed of light.

13 years of discoveries

Thousands of scientists and engineers have worked for decades to prepare and execute Cassini’s scientific campaign, which lasted for 13 years at Saturn.

The orbiter and its piggyback probe, the European-built Huygens lander, have revealed rivers and lakes of hydrocarbons flowing on the surface of smog-covered Titan, Saturn’s largest moon.

Cassini’s readings have confirmed the presence of watery geysers spouting up from cracks in the icy surface of Enceladus, another Saturnian moon. Enceladus’ hidden ocean may even harbor life.

“Enceladus’ discoveries, made by Cassini, are certainly one of the most astonishing sets of discoveries for planetary science,” said JPL’s Linda Spilker, project scientist for the Cassini mission. “To find that there’s an ocean world so tiny, with the possibility of life so far from the sun, 10 times farther from the sun than the Earth, has opened up our paradigm of where you might look for life.”

The mission also gave scientists a closer look at tiny shepherd moons such as Pandora and Prometheus that keep Saturn’s rings in line, and mysterious structures in the rings that may one day spawn new moons.

To celebrate the mission’s end, NASA has assembled Cassini’s greatest hits into an ebook that’s freely downloadable in formats ranging from PDF to Kindle and iBooks.

What lies ahead

Intentionally destroying Cassini might seem like a waste, but it’s actually part of NASA’s plan to make sure the probe has no chance of crashing on Titan or Enceladus. Scientists want to make sure those moons, which could conceivably harbor traces of alien life, remain uncontaminated.

“We must protect those bodies for future exploration,” said Jim Green, director of NASA’s Planetary Science Division.

Throughout the mission, engineers have taken advantage of Titan flybys to adjust Cassini’s orbit, and the last flyby – known as “the kiss goodbye” – was executed on Monday, sealing the spacecraft’s fate. Images from that flyby were released during Wednesday’s briefing.

“What’s going to happen on Friday is absolutely inevitable,” Maize said.

Titan image
This unprocessed image of Titan was taken by NASA’s Cassini spacecraft on Monday during the mission’s final, distant flyby, known as “the kiss goodbye.” (NASA / JPL-Caltech / Space Science Institute)

The probe will blaze through Saturn’s thin upper atmosphere at a speed of 76,000 mph – a dive that will subject it to unsurvivable stresses. There’s a slight chance that observatories in places like Australia, India, China and Taiwan will be able to spot the flash when Cassini is vaporized, Spilker said.

It’ll take another year for Cassini’s science team to put all the data returned by Cassini into shape for NASA’s archives, and the readings are likely to fuel scientific studies for decades to come. What’s more, the observational techniques developed for Cassini are certain to be adapted for future missions.

For example, NASA’s Europa Clipper – a spacecraft that’s due for launch in the 2020s – will study Europa, an icy moon of Jupiter, using the same multiple-flyby technique that was pioneered by Cassini for mapping Titan.

So when will the next mission to the Saturnian system be added to NASA’s to-do list? In response to a question, Green hinted that it might not be long.

“The observations by Cassini have been so remarkable for Enceladus and Titan that indeed last year we announced the inclusion of those two objects in our focused science program called New Frontiers,” Green said. “Those proposals are in and currently under evaluation, and they do indeed include proposals to go back to Titan and Enceladus. So we’ll look through this competition and see what happens.”

NASA TV will air live commentary on the end of the Cassini mission from 4 to 5:30 a.m. PT Friday, followed by a post-mission news conference at 6:30 a.m. PT.

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