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What you need to know for 09/22/2017

Cassini's grand finale at Saturn: What to expect

Cassini's grand finale at Saturn: What to expect

Spacecraft orbited planet for 13 years
Cassini's grand finale at Saturn: What to expect
An artist’s illustration of the Cassini spacecraft above Saturn’s northern hemisphere.
Photographer: NASA/JPL-Caltech via The New York Times

PASADENA, Calif. — Cassini is accelerating to its end.

Early Friday, NASA’s Cassini spacecraft, which has been studying Saturn, its rings and its moons for 13 years, will dip a bit deeper into the planet’s atmosphere.

High above the cloud tops, the atmosphere is still thin, nearly a vacuum. “The analog of that on Earth might be where the International Space Station is,” Earl Maize, Cassini’s project manager, said at a news conference on Wednesday.

But at the speed Cassini will be flying, about 76,000 mph, the force of even a few molecules from Saturn’s atmosphere will be enough to tear the spacecraft to pieces.

“Cassini will be vaporized in maybe two minutes,” Maize said. “But I think more like one. It’s just inevitable.”

That is exactly as he and his team planned it.

Why is the Cassini mission coming to an end?

With Cassini’s fuel running low, NASA is cleaning up after itself, leaving the Saturn system as pristine as it found it. Any spacecraft, even one launched in 1997, has unwanted microbial hitchhikers aboard. In particular, planetary scientists want to ensure that there is zero chance of the spacecraft crashing and contaminating Titan or Enceladus, two moons that could be hospitable for life, with hidden passengers from Earth.

SCI_SATURN_CASSINI_6.JPG
The vortex at Saturn's north pole in an image from the Cassini spacecraft. (NASA/JPL-Caltech via The New York Times)

NASA did the same thing with its Galileo orbiter in 2003, sending it plunging into the clouds of Jupiter to protect Europa, another moon where scientists think life could exist.

How will the Cassini mission conclude?

The beginning of the end was Monday, when Cassini flew close to Titan, the biggest of Saturn’s moons, for the 127th time. The flybys have provided a close-up examination of an intriguing haze-shrouded world; Cassini’s navigators on Earth have also enlisted the flybys as gravitational kicks to send it to the next target.

This last flyby was “just close enough, just the right orientation to seal Cassini’s fate,” Maize said.

On Wednesday and Thursday, Cassini started taking a final set of photographs, of the rings, Enceladus, Titan and Saturn itself. One image will be the spot where Cassini will disintegrate.

At 5:45 p.m. Eastern time on Thursday, that final stream of images will start arriving on Earth. When that is complete, more than 10 hours later, “We will then reconfigure Cassini for its very final transmissions,” Maize said.

For most of the mission, Cassini collected observations and stored them in its memory to transmit to Earth late. On Thursday and Friday, there will be no time. Instead, Cassini will keep its main antenna pointed toward Earth and send data back almost as soon its instruments collects it. The transmission is too slow for photographs, so the camera will be turned off during those final hours.

SCI_SATURN_CASSINI_5.JPG
The moon Rhea that orbits Saturn in an image from the Cassini spacecraft. (NASA/JPL-Caltech via The New York Times)

NASA TV will broadcast live commentary online of Cassini’s end, beginning at 7 a.m. Eastern time on Friday.

Cassini’s last radio transmissions will disappear at 7:55 a.m., according to calculations by NASA engineers. The time of death at Saturn will have actually been one hour, 23 minutes earlier, but that is the time it takes the signals, moving at the speed of light, to travel the 1 billion miles that currently separate Saturn and Earth, picked up by radio telescopes in Australia and then sent to NASA’s Jet Propulsion Laboratory here.

Then, for the foreseeable future, there will be no new data coming from Saturn.

Scientists will be studying for years the information that Cassini gathered. But the engineers working on the mission will disperse to new projects.

“It’s a mix of sadness of Cassini ending, saying goodbye to this Cassini family we talk about,” said Linda Spilker, the project scientist. “We’ve been together, for lots of us, for multiple decades.”

Although the orbital maneuvers that will bring the mission to an end have been a top priority in recent weeks, Cassini’s scientific mission has carried on.

Since April, Cassini has been making daring plunges between Saturn and its innermost ring. For almost all of the 13 years there, Cassini has steered far from the rings to avoid the possibility of a fatal collision with a piece of rock or ice. With the impending end, now was the time to take bigger risks.

Cassini passed through the gap, unharmed, 22 times, providing a closest view yet of the rings. Along the way, it took a few final pictures of old favorites, Enceladus and Titan.

For the last five orbits, the spacecraft has dipped into the upper wisps of Saturn’s atmosphere, its thrusters firing to keep the spacecraft oriented in the right direction.

Then will come the final plunge, and this time Cassini will not emerge on the other side.

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