- NASA has sent its $US3.26-billion Cassini Saturn mission into a death spiral.
- Called the “Grand Finale,” the months-manoeuvre is meant to protect possible alien life on Saturn’s ocean-hiding moons.
- Cassini will be destroyed in Saturn’s clouds around 8:32pm AEST on Friday.
The end is nigh for Cassini, a spacecraft that launched in 1997 and has explored Saturn and its moons for 13 years.
However, scientists will squeeze every last discovery they can from the probe during its final moments.
NASA is destroying the nuclear-powered robot early Friday morning because it has run very low on propellant. Burning that fuel has led to countless discoveries, including a giant hexagon on Saturn’s north pole and a vast ocean of liquid water — and possibly alien life — below the icy crust of the moon Enceladus.
But it’s also created a problem, since the spacecraft is dusted with earthly microbes.
“Cassini’s own discoveries were its demise,” Earl Maize, an engineer at NASA’s Jet Propulsion Laboratory (JPL) who manages the Cassini mission, previously told reporters. “We cannot risk inadvertent contact with that pristine body.”
Instead of chucking Cassini into the void of space, mission managers in 2010 decided to squeeze every last ounce of the probe’s fuel tanks at Saturn.
With one last big burn in April 2017, the probe began a spectacular death spiral called the “Grand Finale.” This manoeuvre slipped Cassini through a veritable cosmic keyhole: a small gap between Saturn and its rings.
So far, the probe has made 22 ring crossings. On Friday, it will make one final orbit, plunge into the Saturnian atmosphere, and burst into light as an artificial meteor.
“Cassini has got to be put safely away,” Maize previously said. The decision was made at the recommendation of NASA’s planetary protection office.
With just days left in the mission, here’s what to expect and when, according to NASA JPL.
Note: Since beams of light (and data transmissions) take more than an hour to reach Earth from Saturn, all times are from Cassini’s vantage unless otherwise noted.
Final Grand Finale data download
When: Friday, Sept. 9 — 11:07pm AEST
What: Cassini beamed back images and other data from the probe’s final crossing between Saturn and its rings.
Titan’s ‘goodbye kiss’
When: Tuesday, Sept. 12 — 5:04am AEST
What: A flyby of Titan, a moon the size of planet Mercury, that put Cassini on course to slam into Saturn.
“Instead of passing safely into and out of Saturn’s outermost atmosphere,” NASA JPL wrote on its site, “Cassini will instead dip so deeply into the atmosphere that the spacecraft will burn up like a meteor.”
Soaring before the plunge
When: Tuesday, Sept. 12 — 3:27pm AEST
What: Cassini reached its farthest point from Saturn, called an orbital apoapsis, before beginning its final descent toward the planet. The spacecraft was about 800,000 miles away from Saturn.
Last images of Titan
When: Wednesday, Sept. 13 — 9:56am AEST
What: All of the images Cassini took of Saturn’s moon Titan during its “goodbye kiss” were sent home.
Speeding toward doom
When: Tuesday, Sept. 12 – Friday, Sept. 15
What: Cassini will gradually accelerate as Saturn — a planet 95 times the mass of Earth — drags the probe toward its destruction. The robot will reach a speed of about 78,000 mph before splintering into glowing, meteoric pieces.
Cassini’s final photos
When: Friday, Sept. 15 — 5:56am AEST
What: The probe will take one final image before shutting down its camera system. That picture will be of Saturn “looking toward the dark side of the planet at the impact location” in infrared light, plus — in visible light — “a fairly dark observation showing in the area lit by reflected light from the rings,” Preston Dyches, a spokesperson for NASA JPL, told Business Insider in an email.
Dyches added: “Prior to that, the last image will be of one of the propeller features in the rings.”
Live transmission begins
When: Friday, Sept. 15 — 6:22am AEST
What: Cassini reorients itself so that its big, non-moving antenna dish is pointed toward Earth, allowing NASA to download all data it’s recorded (included the final photos). The spacecraft will fight to maintain this position for the next 14 hours and 30 minutes — right until the moment it burns up — so that it can transmit atmospheric and other data in real-time.
Australia tracks Cassini
When: Friday, Sept. 15 — 1:15pm AEST
What: A Deep Space Network station in Canberra, Australia, will point its giant radio dishes at Saturn to receive track Cassini and receive its final signals until the probe dies.
The Grand Finale — Friday, Sept. 15
Spacecraft roll, live transmission starts
When: 5:14pm AEST
What: A few hours before burning up, and while keeping its antenna pointed at Earth, Cassini will take five minutes to roll itself into a new position. This will point an instrument called the ion and neutral mass spectrometer, or INMS, toward Saturn — allowing NASA to “sniff” the planet’s atmospheric gases.
As Cassini rolls, its computer will reconfigure for live transmission of atmospheric data. The bandwidth will be just 3.4 kilobytes per second — about 830 times slower than the average download speed of a US mobile phone — but it will be enough to get crucial data about the composition of Saturn’s gases home to Earth.
Atmospheric entry, thrusters fire
When: 8:31pm AEST
What: Cassini starts to plunge into the outer fringes of Saturn’s thick atmosphere — something the probe was never designed to do. It won’t go silently to its death, though: Cassini will start firing its position-changing thrusters at 10% of “open throttle” to keep its antenna dish pointed at Earth as gases buffet the spacecraft.
Loss of contact
When: 8:32pm AEST
What: About a minute after ramming into Saturn’s atmosphere at up to 78,000 mph, Cassini’s computer will boost the thrusters to 100% to keep the live transmission going.
However, the robot won’t win this battle. Maize and Julie Webster, an aerospace engineer and manager of the Cassini spacecraft, told reporters during an August 29 teleconference that the probe has about 60 lbs of propellant out of the 6,900 lbs it started with to use — not enough to right the antenna during the entire descent. At some point Cassini will begin to tumble toward its doom.
When: Seconds to minutes after signal loss
What: Cassini will heat up more rapidly the deeper and faster it dives. In fact, “temperatures around the spacecraft will increase by 30-to-100 times per minute” as it descends, NASA said.
Insulating gold-coloured blankets will char and break off first, followed by Cassini’s antenna, 30-foot-long magnetometer boom, and other loose or fragile parts. Carbon blocks full of plutonium-238 fuel will last the longest.
“It will basically disintegrate … long before we hit any real surface of Saturn,” Webster said. “Not too long after we lose signal, we’ll have already started to be 200 to 500 degrees centigrade — within seconds. We’ll start to melt. All parts of it.”
“The deepest it could possibly go in the atmosphere is about 200 kilometers, or 120 miles,” Eric Strum, a Cassini mission planner, said on the call. He added that Cassini will disintegrate “thousands of kilometers” above what scientists consider the “surface” of Saturn — where air pressure is the same as on Earth’s surface.
Once fully melted, NASA said, “Cassini’s materials will sink deep into Saturn and mix with the hot, high-pressure atmosphere of the giant planet to be completely diluted.”
Photos of Cassini exploding(?)
When: 1 hour, 23 minutes, and 28 seconds after Cassini is destroyed
What: This is about how long the bursts of light caused by Cassini’s death will take to reach Earth. (Saturn will be some 932,822,000 miles away at that moment, according to “NASA’s Eyes on the Solar System” software.)
The Hubble space telescope might have recorded Cassini’s death in ultraviolet light — the strongest signal the probe will emit as it burns up. But Linda Spilker, a Cassini project scientist and a planetary scientist at NASA JPL, told Business Insider that Hubble won’t be in a position to see Cassini die.
Spilker is hopeful that professional observatories and hobbyist astronomers in the southern hemisphere catch a glimpse, especially if several kilograms of Cassini’s hydrazine fuel explodes brightly in the final moments.
“We’ve got sort of the double-whammy of a little tiny spacecraft that’s really not that massive hitting on basically the day side of Saturn. So it’s unlikely, but it’s definitely worth looking,” Spilker told Business Insider. “It’s gonna be tough, but I’m hopeful.”
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