Richard A Lovett
NASA’s 20-year-old Cassini mission is about to make a final orbit correction that will cause it to crash into Saturn and burn up in September.
Before it meets its fiery demise, however, the long-lived spacecraft, which left Earth in 1997 and first entered Saturn orbit in 2004, will make a series of 22 close flybys, diving within 2,000 km of the giant planet’s cloud tops. Its final passes, in fact, would be so close that it would be able to sample the outer fringes of the planet’s atmosphere, Cassini project manager Earl Maize said at a press conference on April 4 in Pasadena, California.
This Grand Finale, as NASA is dubbing it, begins on April 22 when the spacecraft makes a flyby of Titan, using that moon’s gravity to turn toward Saturn. There it will dive through the gap between the innermost of Saturn’s rings and the giant planet’s atmosphere.
At each of these close approaches, the spacecraft will be traveling 122,000 km/h – fast enough that hitting anything more substantial than a speck of dust could damage it irreparably. “We would never take a flagship mission on that kind of course at any other time in the mission except when it’s about to end,” Maize said.
Cassini’s mission is about to end one way or another, because the spacecraft is on the verge of running out of maneouvring fuel. Project leaders decided years ago that they did not want to leave the spacecraft drifting without maneouvring power, for fear it might eventually crash into one of Saturn’s moons, potentially contaminating it with microbes that had hitchhiked all the way from Earth.
The Grand Finale does more than simply dump the spacecraft safely into Saturn’s atmosphere, where conditions are unsuitable for Earth life. It would also produce some exciting science, said Linda Spilker, the mission’s project scientist: “I would not be surprised if some of the discoveries will be the best we’ve obtained.”
One goal is to measure the mass of the rings more accurately than has previously been possible. That can help determine how old they are because ring material is being constantly eroded away by micro-meteorite bombardment from outside the ring system. Massive rings survive longer than less-massive ones, and thus might also be older.
It would also be possible, Spilker said, to determine what the rings are made of by studying the impacts of harmless, smoke-sized dust particles with Cassini’s cosmic dust analyser. “We know they are 99% water ice,” she said, “but we’re not sure about the other 1%. What is it? Iron, silicates, organics, a mix of all three, or something else we’ve not thought of?”
There will also be the best-yet close-up views of Saturn’s poles, which have giant hurricanes and a mysterious hexagonal feature Spilker called a “six-sided jet stream, two Earth diameters across”.
Some of the passes will even come close enough to measure trace constituents in Saturn’s upper atmosphere.
Measurements of Saturn’s gravity field will also allow the scientists to peel back its atmosphere and determine the size of its rocky core.
“The Grand Finale is like a brand new mission,” Spilker said.
Even on the final dive into the atmosphere, the spacecraft will continue to return data as it fights to keep its antenna pointing in the right direction. Not that it will be able to do this for long, because this is a flight for which it was never designed. Eventually, Maize said, it will lose contact, break up and vaporise.
There is, of course, a risk that Cassini will hit something big enough to damage it. Maize put the odds of successfully completing all 22 passes at 98.8%, based on the best models of the density and size of dust particles in the not-entirely-empty gap. “Our most dire models put us at 97%,” he said.
“If we get surprised, we have a bunch of contingency plans,” he added. Even if the spacecraft is knocked entirely out of commission, it is not at risk of hitting a moon, because from the moment it lines up for its upcoming Titan flyby, its orbit is determined by the laws of physics. “Cassini will still end up as planned, but we’ll get less science,” Maize said.
When the end comes, it’s going to be an emotional moment for the project team. “I’ve worked on it for almost three decades,” Spilker said of the mission. “My oldest daughter started kindergarten when I started working on Cassini. Now she’s married and has a daughter of her own. It’s really going to be hard to say goodbye to this plucky, capable spacecraft that has returned all of this great science.”
Cassini had also made its mark in the planning for other missions, added Jim Green, director of NASA’s Planetary Science Division. Just to start with, he said, the upcoming Europa Clipper mission (scheduled for launch in the 2020s) would use an approach similar to that which Cassini used to study Titan. Rather than orbiting Europa, the Europa Clipper will spend as little time as possible in Jupiter’s dangerous radiation belts by instead making repeated flybys of Europa.
“We’re taking a page out of Cassini’s book,” Green said.
