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PASADENA, Calif. – A billion-dollar spacecraft named Cassini is about to burn up as it plunges into the atmosphere of Saturn this month. That’s the plan, exquisitely crafted. Cassini will transmit data to Earth to the very end, squeezing out the last drips of science as a valediction for one of NASA’s greatest missions.
Dreamed up when Ronald Reagan was president, and launched during the tenure of Bill Clinton, Cassini arrived at Saturn in the first term of George W. Bush. So it’s old, as space hardware goes. It has fulfilled its mission goals and then some. It has sent back stunning images and troves of scientific data. It has discovered moons, and geysers spewing from the weird Saturn satellite Enceladus. It landed a probe on the moon Titan.
It has also run out of gas, basically, though precisely how much fuel is left is unknown. Program manager Earl Maize says, “One of our lessons learned, and it’s a lesson learned by many missions, is to attach a gas gauge.”
Did You Know?
- The maximum speed clocked by Cassini was 98,346 mph (44 kilometers per second) relative to the sun on June 25, 1999.
- Relative to Saturn, the spacecraft reached 68,771 mph (30.7 kilometers per second) during the Saturn Orbit Insertion maneuver on July 1, 2004.
- With respect to Earth, the maximum speed reached by Cassini is 42,561 mph (19.0 kilometers per second) on Aug. 18, 1999, as the spacecraft flew past our home planet at an altitude of 727 miles (1,171 kilometers).
- During the second Venus flyby, Cassini zoomed by the planet at 13.6 kilometers per second (30,523 mph) on June 24, 1999.
- The spacecraft flew by Jupiter at a speed of 25,951 mph (11.6 kilometers per second) on Dec. 30, 2000.
- By the way, Saturn's average orbital speed around the Sun is about 34,700 kilometers (21,561 miles) per hour.
About the Mission
After two decades in space, NASA's Cassini spacecraft is nearing the end of its remarkable journey of exploration. Having expended almost every bit of the rocket propellant it carried to Saturn, operators are deliberately plunging Cassini into the planet to ensure Saturn's moons will remain pristine for future exploration—in particular, the ice-covered, ocean-bearing moon Enceladus, but also Titan, with its intriguing pre-biotic chemistry.
Beginning in 2010, Cassini began a seven-year mission extension in which it completed many moon flybys while observing seasonal changes on Saturn and Titan. The plan for this phase of the mission was to expend all of the spacecraft's propellant while exploring Saturn, ending with a plunge into the planet's atmosphere. In April 2017, Cassini was placed on an impact course that unfolded over five months of daring dives—a series of 22 orbits that each pass between the planet and its rings. Called the Grand Finale, this final phase of the mission has brought unparalleled observations of the planet and its rings from closer than ever before.
On Sept. 15, 2017, the spacecraft will make its final approach to the giant planet Saturn. But this encounter will be like no other. This time, Cassini will dive into the planet's atmosphere, sending science data for as long as its small thrusters can keep the spacecraft's antenna pointed at Earth. Soon after, Cassini will burn up and disintegrate like a meteor.
To its very end, Cassini is a mission of thrilling exploration. Launched on Oct. 15, 1997, the mission entered orbit around Saturn on June 30, 2004 (PDT), carrying the European Huygens probe. After its four-year prime mission, Cassini's tour was extended twice. Its key discoveries have included the global ocean with indications of hydrothermal activity within Enceladus, and liquid methane seas on Titan.
And although the spacecraft may be gone after the finale, its enormous collection of data about Saturn—the giant planet itself, its magnetosphere, rings and moons—will continue to yield new discoveries for decades.
The spacecraft is tracked in the Charles Elachi Mission Control Center of the Jet Propulsion Laboratory in Pasadena, California. Mission Control is a darkened chamber with no external windows. The room (named after a retired JPL director) is dominated by glowing screens and people peering into consoles. Someone wandering into the place by accident would think: This looks like the kind of place where they fly spaceships.
On the far wall is a screen showing the operations of the three huge radio antennae – in the California desert; near Madrid; and in Canberra, Australia – that together make up NASA’s Deep Space Network. As Earth turns, there’s always a big dish looking out for Cassini and for JPL’s other spacecraft roaming the solar system.
The navigators have a computer model that tells them where the spacecraft probably is and probably will be.
“We need to be able to point instruments to objects. Nothing is static. Everything is moving. The timing is critical,” said navigation team leader Duane Roth. “We don’t know exactly where Titan is at any given moment, or where Saturn is, or where Cassini is. When you want to propagate that out to some future time, all our errors grow.”
But they’re getting it done.
Cassini’s final orbits have taken it, amazingly, inside the rings of Saturn, where the spacecraft practically skims the tops of the planet’s clouds. These orbits can plausibly be compared to Luke Skywalker flying into that narrow trench on the Death Star.
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