After 20 years and countless orbits, NASA's Cassini mission to Saturn has come to its fiery end. The space agency has confirmed that its Deep Space Network (DSN) tracking station at Canberra lost all radio contact with the unmanned deep space probe today at 7:55 am EDT as the spacecraft hit the atmosphere of the giant ringed planet at 70,000 mph (113,000 km/h), tumbled out of control and burned up like a meteor.
Today's dramatic end of the US$3.26 billion mission to explore Saturn, its rings, and its moons came after a five-month Grand Finale to mark the end of the spacecraft's operational life. This saw Cassini make 22 close orbits of Saturn with four orbits through Saturn's innermost ring and five orbits skimming the outermost layers of Saturn's atmosphere.
On September 11 at 12:04 pm EDT, Cassini made its last flyby of Saturn's largest moon, Titan. This slowed down the orbiter's trajectory, sending it on an irreversible collision course with Saturn. On September 14 at 12:58 pm PDT, Cassini took its last images and at 2:45 pm EDT the same day it executed a complete download of its computer memory to mission control in Pasadena, California via the DSN. This morning at 1:34 am EDT, the spacecraft reconfigured itself to send back live data and telemetry of its last moments at a rate of 3.4 kilobytes per second.
Because Saturn is 933 million miles (1.5 billion km) from Earth, it took 83 minutes for radio signals to reach us from Cassini and it was the cut off of these signals that marked its end. It struck the atmosphere one minute before contact loss.
During the final seconds of its life, with a somber mission control looking on, the spacecraft used its thrusters to remain stable and keep its main antenna directed at Earth. After its thrusters reached full power, it could no longer control its attitude and contact was lost as the antenna went a fraction of a degree off target. Cassini then began to tumble and break up, burning up completely like a meteor while still hundreds of miles above the cloud tops.
The purpose of today's event was to protect any of Saturn's potentially habitable moons from being contaminated with terrestrial microbes. Cassini had almost exhausted the propellant needed to control its trajectory and NASA saw a danger of the derelict, unsterilized spacecraft one day crashing on one of the moons.
Cassini was a joint NASA, ESA, and Italian mission launched on October 15, 1997 from Cape Canaveral Air Force Station atop an Atlas IV booster. After spending almost seven years in transit, that included flybys of Earth, Venus, and Jupiter, the nuclear powered probe went into orbit around Saturn, where it remained on station for 13 years. During this time, it made many close flybys of Saturn's rings and moons. On January 15, 2005, it released the Huygens probe, which made the first landing on the moon Titan.
Though Cassini is now a scatter of molecules floating in the atmosphere of Saturn, the masses of data it sent back to Earth will keep scientists busy for generations to come.
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