When Cassini ends its 13-year tour of the Saturn system with an intentional plunge into the planet on Friday, the spacecraft's speed will be approximately 113,000 kms per hour, NASA has said. The final plunge will take place on the day side of Saturn, near local noon, with the spacecraft entering the atmosphere around 10 degrees north latitude, NASA said. The spacecraft's fateful dive is the final beat in the mission's Grand Finale, 22 weekly dives, which began in late April, through the gap between Saturn and its rings. No spacecraft has ever ventured so close to the planet before.
The mission's final calculations predict loss of contact with the Cassini spacecraft will take place on Friday at 7:55 a.m. EDT. Cassini will enter Saturn's atmosphere approximately one minute earlier, at an altitude of about 1,915 kilometres above the planet's estimated cloud tops. When Cassini first begins to encounter Saturn's atmosphere, the spacecraft's attitude control thrusters will begin firing in short bursts to work against the thin gas and keep Cassini's saucer-shaped high-gain antenna pointed at Earth to relay the mission's precious final data.
As the atmosphere thickens, the thrusters will be forced to ramp up their activity, going from 10 per cent of their capacity to 100 per cent in the span of about a minute. Once they are firing at full capacity, the thrusters can do no more to keep Cassini stably pointed, and the spacecraft will begin to tumble. When the antenna points just a few fractions of a degree away from Earth, communications will be severed permanently, NASA said. The predicted altitude for loss of signal is approximately 1,500 kilometres above Saturn's cloud tops.
From that point, the spacecraft will begin to burn up like a meteor. Within about 30 seconds following the loss of signal, the spacecraft will begin to come apart. Within a couple of minutes, all remnants of the spacecraft are expected to be completely consumed in the atmosphere of Saturn, NASA said. Due to the travel time for radio signals from Saturn, which changes as both Earth and the ringed planet travel around the Sun, events currently take place there 83 minutes before they are observed on Earth.
This means that, although the spacecraft will begin to tumble and go out of communication at 6:31 a.m. EDT at Saturn, the signal from that event will not be received at Earth until 83 minutes later. "The spacecraft's final signal will be like an echo. It will radiate across the solar system for nearly an hour and a half after Cassini itself has gone," said Earl Maize, Cassini project manager at NASA's Jet Propulsion Laboratory in Pasadena, California. "Even though we'll know that, at Saturn, Cassini has already met its fate, its mission isn't truly over for us on Earth as long as we're still receiving its signal," Maize added.
Live mission commentary and video from Jet Propulsion Laboratory Mission Control will air on NASA TV and the agency's website from 7 to 8:30 a.m. EDT (4.30 to 6 pm India time)on Friday. Launched in 1997, Cassini arrived in orbit around Saturn in 2004 on a mission to study the giant planet, its rings, moons and magnetosphere.
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