NASA's Cassini probe on course for fateful plunge into Saturn

NASA's Cassini spacecraft is on course to its final approach to Saturn, hurling towards the ringed planet at the speed of 113,000 kilometres per hour, to meet the fateful end of its 20-year-long epic journey, the US space agency said. Cassini is on course to dive into Saturn to ensure that the planet's moons - in particular Enceladus, with its subsurface ocean and signs of hydrothermal activity - remain pristine for future exploration.

The spacecraft's fateful dive on September 15 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 a minute after it reaches an altitude of about 1,915 kilometres above the planet's estimated cloud tops. During its dive into the atmosphere, the spacecraft's speed will be about 113,000 kilometres per hour.

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. 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 the Earth, communications will be severed permanently.

The predicted altitude for loss of signal is about 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 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. 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.

"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 (JPL) in the US. "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," said Maize. Cassini's last transmissions will be received by antennas at NASA's Deep Space Network complex in Australia.