September 15th, 2017, Published in Articles: EE Publishers, Articles: EngineerIT

by Hans van de Groenendaal, features editor, EngineerIT

The Cassini spacecraft is on its final approach to Saturn, following confirmation by mission navigators that it is on course to dive into the planet’s atmosphere at close to 12h55 SA time today (15 September 2017).

Cassini is ending its 13-year tour of the Saturn system with an intentional plunge into the planet to ensure Saturn’s moons – in particular Enceladus, with its subsurface ocean and signs of hydrothermal activity – remain pristine for future exploration.

The spacecraft’s planned 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 today. Cassini will enter Saturn’s atmosphere approximately one minute earlier, at an altitude of about 1915 km above the planet’s estimated cloud tops (the altitude where the air pressure is 1-bar, equivalent to sea level on Earth). During its dive into the atmosphere, the spacecraft’s speed will be approximately 113 000 km 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°N 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% of their capacity to 100% 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. The predicted altitude for loss of signal is approximately 1500 km 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. This means that, although the spacecraft will begin to tumble and go out of communication at 12:31 SA time the signal from that event will not be received at Earth until 83 minutes later.

Cassini’s last transmissions will be received by antennas at NASA’s Deep Space Network complex in Canberra, Australia.

Cassini is set to make ground-breaking scientific observations of Saturn, using eight of its twelve science instruments. All of the mission’s magnetosphere and plasma science instruments, plus the spacecraft’s radio science system, and its infrared and ultraviolet spectrometers will collect data during the final plunge.

The main observations being made as Cassini dives into Saturn are those of the ion and neutral mass spectrometer (INMS). The instrument will directly sample the composition and structure of the atmosphere, which cannot be done from orbit. The spacecraft will be oriented so that INMS is pointed in the direction of motion, to allow it the best possible access to oncoming atmospheric gases.

Live mission commentary and video from JPL Mission Control will air on NASA Television and the agency’s www.nasa.gov from 12h00 CAT today.

Cassini, which was built and operated from the Jet Propulsion Laboratory in Pasadena, California, lifted off from Cape Canaveral, Florida, in 1997 atop a Titan IVB rocket. The launch capped months of processing on the spacecraft and booster to make sure everything would work correctly on launch day and Cassini would be set on its correct path. Despite the size of the rocket, Cassini still took seven years to reach Saturn. It flew a precise path through the inner solar system building up speed by passing near Venus and Earth so it could slingshot out beyond Mars and Jupiter to rendezvous with Saturn. Getting on that path correctly at the start was the focus of the launch team.

Cassini also carried a second spacecraft along with it, a probe called Huygens that was released to study the moon Titan up close. Huygens parachuted to the surface of the moon, relaying data on conditions there until its batteries ran out.