NASA reconstructs Cassini's last moments
On September 15, 2017, the Cassini mission came to a fiery end as the unmanned deep space probe slammed into the atmosphere of Saturn. Now, thanks to telemetry sent by the spacecraft during its final plunge, NASA engineers have been able to reconstruct the final dramatic seconds before contact with the explorer was lost forever and answer a minor mystery about Cassini's "comeback."
The end of Cassini was a carefully orchestrated grand finale to a 20-year mission that sent back more data about Saturn and its moons during its 13-year visit than earthbound scientists had collected in four centuries. As the spacecraft ran low on propellant, NASA decided to avoid the remote chance of terrestrial microbes contaminating one of the Saturnian moons by deliberately sending Cassini into Saturn's atmosphere, where it would burn up within minutes.
Part of the plan was to keep Cassini sending back scientific and spacecraft performance data for as long as possible, so eight of the probe's science instruments were left running to study the atmosphere and the craft was oriented to it would strike the sunward side of the planet with its antenna directed at the Earth for as long as possible.
To achieve the latter, mission control at the Jet Propulsion Laboratory (JPL) in Pasadena, California programmed the flight computer to use Cassini's 0.5-newton (about the weight of a tennis ball on Earth) thrusters to fire in short, throttled bursts as it dove into the atmosphere to keep it balanced against the growing aerodynamic pressure.
According to NASA, these thruster corrections were needed even when Cassini was still so high above Saturn that it was flying through a vacuum as hard as that surrounding the International Space Station. The difference is that the station travels around the Earth at about 29,000 km/h (18,000 mph), so the drag from the almost non-existent atmosphere takes years to have any real effect, but Cassini was traveling at 113,000 km/h (70,000 mph), so even that vacuum was like ripping through the stratosphere.
"To keep the antenna pointed at Earth, we used what's called 'bang-bang control,'" says Julie Webster, Cassini's spacecraft operations chief at JPL. "We give the spacecraft a narrow range over which it can rotate, and when it bangs up against that limit in one direction, it fires a thruster to tip back the other way."
Telemetry shows that Cassini was able to keep itself orientated towards Earth within 0.1° by firing its thrusters at low power every few minutes as it began its plunge and only needed to compensate for rotation caused by the tidal forces generated by the giant planet's gravitational field.
About 1,900 km (1,200 mi) above the cloud tops, Cassini hit the atmosphere proper and its 11-m (36-ft) magnetometer boom started to act like a sail, making the spacecraft rotate backward. For 91 seconds, the thrusters had to fire harder, more frequently and longer to make corrections until they were firing continuously at full power for a final 20 seconds before atmospheric pressure became too great and the antenna lost its lock on Earth. However, due to Saturn being 1.5 billion km (933 million mi) from Earth, it took 83 minutes for the last radio signals to reach mission control via NASA's Deep Space Network.
NASA says that the signal wasn't lost instantly, but that the telemetry data went first, followed by the radio carrier wave cutting out 24 seconds later. For a short moment, there was even a mysterious tall green spike that appeared on the screens of the radio monitors that seemed as if Cassini was struggling to make a comeback, but Webster said that this was a side lobe of the radio antenna beam pattern. That is, radio beams aren't coherent rays like laser beams, but form a narrow, lobe-like pattern. What the monitors picked up was the unfocused part of the narrow radio signal as the spacecraft antenna rotated away.
Shortly afterward, Cassini began to tumble and disintegrate, burning up completely like a meteor while still hundreds of miles above the cloud tops.
"Given that Cassini wasn't designed to fly into a planetary atmosphere, it's remarkable that the spacecraft held on as long as it did, allowing its science instruments to send back data to the last second," says Earl Maize, Cassini project manager at JPL. "It was a solidly built craft, and it did everything we asked of it."