Reconstructing Cassini’s final moments
During the Cassini spacecraft’s final plunge into Saturn’s upper atmosphere on Sept.15, 2017, the spacecraft live-streamed data from eight of its science instruments, providing readings from a variety of engineering systems. While it will take time to analyze all of the data from its plunge, engineers with the Cassini team already have a pretty clear understanding of how the spacecraft behaved as it went in.
The data could be helpful in evaluating models of Saturn’s atmosphere that the team used to predict Cassini’s behavior at mission’s end, and it could also aid in the planning of future missions to the ringed world.
Among the most crucial engineering data, or telemetry, received were measurements of the performance of Cassini’s small attitude-control thrusters. Each thruster was capable of producing a force of about half a newton, which is roughly equal to the weight of a tennis ball on Earth.
During her final moments, Cassini dived through an atmosphere that wasn’t all that dense. The air the bus-sized spacecraft tore through had about the same density as the tenuous gas where the International Space Station (ISS) orbits Earth.
While this air pressure was close to being a vacuum, Cassini was traveling about 4.5 times faster than the ISS (the ISS is traveling at approximately 17,130 mph, or 27,568 km/h). The higher velocity greatly multiplied the force that Saturn’s atmosphere exerted on the spacecraft.
During the hour before Cassini entered Saturn’s atmosphere, the spacecraft’s final approach, it was subtly rocking back and forth by fractions of a degree, gently pulsing its thrusters every few minutes to keep its antenna pointed toward Earth. The only perturbing force during this time was the slight tug of Saturn’s gravity which tried to rotate the spacecraft.
“To keep the antenna pointed at Earth, we used what’s called ‘bang-bang control’,” said Julie Webster, Cassini’s spacecraft operations chief at NASA’s Jet Propulsion Laboratory (JPL), Pasadena, California. “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.”
The range of motion was extremely small, just two milliradians, which equals 0.1 degrees. The reconstructed data indicates that Cassini was correcting its orientation in that manner until about three minutes before loss of signal occurred.
Cassini began to encounter Saturn’s atmosphere about 1,200 miles (1,900 kilometers) above the cloud tops. The spacecraft approached Saturn with its 36-foot-long (11-meter) magnetometer pointing out from its side. The dynamic pressure of the tenuous gas started to push against the boom, forcing it backward. The spacecraft responded by firing its thrusters to stop the boom from rotating any farther. The thrusters began firing longer and more frequent pulses over the next couple of minutes.
By firing its thrusters almost continuously, Cassini held its own against Saturn’s atmosphere for about 91 seconds. The thrusters reached 100 percent of their capacity during the final 20 seconds before the signal from the $3.6 billion spacecraft was lost. The last eight seconds of data indicate that Cassini began to slowly tip over backward. When this happened, the antenna’s narrowly focused radio signal started to point away from Earth, and 83 minutes later, the travel time for a radio signal from Saturn to Earth, the spacecraft’s signal vanished from monitors in JPL mission control. The telemetry data disappeared first, leaving only a radio carrier signal. This too fell silent, 24 seconds after the loss of telemetry.
“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,” said Earl Maize, Cassini project manager at JPL. “It was a solidly built craft, and it did everything we asked of it.”
Video courtesy of NASA/JPL-Caltech
Jim Sharkey is a lab assistant, writer and general science enthusiast who grew up in Enid, Oklahoma, the hometown of Skylab and Shuttle astronaut Owen K. Garriott. As a young Star Trek fan he participated in the letter-writing campaign which resulted in the space shuttle prototype being named Enterprise. While his academic studies have ranged from psychology and archaeology to biology, he has never lost his passion for space exploration. Jim began blogging about science, science fiction and futurism in 2004. Jim resides in the San Francisco Bay area and has attended NASA Socials for the Mars Science Laboratory Curiosity rover landing and the NASA LADEE lunar orbiter launch.