August is shaping up to be quite the month for space fans. The Perseid Meteor Shower is peaking this weekend. SpaceX is preparing for another ISS resupply mission. A total solar eclipse will stretch across the entire continental U.S. And NASA’s Cassini spacecraft is about to get a taste of Saturn’s atmosphere.
The folks who have shepherded Cassini since its launch nearly 20 years ago are getting ready for the spacecraft’s final five orbits of the ringed gas giant. Next Monday (August 14), Cassini will cruise just over 1,000 miles above Saturn’s cloud tops.
Cassini won’t be just a Saturn orbiter probe anymore. “Cassini will become the first Saturn atmospheric probe,” says Linda Spilker, Cassini project scientist at JPL. “It’s long been a goal in planetary exploration to send a dedicated probe into the atmosphere of Saturn, and we’re laying the groundwork for future exploration with this foray.”
Cassini might need a little more than gravity for this close flyby over Saturn’s clouds. The team piloting Cassini expect to fly through a dense enough portion of Saturn’s atmosphere to fire the spacecraft’s small thrusters to keep it stable.
This won’t be the first time Cassini has fired its thrusters during a close atmosphere flyby. In June 2010, Cassini flew just 457 miles above Saturn’s moon Titan. The thrusters on board fired about twice as much as the team expected them to. Through it all, Cassini kept pointing the way the team expected it to. And the extra thrusting stayed comfortably within the safety margin.
The June 2010 flyby experience is helping the team set Cassini’s trajectory for Monday’s flyby. “Thanks to our past experience, the team is confident that we understand how the spacecraft will behave at the atmospheric densities our models predict,” says Cassini project manager Earl Maize.
According to Maize, the Cassini team is looking for the thrusters to operate somewhere between 10 to 60 percent of what they are capable of. If the thrusters fire more, it means the atmosphere is thicker than expected and the next orbits will be about 120 miles higher. If Monday’s flyby goes smoothly, the team will look into lowering the altitude of Cassini’s final two orbits by about 120 miles.
That would place Cassini less than 1,000 miles above Saturn’s cloud tops for its last two orbits.
Cassini’s instrument package will be laser focused on the gas giant on Monday. Some instruments will gather observations of Saturn’s auroras and vortexes at the planet’s poles. Others will beam temperature data back to Earth. The radar will look deep into the atmosphere and reveal features just 16 miles wide. Almost 100 times smaller than what Cassini could see before this Grand Finale.
We’ll have to wait and see what happens to the final Cassini orbits after Monday’s flyby. But one orbital adjustment is for certain. On September 11, Cassini will get a gravitational nudge from Titan that will seal its fate. On September 15, Cassini will begin its last fall towards Saturn. And this time, it won’t miss.
Seven instruments will be switched on and beaming measurements back in near real time during this last flyby. But Saturn’s atmospheric drag will eventually overcome Cassini’s thrusters keeping the antenna pointed towards Earth. Moments after that, the spacecraft will break apart as friction from Saturn’s atmosphere rips it apart.
But the data gathered over the next month will fuel research for years to come. Cassini will burn up next month, but its legacy will live on for decades.
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