Cassini to begin last five Grand Finale orbits

Spacecraft's science instruments will probe deeply into Saturn's atmosphere.
By Jeremy Morrow | Aug 26, 2017
NASA's Cassini orbiter will begin the last five of its 22 Grand Finale orbits between Saturn and its innermost rings on Sunday, August 13, making it the first probe to ever explore the giant planet's atmosphere.

On April 22, the spacecraft embarked on its Grand Finale, a series of 22 dives between the Saturn's rings and the planet's atmosphere, each taking approximately six and a half days.

The Grand Finale--and the entire Cassini mission--will conclude on September 15, when the probe will plunge into Saturn's atmosphere, where it will be crushed.

Scientists and engineers chose to end the mission this way to avoid contaminating the planet's potentially habitable moons Enceladus and Titan with microbes carried by the spacecraft from Earth.

In the first of the five final orbits, Cassini will pass within 1,010 and 1,060 miles (1,630 and 1,710 km) of Saturn's cloud tops.

Its previous flights through the thick atmosphere of Saturn's largest moon Titan has prepared mission scientists for the flight above the planet's atmosphere noted project manager Earl Maize of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California.

"Thanks to our past experience, the team is confident that we understand how the spacecraft will behave at the atmospheric densities our models predict," Maize said.

Depending on the density of Saturn's atmosphere, Cassini may need to use its small rocket thrusters to keep on board conditions stable.

Maize reported the mission team expects the thrusters to perform at somewhere between 10 and 60 percent of their total capacity.

If the atmosphere is less dense than predicted during the first three of the five orbits, engineers may lower Cassini's altitude by about 120 miles (200 km), enabling its science instruments to obtain data closer to the cloud tops.

In contrast, if the atmosphere is denser than thought, and the thrusters have to work harder, mission engineers will raise the probe's altitude by about the same degree, approximately 120 miles (200 km).

During these close orbits, Cassini's radar system will look further than ever into the atmosphere to resolve very small features. Cameras will capture high-resolution photos of Saturn's auroras while science instruments will obtain data on temperatures and vortexes at both of the planet's poles.

Four days before the final plunge, Cassini will use distant Titan for a gravitational assist that will send it to an altitude within the planet's atmosphere at which its thrusters are ineffective against atmospheric density.

All the spacecraft's instruments, including its Ion and Neutral Mass Spectrometer (INMS) will be turned on and will return data in real time until the spacecraft breaks up.

"It's been a long 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 first foray," emphasized project scientist Linda Spilker, also at JPL.

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