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martyb writes:

The end is nigh for NASA's Cassini spacecraft, according to a report at NASA (Javascript required -- non-JS version available at Science Daily.)

NASA's Cassini spacecraft will enter new territory in its final mission phase, the Grand Finale, as it prepares to embark on a set of ultra-close passes through Saturn's upper atmosphere with its final five orbits around the planet.

Cassini will make the first of these five passes over Saturn at 12:22 a.m. EDT Monday, Aug. 14. The spacecraft's point of closest approach to Saturn during these passes will be between about 1,010 and 1,060 miles (1,630 and 1,710 kilometers) above Saturn's cloud tops.

The spacecraft is expected to encounter atmosphere dense enough to require the use of its small rocket thrusters to maintain stability -- conditions similar to those encountered during many of Cassini's close flybys of Saturn's moon Titan, which has its own dense atmosphere.

"Cassini's Titan flybys prepared us for these rapid passes through Saturn's upper atmosphere," said Earl Maize, Cassini project manager at NASA's Jet Propulsion Laboratory (JPL) in 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."

If the thrusters operate between 10 and 60 percent of their capability, then things will proceed unchanged. If more than 60 percent thrust is required, that means the atmosphere is more dense than predicted and the engineers will perform a "pop-up maneuver" to boost the altitude of later orbits — likely by 200 kilometers (120 miles). If, on the other hand, the thruster use required less than 10 percent of their capability, then the atmosphere was thinner than predicted and they will perform a "pop down maneuver" to reduce the altitude by about 200 kilometers.

Original Submission

"As it makes these five dips into Saturn, followed by its final plunge, Cassini will become the first Saturn atmospheric probe," said 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 first foray."

Other Cassini instruments will make detailed, high-resolution observations of Saturn's auroras, temperature, and the vortexes at the planet's poles. Its radar will peer deep into the atmosphere to reveal small-scale features as fine as 16 miles (25 kilometers) wide -- nearly 100 times smaller than the spacecraft could observe prior to the Grand Finale.

So, those experiments happen during close flybys... how does NASA plan to deorbit the craft and what will happen?

On Sept. 11, a distant encounter with Titan will serve as a gravitational version of a large pop-down maneuver, slowing Cassini's orbit around Saturn and bending its path slightly to send the spacecraft toward its Sept. 15 plunge into the planet.

During the half-orbit plunge, the plan is to have seven Cassini science instruments, including INMS, turned on and reporting measurements in near real time. The spacecraft is expected to reach an altitude where atmospheric density is about twice what it encountered during its final five passes. Once Cassini reaches that point, its thrusters will no longer be able to work against the push of Saturn's atmosphere to keep the spacecraft's antenna pointed toward Earth, and contact will permanently be lost. The spacecraft will break up like a meteor moments later, ending its long and rewarding journey.

Cassini is currently 1.42 billion km away. So far, in fact, that it takes over 90 minutes to send a message to Cassini at the speed of light... another 90 minutes' wait for a reply.