NASA's Cassini probe is gearing up for its final five orbits, during which it will skim the atmosphere of Saturn before making its final plunge. On August 14 at 12:22 am EDT, the unmanned spacecraft will pass within 1,010 to 1,060 mi (1,630 to 1,710 km) of the cloud tops where it will travel deep enough inside the atmosphere to need to use its reaction control thrusters to maintain stability.

The atmosphere-skimming maneuver is part of NASA's Grand Finale plan to mark the final phase of the Cassini mission. Launched almost 20 years ago, the Cassini orbiter is reaching the end of its service life as its thrusters run short of propellant. To prevent the remote possibility of the spacecraft crashing into a potentially life-bearing moon and contaminating it, the space agency will send the probe into a deliberate dive into Saturn's atmosphere.

As a prelude to this final plunge, Cassini has been carrying out a series of close encounters with the planet by flying inside of Saturn's rings. The final passes will send it into the upper reaches of the atmosphere, where NASA expects it to use its thrusters at 10 to 60 percent capability to maintain stability and to perform a "pop up" maneuver to increase its altitude by about 120 mi (200 km).

According to NASA, how hard the thrusters have to work will measure how dense the atmosphere is. If it turns out that it's less dense than predicted during the first three orbits, the thrusters to perform a "pop down" maneuver to lower the last two orbits by the same amount.

Aside from bringing Cassini into position for its final plunge, the skimming will allow the spacecraft to sample the atmosphere, and to radar probe features as fine as 16 mi (25 km) wide inside the lower atmosphere while laying the groundwork for designing future missions.

On September 11, Cassini will make its final encounter with the Saturnian moon Titan, where it has already made several atmospheric skimming maneuvers. This flyby will alter the orbit of Cassini on its final rendezvous with Saturn on September 15. As it makes its plunge, Cassini's instruments will continue to send back data until the atmosphere becomes too dense for the thrusters to cope. At this point, the spacecraft will begin to tumble and burn up.

"Cassini's Titan flybys prepared us for these rapid passes through Saturn's upper atmosphere," says Earl Maize, Cassini project manager at NASA's Jet Propulsion Laboratory. "Thanks to our past experience, the team is confident that we understand how the spacecraft will behave at the atmospheric densities our models predict."