NASA's Cassini spacecraft has completed the first of 20 close-proximity passes of Saturn's outermost rings. The pass, which took place on Dec. 4 at 8:09 a.m. EST, saw the probe fly just 6,800 miles (11,000 km) from the center of Saturn's F ring. This series of 20 orbits represents the penultimate phase of Cassini's decades-long mission, prior to the beginning of the mission's "Grand Finale."

Thirty minutes prior to crossing the ring-plane, Cassini initiated the 183rd (and possibly final) burn of its main engine. Cassini's handlers believe that the remaining maneuvers required to manipulate the spacecraft into its daring "Grand Finale" orbits can be completed using a set of thrusters ordinarily used for attitude control and minor orbital adjustments.

"With this small adjustment to the spacecraft's trajectory, we're in excellent shape to make the most of this new phase of the mission," said Earl Maize, Cassini project manager at NASA's Jet Propulsion Laboratory, Pasadena, California.

When passing through the ring-plane on Dec. 4, Cassini flew through an incredibly faint ring, which is believed to have formed from debris thrown out as meteors struck the small Saturnian moons Janus and Epimetheus.

Cassini's mission operators believe that, even though the probe passed directly through the faint ring, the dust threat to the spacecraft remains very low, owing largely to the spacecraft's remoteness from the gas giant's main rings. However, as a precaution, Cassini was directed to close its main engine cover as it passed through the ring-plane.

Unfortunately, Cassini's first ring-grazing orbit won't produce any stunning imagery of Saturn's rings, as the probe's cameras were inoperative during the pass.

Image of Saturn and its rings as captured by Cassini in mid-2004(Credit: NASA/JPL-Caltech/Space Science Institute)

This measure was taken to allow the spacecraft and its handlers to prioritize the engine burn, and focus on the performance of other instruments, such as Cassini's radio science experiment, which worked to observe the structure of the rings.

During the remaining 19 ring-skimming orbits, Cassini's cameras will be back in action capturing the most detailed views ever of a number of Saturn's smaller moons, including Pandora, Atlas, Pan and Daphnis, as well as compiling a complete scan of the gas giant's ring structure.

It is planned that as Cassini makes its closest pass with Saturn (which will see the probe pass only 56,000 miles/90,000 km from the gas giant's uppermost cloud layer) the spacecraft will snap images of Saturn's rings with a resolution of 0.6 miles (1 km) per pixel.

High-resolution images such as these will allow Cassini's science team to discern the nature of small-scale features in Saturn's A ring known as "propellers," which are believed to be created by the gravitational influence of moonlets embedded within the rings.

The next ring-grazing pass is set to take place on Dec. 11, with the final pass taking place on Apr. 22, 2017. Following this pass, Cassini operators will use the gravity of Saturn's moon Titan to manipulate the probe's orbital path to pass between the innermost of Saturn's rings and its surface, thus beginning the final phase of Cassini's great journey – the "grand Finale."

Source: NASA

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