NASA’s Cassini Orbiter Makes Its First ‘Ring-Grazing’ Dive

On Dec. 4, 2016 at 8:09 a.m. EST (5:09 a.m. PST), NASA’s Cassini spacecraft made its first close dive past the outer edges of Saturn’s rings.

This graphic shows the closest approaches of Cassini’s final two orbital phases. Ring-grazing orbits are shown in gray (at left); Grand Finale orbits are shown in blue. The orange line shows the spacecraft’s Sept. 2017 final plunge into Saturn. Image credit: NASA / JPL-Caltech.

Cassini crossed through the plane of Saturn’s rings at a distance of 57,000 miles (91,000 km) above the gas giant’s cloud tops. This is the approximate location of a faint, dusty ring produced by the planet’s small moons Janus and Epimetheus, and just 6,800 miles (11,000 km) from the center of F ring.

About an hour prior to the ring-plane crossing, Cassini performed a short burn of its main engine that lasted about 6 seconds.

About 30 min later, as it approached the ring plane, the robotic orbiter closed its canopy-like engine cover as a protective measure.

A few hours after the ring-plane crossing, Cassini began a complete scan across the rings with its radio science experiment to study their structure in great detail.

“It’s taken years of planning, but now that we’re finally here, the whole Cassini team is excited to begin studying the data that come from these ring-grazing orbits,” said Cassini project scientist Dr. Linda Spilker, from NASA’s Jet Propulsion Laboratory (JPL).

“This is a remarkable time in what’s already been a thrilling journey.”

Each of Cassini’s orbits for the remainder of the mission will last one week. The next pass by the rings’ outer edges is planned for Dec. 11.

During these ring-grazing orbits, or F-ring orbits, Cassini will approach to within 4,850 miles (7,800 km) of the center of the narrow F ring.

These weekly orbits — 20 in all — will continue until Apr. 22, 2017, when the last close flyby of Saturn’s moon Titan will reshape Cassini’s flight path.

With that encounter, the spacecraft will leap over the rings, making the first of 22 plunges through the 1,500-mile-wide (2,400 km) gap between Saturn and its innermost ring on Apr. 26.

On Sept. 15, the mission will conclude with a final plunge into the Saturnian atmosphere.

During the plunge, Cassini will transmit data on the atmosphere’s composition until its signal is lost.