NASA’s Cassini spacecraft aims to skirt Saturn’s innermost ring

Cassini took this image of Saturn and its rings May 13. Credit: NASA/JPL-Caltech/Space Science Institute

Fresh off its fifth passage through a gap between Saturn’s atmosphere and rings, NASA’s robotic Cassini spacecraft is looping toward a close brush with the planet’s innermost ring next week, when the probe will again use its high-gain antenna as a shield against icy particles that may lie in its path.

Cassini made another safe trip through the 1,500-mile (2,400-kilometer) space between Saturn and its rings at 0311 GMT Monday (11:11 p.m. EDT Sunday), according to NASA, marking the fifth time the craft has dived through the previously-unexplored region since late April.

During its latest ring passage, the orbiter’s instruments were expected to measure Saturn’s gravitational field and gather data on the mass of the planet’s rings. Cassini’s Cosmic Dust Analyzer instrument was also supposed to collect and analyze ring particles on this week’s flyby.

“Interplanetary meteoroids bombard Saturn’s main rings, depositing silicates, organics, and metals in the ring particles,” scientists wrote in a description of this week’s flight through the ring gap. “The more of those contaminants the CDA (Cosmic Dust Analyzer) finds, the older Saturn’s rings probably are.”

NASA tweeted from Cassini’s account Monday that the flyby was successful.

This diagram created by Cassini mission planners illustrates the close approaches of the spacecraft during its Grand Finale orbits between Saturn’s innermost (D) ring and the giant planet’s atmosphere. Credit: NASA/JPL-Caltech/Erick Sturm

The plutonium-powered spacecraft is in its final months, with its orbit around Saturn now taking it on weekly plunges through the ring gap.

Data recorded during the close-up flybys will help tell scientists the age of Saturn’s rings, which will indicate their likely origin. More measurements of the rings’ mass and Saturn’s magnetic field, atmosphere and internal structure are also on tap during Cassini’s final months.

The orbiter is heading for a Sept. 15 plunge into Saturn’s atmosphere, a guided dive during which Cassini will be crushed and vaporized. The craft is running low on fuel, and officials want to ensure the probe is destroyed before losing control of it.

Scientists worry that Cassini could eventually collide with one of Saturn’s habitable moons, such as Titan or Enceladus, and spoil future discoveries.

Cassini’s next journey through the ring gap is set for 1422 GMT (10:22 a.m. EDT) Sunday, May 28, when the spacecraft will make its closest brush with the inner edge of Saturn’s D ring, a belt of tiny ice particles encircling the planet.

Artist’s concept of the Cassini spacecraft at Saturn. Credit: NASA/JPL-Caltech

Ground controllers will command the spacecraft to point its dish-shaped antenna in its direction of travel during Sunday’s approach as a safeguard against potentially damaging impacts from ring particles. So far, scientists have detected fewer strikes from icy debris in the ring gap than predicted, allowing engineers to forego using the antenna as a shield during the last four flybys.

Cautious managers directed the antenna to point forward — in its so-called “ram” position — during Cassini’s first flight through the ring gap last month.

Objectives during Sunday’s flyby include unprecedented radar observations of the structure of Saturn’s rings, up-close imaging of the rings, and listening for particle impacts with one of Cassini’s science instruments.

Because the high-gain antenna will not be aimed at Earth, Cassini will be out of contact with mission control during Sunday’s encounter. The craft is scheduled to radio its status back to Earth around 0329 GMT Monday, May 29 (11:29 p.m. EDT Sunday).

Cassini will come close to Saturn’s D ring on three more occasions in June and July, when it will again use its antenna for protection.

Email the author.

Follow Stephen Clark on Twitter: @StephenClark1.