Saturn's two largest moons have been captured together in a new image from the Cassini spacecraft. The image will help mission managers refine their models of the moons' positions to better steer the spacecraft in the future.
The image was snapped on 11 June, when Cassini was about 3.6 million kilometres away from Saturn's second-largest moon, Rhea. It shows the 1528-kilometre-wide Rhea silhouetted against Titan, which is about three times as big.
The bright ring that surrounds Titan is sunlight scattered by Titan's thick atmosphere – the sun lies behind the moons in this image.
But the image was not taken solely for its aesthetic appeal. "It will help mission managers fly the spacecraft," says Alfred McEwen, a member of Cassini's imaging team at the University of Arizona.
Cassini scientists are constantly taking such images to compare predictions of where the moons should be with where they actually are, McEwen says. This allows them to improve models of the moons' orbits, which helps mission scientists plan when to observe them.
"Knowing where the objects are, you can point the instruments at them and capture them properly," he said.
Improved orbital information will also help mission managers plan out Cassini's manoeuvres for several years into the future. That is because the moons – especially large moons – have a gravitational effect on the spacecraft and can move it away from its intended path.
"You need to know how far away the moon will be and how much it will deflect the spacecraft," McEwen says.
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