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Saturn’s tiny Trojan moon

Saturns' moon Helene

Saturn's 33-kilometre-wide moon Helene, imaged by NASA's Cassini spacecraft from a distance of around 19,000 kilometres.

THIS BLACK AND WHITE image shows Helene, one of the smaller of Saturn’s 62 confirmed moons, seen during a relatively close encounter by NASA’s Cassini spacecraft on March 3, 2010. The grey background is the atmosphere of Saturn.

Helene circles Saturn in the same orbit as the much larger moon, Dione, but ahead of it, making it a “Trojan” moon. A Trojan moon is one that is a location where the gravitational pull of the parent planet and another body (Dione, in this case) balance out, so it always stays in the same relative position. That position is known as a Lagrangian point.

The tiny moon was discovered by astronomers Pierre Laques and Jean Lecacheux in 1980 from the Pic du Midi Observatory in France. It was provisionally designated S/1980 S 6 (meaning it was the sixth new moon of Saturn to be discovered that year), and in 1988 was officially named after Helen of Troy, who in Greek mythology was the granddaughter of Cronus (Saturn).

Helene is just 33 kilometres across at its widest point. It orbits Saturn at a distance of 377,000 kilometres (roughly the same average distance between the Earth and the Moon) and takes about 2.74 Earth days to complete one revolution.

At the time Cassini snapped the image, the spacecraft was about 19,000 kilometres from Helene, which means we can see detail down to about 113 metres.

Images courtesy NASA / JPL / Space Science Institute.

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Saturn’s tiny moon Helene

Saturn's tiny moon Helene

Saturn's tiny moon Helene is just 33km wide, and shares an orbit with a larger moon.

The Cassini spacecraft snapped this image during the spacecraft’s closest flyby of Saturn’s moon Helene, on March 3, 2010.

Helene—just 33 kilometres, or 21 miles, wide—leads the much larger Dione by 60 degrees in the two moons’ shared orbit around Saturn. This makes Helene a “Trojan” moon of Dione, named for the Trojan asteroids that orbit 60 degrees ahead of and behind Jupiter as the giant planet circles the Sun.

The lit terrain seen here is on the side of Helene that faces away from Saturn. The southern pole of the moon is in the lower right of the image.

The image was taken in visible light with the Cassini spacecraft’s wide-angle camera. The view was obtained at a distance of approximately 1,900 kilometres (1,200 miles). The scale in the original image was 235 metres (771 feet) per pixel, but the image has been magnified by a factor of two and contrast-enhanced to aid visibility.

Adapted from information issued by NASA / JPL / Space Science Institute.