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Cassini nears Phoebe, Saturn's dark moon


Getting closer: the Cassini spacecraft returned this image of Saturn in May (Image: NASA/JPL/Space Science Institute)

The Cassini space probe will take the closest look yet at Saturn's dark, rough and contrary moon Phoebe when it flies past this weekend.

Cassini will capture data on Phoebe on its way to a four-year orbital mission around Saturn.

The craft's closest pass to Phoebe will take place at 6:55 am AEST on the morning of 12 June (20:56 UT, 11 June).

Phoebe is a bit of an eccentric among Saturn's 31 known moons, orbiting the planet in the opposite direction from most of the planet's other large satellites, at a 30º tilt compared with Saturn's equatorial plane.

"That means it's really odd," said Dr Bonnie Buratti, who has studied Phoebe for 20 years and is a lead scientist on the Cassini mission.

Last observed from space in images snapped by the Voyager probe in 1981, Phoebe is known to be rough and craggy, so uneven that there may be a huge mountain or crater, Buratti said from the Jet Propulsion Laboratory in Pasadena, California.

But Cassini will take pictures from a much closer vantage point: 2000 kilometres as opposed to 2.2 million kilometres with Voyager.

Cassini could also shed some light on what Phoebe's uneven surface is made of and where it originated.

Astronomers already know Phoebe's surface is extraordinarily dark, reflecting only about 6% of the light it gets from the Sun. This darkness could be a sign that it was originally from the Kuiper belt, an area at the far reaches of the solar system that teems with such objects.

Earlier observations indicate that the darkness may indicate the presence of carbon on Phoebe, an important finding as carbon is one of the building blocks of life, Buratti said.

There is also frozen water on Phoebe, another finding of potential interest to astronomers looking for clues to how life came to Earth.

One theory is that materials essential to life, not life itself, were introduced into the planetary area of the solar system as objects carrying them were captured from the Kuiper belt by big outer planets like Saturn.

In fact, Saturn's distinctive rings, hundreds of them, are thought to be made up of shattered bits of comets, asteroids and moons that broke up before they reached the planet.

Cassini will fly by Phoebe on its way to an orbital path around Saturn. The spacecraft is expected to enter Saturn's orbit at the end of June.


Tags: science-and-technology, astronomy-space