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AN AMERICAN astronomer has found a new moon of Saturn, bringing the
planet’s total to 18. It is the first moon to be found orbiting the planet
within its famous set of rings.

The gravity of the moon sweeps aside the bright material of the ring
to produce a dark gap, 300 kilometres wide, called the Encke division after
the 18th century astronomer who first glimpsed it.

Mark Showalter of the NASA-Ames Research Center in California discovered
the new satellite on eight images out of the 30 000 taken by the two Voyager
spacecraft in 1980 and 1981. Showalter tracked it down by analysing the
wavelike patterns that its gravity raises on the edges of the Encke division.

According to Carolyn Porco of the University of Arizona, these patterns
had already made astronomers ‘very confident’ that there was a moonlet in
the Encke division, ‘but we assumed that the images had all been combed
thoroughly’. But Showalter was not content just to look at the Voyager pictures
of the Encke division. His computer program analysed the wave patterns and
calculated where the moonlet should have been when Voyager took each of
its close-up photographs of the rings. He then selected those in which the
moon should appear, and enhanced the raw images until the satellite showed
up: ‘it was very gratifying to see it,’ he says.

Because the new satellite was discovered on images that were taken in
1981, it has been given the temporary name ‘1981S13’: the moon will receive
a permanent name at the meeting of the International Astronomical Union
in Buenos Aires next year. 1981S13 is Saturn’s smallest known moon, only
20 kilometres in diameter. Even so, it is thousands of times larger than
the icy fragments that make up the rings themselves. Porco speculates that
any icy bodies of an intermediate size would have been ground up by collisions
into the smaller particles that form the rings.

According to Porco, there is probably at least one similar moonlet in
the Cassini division, the widest gap in Saturn’s rings. So far, no one has
found any moons in the Voyagers’ images of the Cassini division. Astronomers
will be taking a special look in both the Encke and Cassini divisions when
the next spaceprobe arrives at Saturn. This is an American-European mission
called Cassini, which will be launched in 1996 to arrive at Saturn in 2002.
Instead of just flying by, Cassini will go into orbit around Saturn. While
the Voyagers observed for only a few days, Cassini will have years in which
to survey the planet and its rings.

Its orbit will be so far from Saturn, however, that it will not see
details in the rings finer than 1 kilometre in size – only four times better
than Voyager. For Porco, an expert on the rings, ‘this is a real heartbreaker’;
but she hopes that Cassini will send back a few close-up pictures of the
rings as it sweeps over Saturn on its arrival.