During these crossings, the rings of Saturn appeared "edge-on" to observers on Earth, allowing a search for new Saturnian satellites and studies of the fainter rings. However, these crossings yielded no new discoveries of Saturnian moons. Two moons detected during the first ring plane crossing, designated 1995S1 and 1995S2, (see press release at the bottom of this page) were later found to be the known moons Atlas and Prometheus, respectively. Later discoveries, designated 1995S3 to S7, are thought to be transient clumps of unknown origin in the F-ring. These features were not observed again on later ring plane crossings. They were hypothesized to be shattered moonlets, as described in a NASA press release of 5 October 1995.
Images of the ring plane crossings are available on a 5 CD-ROM set.
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Amanda S. Bosh, of Lowell Observatory, Flagstaff AZ, and Andrew S. Rivkin, also of Lowell Observatory, and the University of Arizona/Lunar and Planetary Laboratory, reported the discovery in the July 27 International Astronomical Union telegram (Circular No. 6192).
"We were excited to see new satellites in the Hubble pictures. This was not a primary goal of our observations, so we were quite surprised," said Bosh.
Two of the satellites seen by Hubble are in orbits similar to those of a pair of moons, called Atlas and Prometheus, discovered in 1980 by the Voyager 1 and 2 spacecraft. "If these two satellites are the same seen by Voyager, then their orbital longitudes are different from what we expected," said Bosh.
Additional Hubble observation of Saturn, taken when the Earth again crosses the ring plane on Aug. 10, will provide more images that will be used to determine whether two of the four satellites detected by Hubble are truly new or not. If all four satellites are new, then the total number of known moons orbiting Saturn will grow from 18 to 22.
Two of the new moons (called S/1995 S1 and S2) lie inside Saturn's thin, eccentric "F" ring; a third moon (S3) lies just outside the F ring; and the fourth moon (S4) is 3,700 miles beyond the F ring. The moons are no bigger than about 45 miles across.
Hubble's sharp view is ideal for detecting faint new satellites that have previously gone unseen. The astronomers identified the new moons by first processing the Hubble pictures to remove a residual light from the ring edge, and noting the locations of Saturn's known satellites. After this was done, the researchers saw four objects moving from frame to frame that did not correspond with any of the known satellites.
"With the exception of S4, which we're having trouble finding in a few frames, we can follow the other three satellites for 10 hours as they go around the planet," said Bosh.
Saturn ring plane crossings happen only once every 15 years and historically have given astronomers an opportunity to discover new satellites that are normally lost in the glare of the planet's bright ring system. Astronomers discovered 13 of Saturn's moons during ring-plane crossings from 1655 to 1980. Other satellites were identified during the Voyager spacecraft flybys of Saturn in the early 1980s.
After the upcoming Aug. 10 ring plane crossing, the next such event that will be visible from Earth will be in the year 2038. Saturn will be edge-on in 2009 and 2025, but will be too close to the Sun, as seen from Earth, to be observed.
The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency.