60 and counting …
Published on Jul 21, 2007 at 11:16 am.
3 Comments.
Filed under extraterrestrial moons.
A recent JPL news release announced the discovery of a 60th moon for the planet Saturn. There’s only four more to go to put Saturn into first place for the number of moons! (Jupiter has 63 known moons.) Now, granted, a number of these moons still need to be officially confirmed, and their orbits need to be determined more completely in order for them to become official. But, these moons seem to be pretty good candidates, and I don’t have much doubt about them. There are three other moon candidates for Saturn that are in the ring system, but they are quite uncertain, perhaps being just clumps in the rings rather than moons.
I have no doubt that more moons will be found. The Cassini spacecraft is currently in orbit around Saturn, and it is continually making discoveries. A number of the moons recently discovered were found by Cassini. But, Cassini isn’t the first spacecraft to discover moons. Voyager 2 observed moons around each of the gas giants. In fact, nearly half of Neptune’s known moons were found by Voyager 2, as were a number of Uranus’ moons. Also, many of Jupiter’s 63 known moons were found by the Galileo spacecraft. A number of moons in the outer Solar System also were found using the Hubble Space Telescope.
I rather like the idea of finding more moons around Saturn. That isn’t for any really good scientific reason, though. It is simply because Saturn has had more known moons throughout most of the last few centuries. Growing up, I learned Saturn as the planet with the most number of moons. Most of the time that I’ve been teaching, Saturn has been the planet with the greatest number of moons. So, I am just used to that concept. But, a few years ago, data from Galileo started coming in from Jupiter, and a large number of small moons were discovered, putting Jupiter in first place in the Solar System for the biggest number of moons.
Galileo (the man, not the spacecraft) discovered the first extraterrestrial moons when he pointed his telescope at Jupiter. He found four “stars” that seemed to move around Jupiter. Almost a half century later, Christiaan Huygens discovered a moon around Saturn (which we now call Titan). Soon, other discoveries pushed the tally of Saturn’s moons up to 8. Jupiter has four very large moons, and the rest are very small. Saturn has one very large moon, Titan, and about a couple handfuls of mid-sized moons, and then the rest are very small. These very small moons could not be seen without much larger telescopes that would not be available to astronomers for many years to come. So, by the early 18th Century, the known tally of moons was 1 for Earth, 4 for Jupiter, and 8 for Saturn. Uranus and Neptune had not yet been discovered. In the 17th and 18th centuries, scientists were always looking for mathematical patterns in their discoveries. It was in this era that the so-called Titius-Bode Law was developed. So, it was natural for astronomers to try to find some sort of pattern to the number of moons that a planet has. Well, there is a pattern here if one number is inserted: 1, 2, 4, 8. Each one is double what comes before. Well, there is a planet between Earth (1 moon) and Jupiter (4 moons). That is Mars. So, there was plenty of speculation that Mars might have two hitherto unseen moons. Interestingly enough, when Jonathan Swift penned Gulliver’s Travels in 1726, he mentioned two moons of Mars nearly 150 years before Asaph Hall actually discovered two moons around Mars! But, of course, that pattern doesn’t really hold, because bigger telescopes show far more moons to Jupiter and Saturn, and certainly Uranus and Neptune don’t fit the pattern at all.
Still, when I was growing up, Jupiter had ten moons and Saturn 11. When I started graduate school, Jupiter had 16 known moons, and Saturn had 17 known moons. Also, Uranus had 5 known moons and Neptune 2 at that time. By the time that I became a college professor, Jupiter still had 16 known moons, but another one had been found around Saturn, bringing it’s number up to 18. By that time, there were also 15 known moons of Uranus and 8 for Neptune. Then, several more moons were found by the Hubble Telescope at Saturn. So, Saturn has just about always had more known moons than Jupiter, except for a brief time after the Voyagers had passed Jupiter but not yet arrived at Saturn. That was until a few years ago, that is, when Galileo began finding moon after moon around Jupiter. That pushed the Jupiter tally up to 63, with Saturn lagging behind, with 30-something moons, and then finally 48 moons, but with Jupiter staying ahead. Now, Cassini has been busy finding moons around Saturn. With Cassini going strong, I am sure that it will find another 3 or 4 moons in the next year or two, putting Saturn back into first place.
This latest Saturn moon, temporarily designated S/2007 S4 was discovered May 30, 2007, in one of Cassini’s images. It is moon number 60 for Saturn (assuming that the discovery holds up, and the moon can be verified and tracked). Searches through earlier Cassini images also found the moon, indicating that it probably really is a moon. Cassini will pass near enough to this newly discovered moon in December 2009, assuming that the spacecraft is still going strong at that time. (The Cassini mission nominally ends in 2008, but no doubt it will be extended if the spacecraft is healthy. NASA almost always extends missions for healthy craft.)
-Astroprof
Images courtesy of NASA
Dunkleosteus on July 23, 2007 at 10:34 am: 1
“A number of the moons recently discovered were found by Cassini.”
Well, Cassini has actually discovered “only” Daphnis, Pallene, Methone, Polydeuces and this new moon. Rest of the new moons are distant irregular moons that orbit in non-circular and highly inclined (and often retrograde) orbits. All of them were discovered using ground-based telescopes, as they’re way too dim for Cassini.
On the other hand, several of the small moons discovered in the beginning of the 80s were also discovered from Earth: our planet crossed Saturn’s ring plane which allowed astronomers to see the moons otherwise hidden in the glare of the rings.
A Ler…-- Rastos de Luz on July 23, 2007 at 1:42 pm: 2
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A Ler…-- Rastos de Luz on July 23, 2007 at 3:13 pm: 3
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