http://astroprofspage.com/archives/498 wrote:
3200 Phaethon: Asteroid or Dead Comet?
Published on Dec 1, 2006 at 3:41 pm.
<<Comets are icy bodies that form in the outermost parts of the Solar System. Sometimes their orbits are disturbed, and they swing close to the Sun. When this happens, the ices (frozen water, frozen carbon dioxide, frozen methane, frozen ammonia, etc) begin to sublimate and shoot off, giving rise to a pretty blue tail. But, they are also composed of rocks and dust. Some of this dust is also blown out when the gases shoot away from the comet’s nucleus. The dust particles then are pushed back by light pressure to form a more yellowish dust tail. But, the larger dust and rock particles continue to orbit the Sun for a while with orbits not too dissimilar to the comet. Occasionally comets that swoop in towards the Sun from the outer Solar System pass too close to Jupiter or Saturn, and those giant planets’ gravity cause the comets’ orbits to shift so that they return to perihelion over and over again in only a few years. These are periodic comets. And, sometimes these comets have orbits that cross the orbit of the Earth. For a comet whose orbit passes near Earth’s orbit, and has done so for a very long time, the dust and rock particles moving along in orbits similar to that of the comet form a swarm of meteoroids that Earth tends to pass through at about the same time every year. These particles, when they strike Earth’s atmosphere, become meteors (we call them meteoroids when they are still traveling through space). When a bunch of meteoroids are striking Earth, then there are more than the normal number of meteors to be seen, and they appear to be coming from a particular direction in space. We call this a meteor shower. Several meteor showers can be directly associated with comets. For example, the Orionid meteors and the Eta Aquarid meteors are associated with Halley’s Comet. The Leonid meteors are associated with Comet Tempel-Tuttle, and the Perseid meteors are associated with Comet Swift-Tuttle.
Now, for the really new part. In a couple of weeks, the Geminid Meteors will peak. I’ll say more about them in a later post. Occurring near the middle of December every year, the Geminids are a pretty reliable meteor shower. So, what comet are the Geminids associated with? Well, as it turns out, none. The Geminids are associated with an asteroid: 3200 Phaethon. An asteroid? What’s going on? Well, obviously we need to take a closer look at Phaethon.
In 1983, Simon Green and John Davies were studying images from the IRAS satellite. They were looking for moving bodies, and they found one. This was the first asteroid discovered from a satellite, and it turned out to be quite an odd one! The newly found object was assigned a provisional designation of 1983 TB. Eventually, it was given a number and a name: 3200 Phaethon. With a perihelion of only 0.140 AU, Phaethon approaches closer to the Sun than just about any other asteroids, making its surface temperature at that point in excess of 1000K. Hence, the asteroid was named after Phaeton, the son of the Greek Sun god Helios. No one has really explained to me the difference in spelling, except that it may be an attempt to avoid confusion with Heinrich Olbers’ hypothetical planet that he called Phaeton that was supposedly between Mars and Jupiter and somehow blew up to form the asteroid belt. If that were the reason, then the strategy failed, because people regularly misspell the asteroid’s name as Phaeton instead of the correct spelling of Phaethon. With an aphelion of 2.403 AU, Phaethon is classified as an Apollo Asteroid (one whose orbit crosses that of the Earth). Phaethon will actually be passing fairly close to Earth just next year: December 10, 2007. On that date, it will be only about 0.121 AU away. In astronomical terms, that’s pretty close, but it is still far by human standards (a bit over 11 million miles).
But, none of this really explains the Geminid meteors. So, what else can we say about Phaethon besides its having a strange orbit? Well, for one thing, Phaethon is one of a rather rare category of asteroids that is carbonaceous (rich in carbon compounds). It is a Type-B asteroid (a subtype of Type-C carbonaceous asteroids with a slightly different UV spectrum and a slightly more bluish color). This is one clue.
There have been several other bodies discovered that were unusually rich in organic compounds for an asteroid. One of these was 2060 Chiron, and more recently 60558 Echeclus. Both of these bodies were found in the outer Solar System, beyond Saturn. They have another similarity, too. They were discovered and named as asteroids (though their location in the Solar System makes them clearly different from the asteroids of the main belt), but several years after discovery they were found to have cometary features (a coma and tail). Both have now also received cometary designations: 95P/Chiron and 174P/Echeclus (making them the only two comets that I know of that are named for other than the one who discovered them, unless you count Halley’s Comet, which he didn’t actually see himself). So, what are these things, comets or asteroids?
It gets more confusing. There are at least two comets that orbit in the asteroid belt that look and act just like asteroids when they are near perihelion. They have comet names, but they have also been assigned asteroid designations. They are 133P/Elst-Pizarro (7968 Elst-Pizarro), and 176P/LINEAR (118401 LINEAR). Upon discovery, and much of the time, they look like rather wimpy comets, but at aphelion they look more like very carbon-rich asteroids. Another comet, P/2005 U1, does something similar, but to my knowledge has not yet been assigned an asteroid designation. And then, there is another comet, 107P/Wilson-Harrington that was discovered in 1949. In 1979, an asteroid was found and given a provisional designation of 1979 VA. Upon closer analysis, it was found that 1979 VA had the same orbit as Comet Wilson-Harrington! So, the comet had apparently turned into an asteroid! It was eventually given an asteroid number, making it yet another body with dual asteroid/comet status: Comet 107P/Wilson-Harrington and Minor Planet 4015 Wilson-Harrington.
Now, we have more pieces of the puzzle. Comets can become asteroids. But, how is this possible? Well, remember that comets are a mixture of ices, frozen gases, rock, and dust. Once the icy part has mostly sublimated away, what is left is rocky material rich in organic molecules (originating from chemical reactions from the ammonia and methane that made up the comet). After comets have spent long enough and made enough passes by the Sun, the volatiles are mostly gone, and what is left looks pretty much like a carbon-rich asteroid. No doubt deep inside these bodies, there is still some ice and frozen gas, but it is buried under many layers of rock and can’t get out unless a major impact fractures the body. So, Phaethon, perhaps, is what is left of a comet that has run out of easily accessible volatiles. Before doing so, though, it would have been a comet spewing material into space like any other comet. Some of that material would have been dust and small rocks: the things that are the source of meteor showers. So, could Phaethon be a dead comet? That seems very likely. Now, that is not the only possible interpretation of the association of the Geminid meteors with Phaethon. Some have suggested that a major impact on the asteroid could have thrown a lot of material out into space in a similar orbit. But, that seems unlikely to me to be the source of a rather regular meteor shower. Such a thing suggests to me repeated events shedding material, like a comet would do. So, I rather think that Phaethon is more likely a dead comet than an asteroid. But, as we have seen, a comet might be a lot like an icy asteroid. It sort of depends on how you look at them.
And Phaethon is not the only dead comet out there. Other asteroids that have a great deal of similarities to Phaethon and to comets are 2101 Adonis and 2201 Oljato. And, the Geminid meteor shower is not the only meteor shower associated with an asteroid. The Arietid meteor shower, which comes in June each year, seems to be associated with the asteroid 1566 Icarus. So, I guess that just complicates the whole classification thing yet more. Hmm. Astronomy used to be so simple before we started finding out how complicated the universe is!>>
Meteor in the Desert Sky
