Geminids 2011

Times of the Geminids (Click image to enlarge)

Radiant of the Geminids (Click image to enlarge)

The Geminids this year will peak on the afternoon of 14th December but unfortunately it is only a few days after full moon, which will seriously interfere with the view. Weather prospects in the UK aren’t that good either but it will still be worth looking out if there is a good clear patch. The Geminids are one of the more reliable showers so although the moonlight will drown out the fainter meteors there are usually several fairly bright ones per hour that should be visible.

The sky chart shows the position of the radiant at midnight, with the moon not far away. Look well away from the moon to reduce the glare, don't forget that meteors can be seen anywhere in the sky.
The grey areas on the graph show when the moon is above the horizon and the yellow bars show the evenings when the greatest Geminid activity is expected. The lighter blue lines show the extent of the shower a few days either side of maximum. Sunset is about 4pm and sunrise about 8am.

© 2011 Richard Fleet

Did the Geminids come from a comet or an asteroid ?

Soon after asteroid numbered 3200 was discovered in 1983 the similarity of its orbit to the Geminid meteor stream was noticed. Meteor streams are usually associated with comets but, despite careful searches, this object has never shown any sign of recent cometary activity.

Subsequently called Phaethon it belongs to an important group of asteroids whose orbit can cross the Earth’s. It turns out to be an unusual member of that group with a different colour suggesting a different composition to most asteroids.

One of the curious things about the Geminid particles is that they are more solid than meteoroids known to come from comets. This is good for meteor watchers; giving us more bright meteors, but it also tends to support the idea of a broken asteroid rather than a typical comet.

Things got even more interesting in 2005 when a small asteroid called 2005UD was discovered in a similar orbit to Phaethon, it has the same unusual colour which points to a common origin. More recently another asteroid, 1999 YC, has been suggested as a member of the group. Phaethon is clearly the major member at about 5 km across, the other two are little more than a kilometre in diameter.

Orbit diagrams from JPL http://ssd.jpl.nasa.gov/?orbits

A collision is one way to break up an asteroid but that doesn’t seem particularly likely in this case. Far more plausible is the suggestion that Phaethon contained some deep pockets of ice that were heated enough to vapourise and fracture the body, breaking off large chunks in the process. Phaethon goes much closer to the Sun than Mercury so the heating is intense, any ice in the outer layers would soon disappear and the deeper layers would eventually warm up.

Small asteroids and comets are probably little more than piles of rubble and dirty ice anyway. Breakups can be messy affairs and there will be a vast number of small pieces and perhaps a few big ones left over.

The total mass of the meteor stream needed to explain the number of Geminids that we currently see is perhaps 10% of the mass of Phaethon. It wouldn’t have to be a complete break up, just breaking off one end might have been enough.

After many orbits the debris gradually separated and now, perhaps 1000 orbits later, it is scattered all around the original orbit. As they separate each particle will experience slightly different planetary encounters so the orbits can diverge significantly.

Fortunately for us the big chunks won’t come anywhere near the Earth, Phaethon’s closest approach this century is 3 million km in December 2093. 2005 UD s closest approach was at its discovery in October 2005 at the very safe distance of 13 million km.

Radar image of Phaethon at its last 'close' approach. Source link

The orbit of the meteor stream also changes over time, in the middle of the 19th century it started to drift across ours and a new but weak meteor shower was noticed. Observations by amateur and professional astronomers over decades have shown that rates have increased as we reach denser parts of the stream. In another hundred years or so it will move off again and we will lose the Geminid meteor shower completely.

There could well be many other chunks of rubble tens or hundreds of metres across left over from the original breakup. These would now be on slightly different orbits to the rest of the meteor stream and are well below current detection levels. It will be interesting to see how the Phaethon-Geminid family of objects grows in the coming years.

We don’t know exactly when Phaethon was deflected into its current orbit. If it was originally an active comet it would have taken many orbits for all the ices to have been lost, and would have been an impressive sight. However it may have been a stony asteroid with pockets of ice. On those first early passes close to the sun any pockets of ice near the surface would have vapourised giving at least some cometary activity. During the actual breakup Phaethon would certainly have looked like a comet, if only for a few days or weeks. It wouldn’t have needed a great deal of ice to fracture the rubble pile so it might not have been as spectacular as an icy comet breaking up.

The answer to the original question really depends on how you draw the line between comets and asteroids, but it does seem rather likely that Phaethon was more asteroid than comet. It may be a long time before we learn from a spacecraft what Phaethon really looks like but those little streaks of light we see each December tell an interesting story.

© 2009 - 2010 Richard Fleet