Astroprof’s Page

The Alpha Aurigid Meteor Shower

Published on Aug 29, 2007 at 12:27 am. 2 Comments.
Filed under meteors, skywatching.

Earlier this month, the Earth passed through a swarm of meteoroids shed by Comet Swift-Tuttle, giving rise to the Perseid Meteor Shower. Comets are a mixture of ice, rock, and dust. As they swing by the Sun, the ices melt (sublimate) and act to cause the comet to shed some of the solid material, too. This material is very small and it is pushed around a bit by the pressure of sunlight. It too orbits the Sun. When Earth passes through the swarm, a large number of the meteoroids (what we call the debris when it is flying around the Solar System) run into Earth, creating meteors (what we call the flash of light as they shoot into the atmosphere, burning up). The more frequently the comet passes by the Sun, the more meteoroid streams become associated with the comet, moving along in similar orbits to the comet. Over time, these meteoroid streams spread out along the approximate orbit of the comet. If the comet’s orbit passes near the Earth’s orbit, then every time that the Earth passes near the comet’s orbit, it passes through meteoroid streams, resulting in an annual meteor shower. That is why we see the Perseid meteors every August when Earth passes near the orbit of Comet Swift-Tuttle. October’s Orionid meteors are debris from Halley’s Comet. Those are mature meteor showers. November’s Leonid meteors are debris from Comet Tempel-Tuttle. However, the Leonid meteor swarms have not spread out as much as those of the the Orionids or Perseids. So, the Leonids are normally a pretty poor meteor shower, with just a few meteors per hour. But, about every three decades, a dense clump of meteoroids from Comet Tempel-Tuttle has run into Earth, yielding a spectacular meteor shower (gravitational perturbations from the planets have gradually shifted the meteoroid streams, so this will happen only one more time near the end of the 21^st Century, and then never again for the Leonids).

But, all of these are fairly short period comets (they have orbits of only a few years up to a few dozen years). There is another class of comets, long period comets, that have orbits of many hundreds to thousands of years long. These comets have much larger orbits, and they come by the Sun less frequently than the short period comets, so the meteoroid streams typically don’t get much of a chance to spread out along the orbit. Thus, even when the long period comets have orbits that cross Earth’s orbit, they seldom produce a meteor shower.

However, in a few days, Earth will be passing near the orbit of the long period comet C/1911 N1 (Comet Kiess). This comet has an orbit with a period of about 2100 years, or so. It has only been observed on one passage by the Sun, in 1911. However, those observations were enough to give an approximate orbit for the comet. It has shed some meteoroids, but like the Leonids, these meteoroids have not spread out along the entire orbit (and likely never will). A few meteoroids have spread out along the orbit somewhat, so each year near the last week of August and the first week of September as Earth passes near the orbit of Comet Kiess, there appear to be a few meteors shooting away from the constellation Auriga. These meteors appear to shoot away from a spot in the sky very near the star Capalla, which has a Bayer designation of Alpha Aurigae, so these meteors are called the Alpha Aurigids (to differentiate from another minor meteor shower that shoots away from Delta Aurigae: the Delta Aurigids). Theta Aurigae is also near the radiant of this meteor shower, so they are sometimes called the Theta Aurigids, but I’ll use the more classic name of Alpha Aurigids for the event. Typically, the Alpha Aurigids are pretty puny. They amount to only a half dozen or so meteors per hour … or less. So, it is no great surprise that they are seldom observed. However, there have been a few instances in which the Alpha Aurigids have surprised observers. In three years (1935, 1986, and 1993) the Alpha Aurigids has brief spurts of activity (only an hour or so long). These periods of activity, corresponding to somewhat denser regions of the meteoroid streams, were at least as active than the most active of the typical major annual meteor showers (like the Perseids, Geminids, or Quadrantids). But, the Alpha Aurigids are normally extremely low activity, so most people don’t go looking for them. For one of the peaks, there were only two reports from meteor observers. The periods of activity were quite short, so this raises the possibility that there may have been other instances of activity that happened in years at times that no meteor observers were watching the sky. The known periods of activity have seemed random, so few observers have gone looking for meteors on just the off chance of an active period.

But, Esko Lyytinen and Peter Jenniskens have studied meteor showers and methods of predicting meteor shower outbursts, and they have come up with an interesting analysis of meteor showers associated with long period comets. They used a technique that was used to successfully predict a very rare outburst of the Alpha Monocerotid Meteor Shower in 1995 to determine potential outbursts of the Alpha Aurigids (Note that they use the other name, Theta Aurigids, for this meteor shower). They have been able to reproduce the earlier observed outbursts. More interestingly, though, they also predict that there may be an outburst this year. The peak activity, according to their calculations, is September 1, 2007, at about 11:36 UT, give or take about half an hour. The Alpha Aurigids, in prior outbursts, have high activity for only short periods of time. That means that you need to be in the right place on Earth to see them. The diagram below shows the portion of portion of the Earth that should be struck by the meteoroids at about the time of peak activity.

Note that most of the planet that gets the best view is covered in ocean. The West Coast of the United States gets a good view, as does Hawaii (the same places that got the best view of the lunar eclipse yesterday morning!). Here in Texas, the Sun is coming up right at the predicted maximum of activity. However, predicting meteor activity associated with long period comets is pretty new, so there is some uncertainty in the calculations. If the peak comes 30 minutes or more early, then we get a good view here. If the maximum comes 30 minutes or more late, then you have to be west of the Rocky Mountains to see the meteors at peak activity. In either case, Hawaii should get a good view. The predicted time of peak activity, 11:36 UT, corresponds to 6:36am CDT here in Texas, 4:36am PDT along the west coast, and 1:36am HST in Hawaii.

The Alpha Aurigid meteors will hit Earth’s atmosphere fairly quickly, at perhaps up to 66 km/s near dawn. This means that the meteors should be fairly bright, ranging from magnitudes +3 to -2. Previous observations suggest that about 3/4 of the brighter meteors leave a persistent train (a glowing path behind the meteor). Also, observations suggest that a fair number of these meteors are blue-green in color. That is interesting, because the only other meteor shower known to be associated with a long period comet that has been widely observed was the 1995 Alpha Monocerotid meteor shower, and that meteor shower also had a large number of blue-green meteors. The Alpha Monocerotids also were observed to penetrate about 5 kilometers deeper into Earth’s atmosphere than the typical meteors associated with annual meteor showers. It will be interesting to see if the same is true of the Alpha Aurigids. This meteor shower will be well observed by both amateur and professional astronomers, since meteor showers associated with long period comets are very rare. Long period comets and short period comets may be somewhat different in composition due to the more frequent heating of the exterior of the short period comets near perihelion as they orbit the Sun. So, naturally, we want to study these meteors, particularly if there might be a more-than-average number of them. This is particularly important this year, because Lyytinen and Jenniskens’ calculations indicate that there will not be another Alpha Aurigid outburst for about 70 years. After that, gravitational perturbations of the comet and meteoroids’ orbits may mean that the meteoroids will after that always miss Earth, so that would be the last chance to observe this meteor shower.

So, what might you expect to see if you go out and look for these meteors yourself? Well, that depends upon where you live. You need to first of all be in the right spot on Earth. That means being in the dark shaded region of the diagram above. But, remember that the diagram was produced for the anticipated peak time. The peak may be earlier (and then more of North America gets to see the meteors just before dawn) or it may be later (so that less of North America, if any, sees the meteors). Also, the Moon will only be four days past full, so the moonlight will interfere significantly with observations. To minimize the effect of the Moon, try to find a location where direct moonlight is blocked (such as by a building, tree, etc). Even a hat with a brim positioned to keep the moonlight out of your eyes will help. The meteors will look like they are coming away from a spot in the sky near the bright star Capella (in Auriga), as seen in the diagram below.

So, for sure, if you are going to be on the west coast, or in Hawaii or Alaska, then you might want to go look for the meteors. I’ll probably be looking here in Texas (weather permitting) right before dawn, but I don’t really expect to see the peak activity. Even so, there may be a handful of meteors shooting away from Auriga in the hour or so before dawn, and that may be true for much of North America (particularly the western half of the continent).

Good luck observing!

-Astroprof

Diagrams courtesy of ARC, NASA