A burst of bolides
Published on Sep 9, 2008 at 4:31 pm.
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Filed under meteors.
Dr. Bill Cooke of NASA’s Meteoroid Environment Office has reported an unusual burst of meteor activity for this past morning. Over a period of four hours, over two dozen bright meteors were recorded appearing to come from about the same part of the sky. These meteors were quite bright, as bright as Jupiter or Venus, so I am calling them bolides or fireballs to differentiate them from the normal dimmer meteors that you often see every night that you are out where it is dark for an extended period of time. The image below is formed from sandwiching frames collected over part of that time. There were not really that many meteors in the sky all at the same time!
Meteors are the flashes of light that you see when meteoroids slam into the Earth’s atmosphere at high speed. These meteoroids compress the air in front of them, which glows white or blue hot for an instant. The meteoroids don’t generally survive the event, and they burn up. But, for observers on the surface of the Earth, we get to see a quick flash of light shooting across the sky.
The radiant of this particular meteor shower seems to be from the area of Perseus. But, there is not normally a meteor shower this time of year. The radiant is not from the part of Perseus when the Perseid meteors of mid August are normally active, and the Perseids don’t normally have bursts of activity like this. So, what is going on?
Cooke is proposing that this may be one of the rare outbursts of the Delta Aurigids (now sometimes called the September Perseids since the radiant is more in Perseus than Auriga). Like many minor showers, the Delta Aurigids normally don’t do much, so we don’t know much about them. With only an occasional meteor now and then, these sort of minor showers are not very interesting, so meteor observers rarely pay much attention to them. But, they can occasionally offer a surprise. If this is an outburst of the Delta Aurigids, then it is only the fourth such outburst known (the other three coming in 1936, 1986, and 1994). That does not mean that there have not been other outbursts, of course. Since the shower is so little observed, other outbursts may have gone unnoticed or unreported. Cooke believes that this current outburst is over, so the chances are that if you go out tonight and look, you won’t see any Delta Aurigids, or if you do it will be only one dim one. But, with a meteor shower so unpredictable, you never really know what it will do for sure.
What causes this sort of thing? Any small stray piece of solar system debris hitting Earth’s atmosphere will cause a meteor. Perhaps hundreds of tons of the stuff hits Earth on an average day, just randomly. But, once in a while, Earth passes through a swarm of meteoroids. For a period of time, observers on Earth see multiple meteors appearing to come from the same part of the sky. This is a meteor shower. There are several major meteor showers that occur every year as Earth passes through a stream of debris, such as the Perseids or the Geminids. Other meteor showers, like the Leonids, are normally pretty subdued with big bursts only at intervals of decades apart. The Delta Aurigids are of this type.
So, where does all this debris come from? Most of the stray sporadic meteors that you see every night are simply bits of debris orbiting the Sun that happen to run into Earth. But, as I said, meteor showers occur when there are swarms of debris moving together. Normally, the swarms are not very tight, so you get a few meteors per hour over several days. Occasionally, the swarms are quite tight, so there are a hundreds or thousands of meteors over several hours (the Leonids are famous for doing that about every three decades). To get all that debris moving together, you need some source for it. A common source is believed to be comets shedding debris as they go around the Sun. That debris then follows along in an orbit similar to that of the comet. For very young comets and debris clouds, the debris is tightly clumped. For more mature debris clouds, the debris is spread out along a path similar to that of the comet. There is no clear candidate, though, for the comet that may be the host of the Delta Aurigids. That should not be a surprise, though, since there are so few observations of the meteors that a reliable orbit for the meteoroid stream would be tough to determine. You may be able to find more about subsequent observations at SpaceWeather.com’s web site.
So, this is a good reminder that astronomy is a dynamic field, and you should be ready for the unexpected discovery. That is part of what makes it fun!
-Astroprof
Image courtesy Bill Cooke, NASA, MSFC
