I was recently asked why the number of meteors that we see on the date(s) of a particular meteor shower's anticipated peak (the time when the greatest number of meteors per hour are expected) varies so much from year to year.
Meteors are not "shooting stars" or "falling stars", but are, somewhat less romantically, debris left behind by a comet or - in at least in one case, an asteroid - as it passes through our solar system, swings around the sun, and then heads back out into deep space.
Astronomers think that the majority of comets come from the Oort cloud, a vast sphere of comets surrounding our solar system at a distance of from 2,000 - 200,000 AUs (an AU is the average distance from the Earth to the sun, roughly 149 million kilometres) that formed in the very infancy of our solar system. It is believed that gravitational influences from passing stars and the Milky Way galaxy itself dislodge many of these comets from the cloud, sending some of them hurtling towards the inner solar system and our sun.
There have been thousands of comets throughout the centuries which have visited our solar system, many of which re-visit us on a regular basis. Perhaps the most famous of these re-visitors is Halley's Comet. Though now known to have been observed and recorded (though not named) by ancient astronomers as far back as 250 BC, the comet was the first comet to have its predicted return verified, when, in 1705, English astronomer, Edmond Halley re-discovered the comet based on his calculations. The comet, which has a 75-76 year orbital period (meaning it returns every 75-76 years) now famously bears his name.
If you picture a comet as a "dirty snowball" made up of various ices and rocky material that it gathered at some distant time in its creation when our solar system was forming, you can better understand that, as the comet draws closer to the sun, the ice particles sublimate (go directly from an icy state to a gaseous state), freeing the trapped small rock particles. The sun's solar wind pushes the gaseous particles off the comet's surface in the form of a gas tail, and the rocky material off as a dust tail.
Most comets have distinct gas and dust tails. As the rocky material is pushed off and trails behind the comet in its journey towards the sun, it gets spread out in a continuous, conveyor belt-like stream behind the comet. Certain parts of the debris stream are denser than others, particularly as the comet gets closer to the sun, and larger quantities of gas and rock are blown free of the comet's surface.
Over time, the conveyor belt stream of debris stretches (in a large oblong shape) the entire distance of the comet's orbit around the sun, and actually carries the comet's orbital motion with it, such that the momentum of the stream slowly moves it along the comet's orbital path. The result is that when the Earth intercepts a particular comet's debris stream at a certain (and fairly consistent) time of the year (in the case of the Perseids, mid-August), it passes through either a dense portion or a less dense portion of that stream.
The density of the stream portion the Earth passes through thus varies from year to year. If it passes through a dense portion, we can observe more (in some cases, thousands more) meteors than when the Earth passes through a less dense portion. Astronomers who study meteors and meteor showers can usually predict fairly accurately, from year to year, if the Earth will pass through a dense or less dense portion of a comet's debris stream. However, this is not an exact science, and many things can influence a given meteor shower's actual meteor count each year, the most notable being the location of the outer planets, particularly Jupiter, relative to the meteor stream path.
Thus, though predictions may forecast a low hourly rate of meteors during the peak period of a given shower, there is always the possibility that the astronomers got it wrong, and that a shower can, in actuality, turn into a storm, with many, many more meteors appearing during the peak period than forecast.
Clear skies.
Glenn K. Roberts lives in Stratford, P.E.I., and has been an avid amateur astronomer since he was a small child. His column, Atlantic Skies, appears every two weeks. He welcomes comments from readers at [email protected].
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