Early August always brings with it the promise of a spectacular show in the form of the Perseid meteor shower. This shower, which peaks August 11-13, is one of the most reliable and active meteor displays throughout the year. A new NASA study also shows that more Perseid meteors are fireballs (averaging over 100 per year) than in any other meteor shower.
The Perseids have appeared in historical records over the past two thousand years, with the earliest known observation occurring in 36 AD. Perseid meteors appear to originate at a radiant located in the constellation of Perseus, hence shower's name.
A meteor shower occurs when the Earth passes through a stream of debris, usually, but not always, resulting from the passage of a periodic comet. The Perseid meteors are associated with the comet Swift-Tuttle, which has an orbital period of 133 years, and which made its last passage through the inner Solar System in 1992.
The rate at which bright Perseids are seen is larger when the Earth passes through this new filament of debris, leading to a double peak in the Perseid rate. This year, the first peak is expected at 1:00 p.m. UTC on August 12, and the second peak at 2:00 a.m. UTC on August 13. The first peak is well placed for western North America and the Eastern Pacific, while the second favors Europe and Africa.
Owing to the large eccentricity and the sizable angle between Swift-Tuttle's orbit and the plane of the Solar System, the Perseid meteors tend to be very fast - some 60 km/s (38 miles per second). This gives a meteoroid of a given size more kinetic energy, which in turn can produce a meteor having a larger peak brightness.
At present, about 20 Perseids per hour are being seen in dark skies. NASA's Fluximator expects this rate to increase to about 90 per hour in the early morning hours on the nights of the August 11 and 12. (A waxing crescent Moon will set before midnight local time during the peak.) This is the rate at which meteors might be seen in dark skies. In the suburbs the estimated rate is closer to 30, while in a downtown area you would be doing well to see six or seven per hour.
Most meteors observed during a shower are no larger than a few millimeters in size. However, brighter meteors, known as fireballs and bolides, appear occasionally. A fireball is not precisely defined, but generally refers to a meteor with a peak brightness equal to or greater than any of the planets. A bolide is a meteor brighter than magnitude -14, or about four times brighter than the Moon. On occasion, bolides will survive the trip through the atmosphere to land as meteorites.
The new NASA study referred to earlier found that the average brightness of a Perseid fireball is -2.7, which is midway in brightness between Jupiter and Venus at their brightest. (The dimmest star you can see in a dark sky is magnitude +6, Venus at its brightest is magnitude -4.) The numbers seen in past showers suggests that a Perseid fireball might be visible at a given location every hour at the peak of the shower. Already two to three Perseid fireballs are appearing each night, with brightness as large as magnitude -6.
What can you do if your skies are either very bright, or you are clouded out? There are a variety of skycams which will be available online. There are too many to list here, but a particularly nice setup is available from the UK Meteor Observation Network.
Another approach is to set an FM radio to listen to a high power station located on a clear channel between 400 to 1,200 miles distant. If you listen to this frequency, you will hear mostly silence. However, when a meteor passes over the right area, you will hear a short (typically from one to perhaps thirty seconds long) burst of the distant station's signal.
The signal is bouncing off the ionized meteor trail, just as lower frequency signals bounce off the ionosphere. Just as in visually observing meteors, the shower peak will be in the wee morning hours, with a peak just before dawn. One advantage to radio observation is that you may be able to observe meteors in the daytime. Try it, and good skies to all.