For as long as records exist, the Perseid meteor showers have always been strong. This summer's Perseid shower will be exceptional. The moon is mostly out of the way later in the night, and higher-than-normal activity rates are expected over the United States.
The Perseid shower's parent body, comet 109P/Swift-Tuttle, is notable in being a comparatively huge comet in an orbit that passes close to Earth's orbit frequently. It measures 24-31 kilometers in diameter, 2 to 3 times the size of comet Halley, and is so big that the continuous ejection of water vapor and dust during its approach to the Sun does not move the comet much off course. It has spewed dust for at least 5,000 years and most likely thirty times longer. It has built a massive meteoroid stream, most of which is located just outside of Earth's orbit. Earth passes through the outer regions of that stream in July, and hits the center on August 12.
At that time, the annual Perseid shower peaks at 80 meteors per hour under ideal circumstances (no clouds or moon, dark sky, stars of magnitude +6.5 just visible).
When comet 109P/Swift-Tuttle was rediscovered in 1992, scientists noticed in alarm that a delay of 17 days in the next projected return of the comet in 2126 could cause it to collide with Earth. That fear dissipated when the orbit was recomputed using data from sightings in 188 A.D. and 69 B.C. The more precise orbit has the comet approach Earth in 2126 to within only 23 million kilometers, but there's no danger of us being hit. In September 4479, the comet will approach Earth even closer, to within about 6 million kilometers. It will then be as bright as Jupiter (-2.1 magnitude) in the sky.
The dust released will spread along the comet orbit because some dust grains make wider orbits than others and return later. When Earth encounters these dust trails, a meteor storm may be observed. But only if the very narrow trail is steered smack in Earth's path by perturbations of the planets. Most dust does wander far from the comet, which is why the best showers are observed in the years following when the comet returns, while lesser outbursts occur when dust further along the comet orbit wanders in Earth's path.
The next big shower is not expected until the next return of the comet.
For now, a nice outburst is projected for Aug. 12, 2005, at 08:18h UT (= 04:18 EDT and 01:18 PDT), when Earth will encounter the dust ejected in the return of 1479. Rates can go up four fold to about 240 per hour on top of the 80 per hour annual activity, for a brief period of time (approximately 1.2 hours).
In addition, rates may increase again around 13h UT, when Earth is slated to encounter the Filament component, rising to less than 86 per hour on top of normal, annual activity. That Filament is older dust presumably in mean-motion resonance with Jupiter.
- Top 10 Perseid Facts
- 2005 Perseid Viewer's Guide
- How Meteor Showers Work