The Perseid meteor shower is named for the constellation Perseus, from where the meteors appear to originate. The Perseid meteor shower is one of the most prolific showers of the year, with an average peak rate of 50- 80 streaks per hour, in darker skies. Meteors are the visible paths of vaporizing space debris as it encounters our planet’s atmosphere.
This debris, known as meteoroids, ranges in size from dust particles to small pebbles, and occasionally larger stones. As a meteoroid enters the Earth’s atmosphere, it is heated by friction, which vaporizes the debris and causes the gases (both atmospheric and meteoritic) to glow. Most meteoroids disintegrate at about 30-60 miles above the surface, but become visible at about 40-75 miles.
Meteoroids orbit the Sun just like planets, comets, and asteroids. They travel at speeds of about 26 mps, but, when combined with Earth’s orbital speed of about 18 mps, enter our atmosphere at a velocity rate of about 44 mps. The meteoroids associated with the Perseid meteor shower enter the Earth’s atmosphere at about 37 mps. Our planet encounters space debris every day, thus meteors are actually visible all year long.
Occasionally, Earth passes through thicker patches of debris, known as streams or swarms, resulting in a meteor “shower.” Meteoroid streams, or swarms, have orbits similar to those of comets, thus are believed to be fields of comet debris resulting from a comet’s closing approach of the Sun.
The Perseid meteor shower has been associated with the ancient debris field of Comet 109/Swift-Tuttle. Comet Swift-Tuttle leaves new debris each time it passes our planet – every 130 years. This debris field has the appearance of several streams, each measuring millions of miles long.
The Swift-Tuttle debris streams are comprised of small widely-spaced particles. Most of the meteoroids are about the size of sand grains, but some may be as large as small pebbles. With a core diameter of about 26km, comet Swift-Tuttle is the largest known object, and one of the oldest comets, to regularly pass closely to our planet.
Comet Swift-Tuttle was originally recorded by Chinese astronomers in 69 BC and 188AD, but was formally discovered in 1862, by Lewis Swift on July 16, and by Horace Parnell Tuttle on July 19. Three others also independently discovered this comet: Dudley Observatory’s Thomas Simons; Antonio Pacinotti and Carlo Toussaint from Florence, Italy; and Danish Astronomer Hans Schjellerup. Comet Swift-Tuttle was “rediscovered” in 1992 by Tsuruhiko Kiuchi, ten years after its expected return of 1982.
That year, the comet reached 5th magnitude, making it easily visible through binoculars. Comet Swift-Tuttle will pass within 14-million-miles of our planet when it next returns in 2126. Scientists believe that the comet will be even brighter than the 1992 pass, and likely readily visible to even unaided eyes.
Astronomers once believed that comet Swift-Tuttle might, in the relatively near future, pass close enough to actually impact Earth or the Moon. While continued observations and recalculations have dispelled that concern for at least the next 2,000 years, this comet remains one of the greatest known solar system threats to our planet.
Source Material: NASA Worldbook JPL’s Solar System Dynamics Gary Kronk’s Cometography Astronomical Society of the Pacific Space.com Wikipedia
By Tavi Greiner. See more from Tavi at her site A Sky Full of Stars and follow her on Twitter @TaviGreiner