The Perseid Meteor Shower Explained

Tonight is the peak of the Perseid Meteor Shower, one of the most popular meteor showers of the year. Folks lay out in the dark and count the meteors as they enter the Earth's atmosphere and burn up. Some are green, some are yellow, some are orange. Some meteors could be called fireballs-fireballs can make a whistling noise, make a sonic boom, or may even leave a smoke trail. It's hard to believe, but the Perseid meteors are pieces of a comet called Swift-Tuttle.

Picture, if you will, a model of the solar system in your head. Swift-Tuttle is a 6 mile wide comet(about the size of the one that supposedly killed the dinosaurs) named for the two astronomers, Lewis Swift and Horace Tuttle, who discovered it back in 1862, during the Civil War. The comet had actually been observed for a couple of thousand years, but it wasn't until 1862 when it's elliptical orbit was calculated and they discovered it's period to be 120 years or so. They predicted the return of the comet for sometime around 1982. Comets get subjected to all kinds of forces and therefore it is hard to predict when they will return. The conventional theory that comets are dirty snowballs is under debate-I personally don't subscribe to the dirty snowball theory, but for the sake of this blog, I will stick to convention. Anyway, as the snowball comet approaches the sun, it begins to outgas and sling particles off it's surface, leaving a tail, a trail, that is often seen by observers. Once the gasses have dissipated, blown away by the solar wind, the small rocky particles still remain, floating along a debris trail in space. Some particles are large, some are almost microscopic. Each time the comet makes a pass, it adds a few more lines of debris, like a dog tracking mud into the house for the third time. Every year, the Earth, as it orbits the sun, passes though the dirt trail left by Comet Swift Tuttle, and as those pieces of dirt hit our atmosphere, we call them meteors. From Earth, it you look to space to determine exactly where the meteor stream originates, you will see that if you trace them back, they will all form a line back to a general spot in the sky, in this case, the constellation of Perseus. All meteor showers have a radiant constellation. The Geminids, in December, can be seen by looking in the direction of the constellation of Gemini. The Taurids originate from the constellation of Taurus. Each meteor shower is named for the radiant constellation, not, ironically, for the comet that created the debris trail. Some years, Earth travels through a part of the debris trail that is thick, and some years it travels though an area where there is not much debris, areas we have swept clean on prior years. If we are lucky, the comet has passed through laying down a new debris trail for the planet to pass through, creating a show that will become an active meteor shower.

As I mentioned before, the comet was due to return around 1982, but it did not show up until 10 years later in 1992. The reason for the delay could have been because it was affected by the gravity of other planets or moon, something in the Kuiper Belt, or a change in mass causing a loss of velocity from the outgassing and particle loss. There are a lot of variables that affect comet prediction. An astronomer named Brian Marsden solved the mystery in 1973. He correctly proposed that the outgassing would elongate the comets orbit creating a 130 year period. He correctly predicted the return of Swift Tuttle in 1992. Marsden made predictions based on the new orbit and velocities of Swift Tuttle, and August 14, 2126. But if the actual date of perihelion (when the comet makes its closest approach to the sun) was off by 15 days from his prediction (as the 1992 perihelion had been off by 17 days), the comet and the Earth might be in the same place in space at the same time. Since Comet Swift-Tuttle is thought to be about 10 kilometers or six miles across, the same size of the asteroid that killed the dinosaurs, it must be taken seriously and watched closely for any additional changing variables.

Marsden continued to refine his work, and he traced the comet's path back 2000 years matching observations in 188 AD and 69 BC. Marsden concluded that it is highly unlikely the comet will be 15 days off in 2126, and he called off his warning of a possible collision. His new calculations show Comet Swift-Tuttle will pass a comfortable 15 million miles from Earth on its next trip to the inner solar system. The thing is, there is no way to really know what will happen with so many variables in play.

Worry not about Swift Tuttle colliding with Earth, just grab a bottle of wine, some blankets, some fruit, cheese, chocolate, and most importantly, those you love, and head off to a dark place. Lie on your back and watch Nature's Fireworks. Have fun! I'm socked in the clouds here. Bummer, but at least we have seen more than a few Perseids during the last few weeks. I'll take what I can get, I suppose. I'll be watching for them over the coming weeks. Just becuase you miss the peak doesn't mean the excitement is over. I have seen a lot of really cool fireballs after peak time.