THE PERSEID METEOR SHOWER
Hi everyone, I’m Darrell Heath with the UALR College of Arts, Letters, and Sciences: welcome to The Night Sky.
On any given day of the year it’s estimated that the Earth collides with about 19,000 meteors weighing over 3.5 ounces. The majority of these burn up in our atmosphere but a few do make it to the ground but fewer than 10 of these are ever recovered each year. On the other hand, some 100 tons of dust-sized micrometeoroids rain down on our planet each day. We occasionally see these bits of cosmic debris burning up in our atmosphere as a sudden streak of light in our night sky. We often call them “shooting stars” but of course they aren’t really stars at all. Most of this stuff is the random fragments that were left over from the birth of our solar system, but throughout the year there are certain nights when, rather than just seeing a few sporadic meteors every few hours, we can see anywhere from dozens to around a 100 meteor streaks per hour. We call these events “meteor showers” and their source material often comes from comets.
One of the very best of our annual meteor showers occurs this month: the Perseids, so named because when you trace back all the meteors you see on the peak night they appear to originate from a point in the sky where we find the constellation of Perseus. The source for the Perseid meteors is comet 109P Swift-Tuttle, which completes one orbit around the Sun every 133 years. Comets are relatively small bodies left over from the formation of the solar system and are composed of various kinds of frozen gases along with bits of rock and dust. Whenever Swift-Tuttle makes a close approach to the Sun, as it last did back in 1992, it begins to defrost and sheds large amounts of gas and dust in its wake. These streams of comet material will follow along in the same orbital path as the object that produced them and eventually spreads out over time. Like clockwork the Earth will plow through it every year and we get treated to a celestial light show when this comet dust burns up in our atmosphere some 60 miles overhead. Now, Swift-Tuttle is big, its nucleus is 16 miles across: more than twice as big as the object thought to have wiped out the dinosaurs 66 million years ago; but the stuff you see creating those meteor trails are no bigger than a grain of sand. How can something so tiny create something so bright and dramatic? Well, meteoroids (which is what we call the bits of interplanetary material that creates a meteor streak) are travelling really fast: a piece of Comet Swift-Tuttle may be screaming into our upper atmosphere at a velocity of around 133,200 mph (many times faster than a rifle bullet). The meteoroid has a lot of kinetic energy (the energy of its motion) associated with it and once it meets the resistance of our atmosphere (and suddenly slows down) that kinetic energy gets converted to thermal energy. A fast moving meteoroid violently compresses a column of air out ahead of it as it slams into our atmosphere, this sudden shock to the air molecules makes them heat up and glow. What we see as the sudden appearance of a meteor streaking across the sky is actually that superheated and glowing column of air being violently squeezed out ahead of the meteoroid.
So, when and where should you be outside and looking up for Perseid meteors? The Perseid meteor stream is very wide and you can start seeing meteors from July 17th up to August 24th. This year the peak time is the night of August 12th and into the morning hours of the 13th. But, if the weather proves to be uncooperative then viewing will be just dandy during the morning hours from August 11th through the 14th. The best time to be on the lookout for the highest hourly rates (which on the peak nights can be anywhere from 50 to 100 meteors per hour) are from midnight until dawn. The reason for this is that it’s during these hours that the side of the Earth we are on is facing in the same direction that our planet is orbiting around the Sun. It’s a bit like being in a car and looking out through the windshield into an oncoming snowstorm. Earlier in the evening the meteors will have to catch up with the Earth to collide with our atmosphere but in the wee hours of the morning we are driving head on into the meteor stream. But, don’t rule out looking during the early to late evening hours. At this time the shower’s radiant (the constellation Perseus and the spot from which the meteors all appear to radiate from) is low in the sky and if you’re lucky you might see an “earthgrazer” meteor. These are bright and very slow moving meteors traveling in a horizontal, overhead path.
The best way to see any meteor shower is to go the best dark sky site you can find, this often means getting away from your neighborhood or city in order to escape light pollution.
Get comfortable. Lie out on a blanket or a reclining lawn chair and simply look up. While the constellation of Perseus is the radiant point and is in the northeast part of the sky you will still see meteors in all directions, so don’t focus on just one area of sky.
Expect lulls in meteor activity punctuated by sudden outbursts.
Keep insect repellant on hand as well as lots of water to stay hydrated. Snacks won’t go amiss either.
Invite family and friends. A meteor shower party is a great bonding experience and even if you don’t see many meteors, you’ll at least have spent some quality time with people you care about and maybe even have created a few memories that will last you and them a lifetime.
All you really need are your eyes to see meteors but you might consider a pair of binoculars as part of your equipment. Use them to scan the rich star fields of the Milky Way, which is arching high overhead by midnight. Pay close attention to the constellations of Sagittarius and Scorpius low upon the horizon in the southwest at midnight. These star patterns are especially rich in star clusters and nebulae, many of which are easily seen with binoculars. Use a star map found in the latest issue of an astronomy magazine, an app, or download one for free from skymaps.com in order to find constellations and the deep-sky treasures hidden within them.
Also, be sure to watch the Central Arkansas Astronomical Society’s web site for announcements of meteor shower observing nights at their River Ridge Observatory.
Most of all just get outside and look up because I guarantee that once you start seeing meteors you’ll be looking up in both awe and wonder.