Just kidding about those tinfoil hats. It's pretty much a guarantee that no Perseid meteoroids will make it to the ground. That's because they're small and fluffy. Perseids range in size from sand grains to peas with a few larger ones up to about 3/4-inch (20 mm) across. When they strike the atmosphere at the incredible speed of 133,000 miles an hour (60 km/sec) — 242 times faster than a jet airplane — they incinerate into fine dust.
Right at the outset, we should distinguish between the terms meteoroid, meteor and meteorite. A meteoroid is a particle shed by a comet or asteroid in space. If it happens to enter Earth's atmosphere, it heats up, makes the air glow around it, and we see a meteor trail. A meteorite is a meteoroid that's big enough and dense enough to survive passage through the atmosphere and land on the ground.
Perseids are small and friable because they come from a comet, a porous, lightly-compacted object composed primarily of ice mixed with dust. A typical asteroid on the other hand is made of denser rock. Each meteor shower has its own "parent" object. For the Perseids it's Comet 109P/Swift-Tuttle, discovered in 1862 by American astronomers Lewis Swift and Horace Tuttle.
Whenever a comet passes near the sun, solar heat vaporizes (the technical term is sublimes) ice from the icy core of the object called the nucleus. Dust and small rock fragments are embedded in the icy are released into space and form the comet's glowing head, called the coma, and tail.
As the comet continues to circle the sun it drops "breadcrumbs" of debris that spread along its orbit. For about four weeks beginning in mid-July and continuing through mid-August, the Earth coincidentally passes directly through the debris trail of Comet Swift-Tuttle. Like driving through a snowstorm at night, only a few flakes bound off your windshield at first. But as you approach the core of the storm, so many bombard the car it can be difficult to focus on the road.
When you finally come out on the other side of the storm, the number of flakes decreases until it drops back to zero. Replace the car with planet Earth and the snow with Perseid meteoroids and you have a good idea of what happens during the Perseid meteor shower. A few Perseids are visible each night in late July. We plow through the center of the comet's dust trail on the nights of August 11-12 and 12-13, and then slowly depart until we're in the clear again.
Perseids strike the air between the altitudes of 50 and 75 miles (80-120 km). Incoming meteoroids glow from heating but also because they ionize the air around them. When air slams into the incoming meteoroid it gets really, really hot, reaching temperatures as high as 3,000° F (1,727° C). The intense heat excites the atoms of both the air molecules and the material in the meteoroid that's being stripped away by the hot air. The atoms then release this pent-up energy as light, and we see a bright streak or meteor.
The average meteor trail is about 20 miles long but just 3 feet in diameter (32 km x 1 meter) and lasts a second or two. Streaks from larger meteoroids are brighter and last up to several seconds. They're known as trains. Perseids are rich in bright meteors, called fireballs, that leave trains. Be sure to watch for them.
Different elements glow (or ionize) in different colors. Magnesium (green), sodium (orange), iron (blue) and calcium (purple) are common in dust shed by Swift-Tuttle, while oxygen and nitrogen in Earth's atmosphere create the red hues. When all occur at once the brightest part of the trail will appear white. Most of the bright meteors I see are colored green from magnesium. During the passage of a bright fireball multiple elements often get into the act, with several colors visible across the length of the streak.
The Perseid shower will be best on Wednesday night, August 11-12 and extend into the next night as well. That's great news in case of uncertain weather. The moon is a crescent and will set around 10, so it won't detract from the show. From a country sky you might see 50-80 meteors per hour at peak in the wee hours of the 12th. Since most of us live with light pollution, figure about 30-40 per hour from the suburbs.
Give the shower a little time to show its stuff. If you can spare an hour, that's great. Two hours is even better. Rates vary. Sometimes a few meteors will fly by in the space of a minute. Other times, you'll have a 5-minute gap between one and the next. I always keep an informal meteor count and pick a number to end my watch. That might be 30 meteors, but inevitably I push past it. 30 becomes 40 and then 50 before I finally slog to bed. I can never get enough. During last year's Geminid meteor shower I saw more than 100 tear across the sky.
Perseids radiate or stream from a radiant in Perseus the Hero, the reason they're called Perseids. That makes distinguishing shower member from random meteors easy: it you can trace a trail back to Perseus it's a Perseid! The radiant is located just below the left end of the familiar W of Cassiopeia in the northeastern sky.
You can start your shower watch as early as 10 p.m. local daylight time when the radiant stands about 20° high in the northeastern sky. But the later you stay up the more meteors you'll see. That's because Perseus rises higher and higher through the night and fewer meteors get cut off by the horizon. So the best time is really from about 1 a.m. till dawn.
Go out when you feel like it. Bring a reclining chair and blanket to keep warm, and face southeast or north. Bring the kids, too. Let 'em stay up late.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.