The annual Perseid meteor shower — the most popular one of the year — has already begun, with the most shooting stars expected to fall around Aug. 11 as the shower reaches its peak.
The Perseids occur each year when the Earth plows through a stream of space rubble left behind by a large comet called Swift-Tuttle. Bits of debris going over 100,000 miles per hour strike the planet’s atmosphere, creating quick streaks of light and sometimes even dramatic fireballs.
Here’s what to know, and how to take in the show.
Aim for dark skies, and adjust your eyes
This time around, the greatest activity is expected to occur on the night of Aug. 11 and into the early morning hours of Aug. 12, says Hunter Miller, a public observing educator at the Adler Planetarium in Chicago, who notes that viewing is best under dark skies, away from artificial light and the bright moon.
“This year isn't necessarily the best year when it comes to the moon, but it's not so bad because the moon will be far away from where the Perseids are located in the sky,” says Miller.
Plus, the moon will set around midnight on the day of the shower’s peak, he adds.
“As late as you can be out, the better the views will get, the darker the sky will be,” he says. “As the moon sets, you'll have a pretty nice dark sky.”
He recommends looking northward and giving your eyes at least 30 minutes to adjust to the darkness — and warns that looking at the bright screen of a phone “will basically mess up all of the time that you spent trying to let your eyes get dark-adjusted.”
“You want as big of a view of the sky as possible,” says Miller. “Sit back, get your lawn chair out. My favorite part about meteor showers is that it doesn't really require too much.”
If clouds keep people from watching the Perseids at their peak, the views should also be good in the days immediately before and after, says Peter Brown, a researcher at the University of Western Ontario who studies meteors.
“With some meteor showers, if you miss the night of the peak or even an hour or two off the peak, there's nothing. Not so with the Perseids,” he says. “The Perseids are so large and they're so spread out that you can see them within a day or two of the peak, and you still see a really impressive show.”
Little rocks, big flash
Almost all of the debris streaming off the comet is tiny. Still, even something the size of a grain of sand can create a streak of light visible from the ground, some 60 miles below, because it’s coming in at such a high speed.
“You're not seeing the actual rock, but you're seeing all the light and heat that's being produced as a result of its passage through the atmosphere,” says Brown.
Typically, he says, the biggest incoming rock that someone might see during the Perseids is on the order of a few grams, and less than a half-inch across.
But this meteor shower does occasionally include larger rocks.
To see the full range of sizes that hit the Earth during the Perseids, Brown and some colleagues recently used a lightning mapping instrument on a satellite to observe the flashes of light that occurred during this meteor shower.
“We detected really bright Perseid fireballs over the whole Earth,” says Brown.
He and his colleagues calculated that the very biggest Perseids would be in the range of 20 pounds or so. Given the density of meteors, these space rocks would be smaller than soccer balls.
“That’s the absolute biggest,” he says, adding that rocks that size would entirely burn up in the atmosphere, creating a meteor streak that’s brighter than the full moon.
These largest Perseids mostly go unobserved, however, since they’re uncommon and much of the planet is covered with water and uninhabited land.
“Very, very rarely do people ever see anything this bright, but it is not unheard of,” says Brown. “If you go out, you observe year after year, you may eventually see a Perseid that rivals the full moon. But those are rare.”
More typically, a decent-sized meteor streak seen during the Perseids comes from a rock that weighs about a gram, or about as much as a raisin. These meteors can rival the planet Venus in terms of brightness.
“There are a fair number of those,” says Brown. “In fact, at the peak night of the Perseids, you might see a handful of those — three, four or five of those — an hour. ”
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