What can be so small you couldn't see it on the ground a few feet away, yet so bright it outshines the stars? A meteor. Some are large enough to turn into fireballs in the sky, but even a meteor weighing less than one gram (a paper clip weighs about a gram) can leave a bright trail as it falls through the Earth's atmosphere. One of the best opportunities to watch for these "shooting stars" will be just before dawn on Wed., Aug. 13. That's the peak of the annual Perseid meteor shower, which runs from July 17 to Aug. 24.
Meteors come from different sources, including asteroids, comets, and even the Moon and Mars. They enter the Earth's atmosphere as meteroids, and many are never seen at all. They enter during the day or are so small they don't light up the sky. Millions or even billions fall to earth each day, adding perhaps 100 tons to our planet. That sounds like a lot, but not compared to the Earth's estimated mass of 6.6 billion trillion tons (6.6 with 21 zeroes after it). Nearly all meteorites are tiny, weighing only thousandths of a gram.
When meteroids are big enough and fast enough, they light up the sky. At this point they're called meteors. Most burn up from the energy created by their collision with air molecules in our atmosphere, but a few make it to the ground. Meteors that actually hit the ground are meteorites.
How can something as small as a grain of sand create such a bright light? The answer is in its speed. Many are traveling at 60 to 70 kilometers per second (134,000 to 156,000 miles per hour) as they fall through the air. To compare, a space shuttle orbits at a relatively poky 8 kilometers per second (18,000 miles per hour).
On most nights you could see a few meteors an hour from a clear, dark location. To see a real show, you need a meteor shower. Several happen regularly, including the Perseid shower. This one is the result of a comet called Swift-Tuttle that orbits the sun and leaves a trail of debris behind it. Earth passes through this debris - bits of rock and ice - every year around this time. The particles come shooting through the Earth's atmosphere. During the peak of this shower you might see 60 meteors an hour. That's about one every minute. It won't look like a huge fireworks display, but in its own way it's even more breathtaking. It's a natural and amazing event happening far above us.
The Perseid shower gets its name from the constellation Perseus, where the meteors seem to originate. But you will be able to see the meteors falling through all parts of the sky if you are in a good location. Here are some tips for good meteor viewing:
Find a location away from lights and high trees. The darker the sky, the more meteors you will be able to see. It will take about 10 minutes for your eyes to adjust to the darkness and allow you to see the dimmest meteors. Don't try to use a telescope or binoculars. They will limit your vision to a tiny portion of the sky, and the meteors will be spread across a wide area. Avoid looking at the moon (or any bright lights). If you can, situate yourself so the moon is blocked out by a tall tree or building.
Check the weather before you go. If the sky will be cloudy, you won't see much. Keep in mind that you will be looking up at the sky. Take a blanket or a lounge chair so you can lie down to watch.
The best time to see meteors is between midnight and dawn. (What a great excuse to ask your parents to let you stay up late!) But there will still be meteors to see earlier in the evening, once it's dark. Some people plan "meteor parties" for these regular showers.
The Perseid shower has occurred for thousands of years, but these meteors weren't exactly party material hundreds of years ago. An early Christian now known as Saint Lawrence was tortured and killed on Aug. 10, 258, when the Perseid shower was near its peak. For centuries English and German peasants believed that St. Lawrence wept tears of fire that fell from the sky every year after that.
For a long time, astronomers paid little attention to meteors. They thought they were weather phenomena, like lightning. But when a spectacular meteor shower occurred in November 1833, astronomer Denison Olmsted determined that they must have arrived from a distant region of space. Astronomers searched historical records to find if there had been other mid-November showers. The answer: Yes, indeed. Today the Leonid meteor shower (the meteors seem to originate in the constellation Leo) is well known.
Another American who was not a scientist also became interested in meteor showers. Edward Herrick never went to college. He worked at a bookstore near Yale University in New Haven, Conn., in the 1820s and 30s. On Aug. 9, 1837, he noticed a lot of meteors in the night sky. He had always enjoyed talking about astronomy with Yale professors who came into the bookstore. Now he wondered if perhaps this event might be an annual shower like the Leonids in November. He searched old records and found mentions of other August showers in the past. Eventually he came across the story of Saint Lawrence's "tears."
By the time Herrick published his theory of the Perseid shower in 1838, a Belgian astronomer, Adolphe Quetelet, had suggested the possibility as well. His findings hadn't become common knowledge in the United States. It was an example of "parallel discovery," when people in different places independently discover the same thing at about the same time.
Herrick continued to study the Perseid shower and also discovered another shower that occurs in early December (the Bielids, after Biela's comet). In 1838, after publishing some scholarly articles on these showers, the former bookstore clerk was given an honorary master of arts degree from Yale. He later became the librarian of Yale's library.
Now a number of annual showers are known, including the Delta Aquarids. (They peaked on July 28 at about 20 meteors hour.)
For a schedule of this year's showers and to learn more about meteors, visit the NASA Space Academy website at: liftoff.msfc.nasa.gov/ academy/space/solarsystem/ meteors/meteors.html.
To learn more about meteorites, see: www.seds.org/nineplanets/ nineplanets/meteorites.html.
While most meteors burn up traveling through the atmosphere, a few manage to reach the ground. If they're big enough, they can make a real impact on the Earth and leave a crater. The NASA Space Academy website includes directions for making your own craters and then studying the impact to learn more about how meteorites affect the Earth.
Here's how to make your own craters:
You'll need a bucket of fine, dry sand and a second cup of flour. You'll also need a flour sifter or sieve and your own 'meteorites' to drop in the sand. (Try ball bearings, marbles, fishing weights, or anything small and heavy.)
Fill the bucket with sand and then shake it to level the sand. Use the flour sifter to put a dusting of flour atop the sand. Stand over the bucket and drop your 'meteorites' from different heights and angles. See what kinds of craters they produce in the flour and sand. Ask yourself the following questions:
Where did the projectile end up? Under the surface or on top of the flour and sand? Does a projectile make a different type of crater if it hits at an angle? Does the size of the crater change with the size of the projectile or the height from which it is dropped?
For more questions and details on this activity, see the NASA Space Academy website mentioned above.
The Perseid meteor shower (July 17-Aug. 24) will peak on the morning of Wednesday, Aug. 13. Think of meteors as outer-space 'bugs' hitting the 'windshield' of Earth's atmosphere. The best viewing time will be between midnight and dawn - the VERY best just before dawn. It will be a full moon, so you may not see many of the fainter meteors. The Perseid meteors are actually debris from the Swift-Tuttle comet, which has been seen only in 1862 and 1992.