UFO'S of UAP'S, ASTRONOMIE, RUIMTEVAART, ARCHEOLOGIE, OUDHEIDKUNDE, SF-SNUFJES EN ANDERE ESOTERISCHE WETENSCHAPPEN

How Often do Meteorites Hit the Earth? — **Thousands of meteorites weighing about a pound are thought to fall to Earth every year, but many of these events go unnoticed, because the small rocks land in uninhabited forests or open ocean waters.**
**Credit: K. Paterson**

Pieces of natural space debris — typically rocky shards of comets or asteroids — occasionally survive their journeys through Earth's atmosphere and strike the ground, but how often does an event like this actually occur?

While large impacts are fairly rare, thousands of tiny pieces of space rock, called meteorites, hit the ground each year. However, the majority of these events are unpredictable and go unnoticed, as they land in vast swathes of uninhabited forest or in the open waters of the ocean, Bill Cooke and Althea Moorhead of NASA's Meteoroid Environments Office told Space.com.

In order to understand meteorite impacts on Earth, it is important to know where the chunks of rock come from. Meteoroids are rocky remnants of a comet or asteroid that travel in outer space, but when these objects enter Earth's atmosphere, they are considered meteors. [Photos: Fireball Drops Meteorites on California]

Most (between 90 and 95 percent) of these meteors completely burn up in the atmosphere, resulting in a bright streak that can be seen across the night sky, Moorhead said. However, when meteors survive their high-speed plunge toward Earth and drop to the ground, they are called meteorites.

The Perseid meteor shower — one of the most popular meteor showers of the year — is expected to put on a particularly breathtaking show Aug. 11 and 12, when the Earth passes through the trail of debris created by Comet Swift-Tuttle. However, viewers should not expect to find any meteorites lying on the ground after this spectacular meteor shower.

"Perseids come from Comet Swift-Tuttle and are very fragile, being an ice-dust mix," Cooke said. "They are not strong enough to survive passage through the atmosphere at 132,000 mph (212,433 km/h) and so never produce meteorites — they are totally vaporized by the time they make it to 50 miles (80 km) altitude." [Perseid Meteor Shower 2016: When & How to See It]

Unpredictable catastrophes

Most meteorites that are found on the ground weigh less than a pound. While it may seem like these tiny pieces of rock wouldn't do much damage, a 1-lb. (0.45 kilograms) meteorite traveling upward of 200 mph (322 km/h) can fall through the roof of a house or shatter a car windshield.

When the Grimsby meteorite landed in Ontario, Canada in 2009, for example, it broke the windshield of an SUV. In another incident, meteorites crashed into the back end of a Chevy Malibu in Peekskill, New York, in 1992, Cooke and Moorhead said. Thankfully, no one was injured during these events.

However, the pieces of rock falling from the sky are not even the greatest concern regarding meteor impacts, Cooke said.

"What causes the most damage is the shock wave produced by the meteor when it breaks apart in [Earth's] atmosphere," Cooke said. "So, you don't have to watch for the falling rocks — you have to worry about the shockwave."

For example, the Chelyabinsk meteor — an asteroid the size of a six-story building that entered Earth's atmosphere in February 2013 over Russia — broke apart 15 miles (24 km) above the ground and generated a shock wave equivalent to a 500-kiloton explosion, Cooke said. It injured 1,600 people.

**This is a sample of a meteorite that was found following the Chelyabinsk event in Russia in 2013. Several small stones were found in the area after the massive impact.**
**Credit: Qingzhu Yin, Univ. California-Davis**

Another major collision was the Tunguska meteorite, which was larger than Chelyabinsk and 10 times more energetic. The meteorite exploded over the Tunguska River on June 30, 1908, and flattened 500,000 acres (2,000 square km) of uninhabited forest. Because of its remote location, the event is an example of a meteorite that would have gone undetected had it not been so large, Cooke and Moorhead explained.

Generally, astronomers are unable to predict meteorite impacts, largely because meteoroids traveling in outer space are too small to detect. However, even large meteorite events that originate from asteroids, which can be tracked in space, are unpredictable.

Fortunately, between 90 and 95 percent of meteors don't survive the fall through the Earth's atmosphere to produce meteorites, Moorhead explained. This is because most meteorites are believed to come from comets, which are more fragile than asteroids.

"Only those meteoroids that happen to be made of stronger material produce meteorites," she said. For example, "if the [meteoroid] is a chunk of an asteroid, instead of a chunk of a comet, it's likely to be a little denser, a little stronger and more likely to produce a meteorite."

Also, if the meteor is approaching Earth at a slower speed, the rock will likely survive its collision with Earth's atmosphere, Moorhead added. In other words, the meteor will not burn up completely, and some remnant meteorites will fall to the ground.

"We actually get a few meteorites from the moon and Mars," Moorhead said. "These [meteorites] are actually chucks of the moon and Mars that have been blown off those planets by impacts and then spent a long time in space before finally hitting Earth as a meteorite."

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Meteor showers can be awesome night sky sights, but how well do you know your shooting star facts? Find out here and good luck!

Although predicting these events is nearly impossible, there are a couple different ways in which researchers can measure how many meteorites fall to the Earth. For example, in uninhabited areas like the Sahara Desert or atop glaciers in Antarctica, it is fairly easy to find meteorites on the ground and date them based on the amount of weathering they've experienced, Moorhead explained. Because these areas are largely undisturbed, the meteorites that lie there provide scientists with a general idea of how many of the space rocks might have hit the Earth over a period of time.

While skywatchers will not be able to hunt for meteorites after this week's Perseids meteor shower, the dazzling trail left behind by Comet Swift-Tuttle will provide spectacular light show, which can best be seen in clear skies of the Northern Hemisphere on the nights of Aug. 11 and 12.

Follow Samantha Mathewson @Sam_Ashley13. Follow us @Spacedotcom,Facebook and Google+. Original article on Space.com.

**Perseid meteor shower time-lapse image**
**Credit: NASA**

Comet bits

Most meteor showers evolve from comets. The cometary parent of the Perseids is a comet known as Swift-Tuttle. This comet takes approximately 130 years to make one trip around the sun. It was discovered in 1862 and last appeared in our skies in 1992. (It's due back in 2126.) Comets are composed chiefly of frozen gases, and meteoroids that are mixed in with that gas — the bits and pieces that have flaked off from the comet's solid center or nucleus.

Like all comets, Swift-Tuttle leaves behind streams of tiny particles ranging in size from large grains of sand to small pebbles, but with the consistency of cigar ash. They're invisible to people on Earth until, upon entering our atmosphere with immense velocity — 37 miles per second (60 kilometers per second) — they blaze up within the span of a heartbeat and streak across the sky in a brief, blazing finale. Comet Swift-Tuttle probably has passed the sun hundreds of times to produce the present stream of Perseid meteors.

**The 2016 Perseid meteor shower will peak overnight on Aug. 11 and 12. They appear to radiate out of the constellation Perseus, from which they take their name.**
**Credit: NASA/JPL-Caltech**

The meteoroids have become distributed all around the comet's elongated elliptical orbit, which takes them out beyond the realms of Pluto and ultimately brings them back to intersect Earth's orbit at almost a right angle, producing a fairly predictable meteor display that lasts for several days in mid-August.

Enter Jupiter

**Learn why famous meteor showers like the Perseids and Leonids occur every year [See the Full Infographic Here].**
**Credit: Karl Tate, SPACE.com contributor**

The Perseids intersect the orbit of Earth more or less directly, but on their way in toward the sun, they also pass other planets. In the case of the most massive planet in the solar system, Jupiter, there is no direct encounter; the dross of Comet Swift-Tuttle passes well above this giant world, never coming any closer than about 160 million miles (257 million km) at 11.8-Earth-year intervals — the time it takes for Jupiter to make a complete orbit around the sun.

But on those occasions when it's passing closest to Swift-Tuttle's trail of debris, Jupiter's powerful gravitational field can still slightly perturb the comet particles just enough to nudge them about 930,000 miles (1.5 million km) closer to Earth. The result is usually a noticeably brighter and stronger Perseid display, producing more than the usual complement of meteors.

Most years when the Earth encounters the Perseids, observers in areas where the sky is very dark and clear might count as many as 60 to 90 meteors per hour. (But here's a necessary disclaimer: If you live in an area where haze, smoke and bright city lights proliferate, or tall obstructions block large areas of the sky, you will likely see a lot fewer.)

When Jupiter has had an influence on the Perseid orbit, the meteor numbers tend to ramp up to above-normal levels. That's exactly what happened in 1921, 1945, 1968, 1980 and, most recently, in 2004.

As it turns out, Jupiter recently passed closest to the orbit of Comet Swift Tuttle in November 2014. But an additional 22 months must then pass for the meteoroid zone affected by Jupiter to arrive in the vicinity of Earth. That indicates that the 2016 Perseids could be another "prime year" for the Perseids; they could indeed prove to be unusually numerous. [Top 10 Perseid Meteor Shower Facts]

What do the Perseid pundits say?

In the 2016 edition of the "Observer's Handbook" of the Royal Astronomical Society of Canada, Margaret Campbell-Brown and Peter Brown noted that, "Some models predict an enhancement in activity from the Perseids in 2016about 7 hours before the traditional peak, but this is from older trails unlikely to produce more than a moderate increase in rates." On their table of meteor showers for 2016, they highlighted the Perseids in bolddue to the possibility of this outburst.

The outburst in question is being forecast by French meteor expert Jeremie Vaubaillon. It shows the Earth moving along in its orbit and encountering a small, albeit concentrated, zone of comet material at around 1 a.m. EDT (0500 GMT) on Aug. 12 that would favor the eastern portions of the U.S. and Canada. But Vaubaillon is cautious. "This trail of material dates is over nine centuries old, making the forecast for a possible outburst less certain," he told me in an email. "Certainly I would look for it, but just keep in mind the uncertainties."

You can see a map charting that interaction here:

In this diagram, French astronomer Jeremie Vaubaillon has charted the interaction of the Earth through this year's stream of Perseid meteoroids. Earth's orbit is plotted in orange. Dates are indicated as Day/Month (Example 11/8 = 11 August at 0 hour UT). Notice the small "blob" of debris that directly encounters Earth's orbit at roughly 1 a.m. EDT (0500 GMT) on Aug. 12. Presumably, that material was shed by Comet Swift-Tuttle in A.D. 1079. If it's still intact, it might produce a sudden outburst of bright meteors. But if it has dispersed significantly, nothing out of the ordinary will be seen.
**Credit: Jeremie Vaubaillon**

Another noted meteor expert, Mikhail Maslov of Russia, forecasts higher-than-normal Perseid meteor rates: "In 2016, Jupiter will cause a significant increase up to 160-180 meteors per hour. Earth will encounter the perturbed part of the Perseid stream, which was shifted closer to the Earth's orbit by Jupiter's gravitation. This means we have to expect heightened Perseid activity." His complete description, including more technical details, can be found here: http://feraj.narod.ru/Radiants/Predictions/Perseids2016eng.html

The "clumping effect"

This year is an auspicious occasion for me, as it marks the 50th summer in which I have watched out for the Perseids. What I (and others) have noted over the years is what some have called the "clumping effect" — that the Perseids appear to come in rapid succession; several or more within a minute or two, followed by a lull of several minutes before the sky suddenly bears fruit once again. Interestingly, while many observers have stated that the Perseids seem to come in bunches, when hourly Perseid observations have been carefully examined using the Poisson distribution formula (explained in statistics textbooks), it has been determined that, more often than not, a random distribution of arrival times has been inferred. Therefore, the "clumping" or bunches may be merely an illusion.

Have a Perseid party

Why not consider organizing a "Perseid party," and have a number of people watching different parts of the sky?

To observe the Perseids, you may need to dress warmly, bring a reclining chair and watch the sky. You need no other equipment. Find a dark observing spot, preferably with a low horizon, and look in any direction. Use this light-pollution map (http://tinyurl.com/zrk7qju) to find dark locations near you.

The Perseids get their name from their apparent radiation from the northern constellation of Perseus. At dusk, the uppermost part of this star pattern is grazing the northern horizon. Not until 11 p.m. local time is this group high enough in the northeast to be conspicuous. At 4 a.m., when Orion has risen, Perseus has reached a lofty perch and is nearly overhead; that's also when you're likely to see the most meteors.

Meteor :

Meteor showers can be awesome night sky sights, but how well do you know your shooting star facts? Find out here and good luck!

But it is not necessary to continuously stare at Perseus itself if you are looking for these shooting stars. A Perseid may appear in any part of the sky, but its trail, if mentally traced backward, invariably leads back to Perseus.

Typically, they appear as bright, swift streaks. A few outstandingly bright meteors (fireballs) occasionally may be seen. Fainter meteors appear white or yellow; brighter ones green, orange or red. About one-third leave luminous trains, which may be spectacular and can persist for many seconds; the most outstanding meteors often end in flares or bursts (called "bolides") and are even capable of casting shadows! Those are the kind of meteors that tend to elicit screams of delight from Perseid watchers.

You can also contribute valuable observational data. If you decide to do so, use the techniques suggested by the International Meteor Organization (IMO) (http://www.imo.net/visual/major). This way, your data will be in a standard format, which can readily be used for analysis. Send your data to the IMO athttp://www.imo.net/visual/report/electronic.

Good luck and here's to clear skies!

Editor's note: If you have an amazing photo of this year's Perseid meteor shower you'd like to share for a possible story or image gallery, please contact managing editor Tariq Malik at spacephotos@space.com.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmer's Almanac and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.