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Comets formed 4.6 billion years ago along with the rest of the planets from the same solar nebula material. They were too small and cold to undergo any geologic activity (they did not differentiate), so they preserve the record of the early solar nebula composition and physical conditions. Those forming near the jovian planets were deflected outward, swallowed up or sent careening inward toward the terrestrial planets and the Sun. Some of the water now present on the terrestrial planets may have come from comets crashing into them. Short period comets make hundreds to thousands of passes around the Sun spewing out gas and dust. Over time a comet will leave bits of dust along its orbit, each piece of dust has an orbit close to the comet's orbit. The dust grains are the size of a grain of sand or smaller. If the Earth passes through the comet's orbit, the dust grains can hit the Earth's atmosphere to make the spectacular displays called meteor showers. After many passages around the Sun, the nucleus has no more volatile material and it becomes "dead." Some asteroids are former comets. The famous Perseid meteor shower in mid-August is due to Earth passing through the orbit of Comet Swift-Tuttle and the Leonid meteor shower in mid-November is due to Comet Tempel-Tuttle. The meteor showers appear to be coming from a particular direction in the sky so the meteor showers are named after the constellation they appear to be coming from. The divergence from a point in the sky (called the "radiant") is from the same geometric effect you see of straight railroad tracks or the edges of a straight road spreading out toward you from a distant point far away on the horizon. The Perseids appear to diverge from the Perseus constellation and the Leonids diverge from Leo. When the parent comet passes through the inner solar system, the meteor shower display is particularly impressive---several hundred meteors can be seen in one hour. Such events are called meteor storms. The last storm was for the Leonids in 2002. While most comet dust particles in meteor showers hit the atmosphere at 30 to 40 kilometers/second, the Leonid particles hit our atmosphere at 72 kilometers/second. The table summarizes the characteristics for several impressive showers. A meteor shower will cover a time period of several days before and after the given maximum date. More on meteor showers from: The meteors not associated with a meteor shower are bits of rock from asteroids. The meteors that ARE associated with a meteor shower are much too fragile to survive their trip through our atmosphere and burn up at altitudes above 50 kilometers. Some of the comet dust intercepts the Earth at much slower speeds than those making the meteors and can make its way to the surface gently. Don Brownlee, an astronomer at the University of Washington has pioneered the collection of this comet dust in the stratosphere. More information about the comet dust samples is available at the Stratospheric Dust web site at the Planetary Materials Curation office of NASA. Go back to previous section -- Go to next section last updated: March 25, 2013
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Suze svetog Lovre, spektakularni godišnji meteorski pljusak na nebu se najjasnije može vidjeti baš ovih dana, između 11. i 13. kolovoza, a zaljubljenici u Perzeide zvane suze svetog Lovre sigurno će uživati u videu koji se munjevitom brzinom proširio Facebookom. Inače, do kiše meteora dolazi kada Zemlja prolazi kroz polje krhotina koje nastanjuju orbitu pojedinog meteora, u ovom slučaju, kometa Swift-Tuttle. Zbog perspektive meteorski pljuskovi ostavljaju dojam da izbijaju iz jedinstvene točke na nebu koja se naziva radijantom. Prosječni meteor nastaje od čestica veličine zrnca pijeska koje izgaraju pri ulasku u Zemljinu atmosferu jureći brzinom od 215 000 kilometara na sat. Iako nas nije poslužilo vrijeme pa i vidljivost nije neka, uživajte u ovom spektakulranom videu. NAPOMENA: Komentari odražavaju stavove njihovih autora, a ne nužno i stavove portala ŠIBENIK.IN. Portal ŠIBENIK.IN zadržava pravo da obriše komentar bez najave i objašnjenja. Zbog velikog broja komentara portal ŠIBENIK.IN nije dužan obrisati sve komentare koji krše pravila.