1. Asteroids are small Solar System bodies that are not comets. The term asteroids historically referred to objects inside the orbit of Jupiter. They have also been called planetoids, especially the larger ones. These terms have historically been applied to any astronomical object orbiting the Sun that did not show the disk of a planet and was not observed to have the characteristics of an active comet, but as small objects in the outer Solar System were discovered, their volatile-based surfaces were found to more closely resemble comets, and so were often distinguished from traditional asteroids. Thus the term asteroid has come increasingly to refer specifically to the small bodies of the inner Solar System within the orbit of Jupiter, which are usually rocky or metallic. They are grouped with the outer bodies—centaurs, Neptune trojans, and trans-Neptunian objects—as minor planets, which is the term preferred in astronomical circles. This article uses the term "asteroid" to the minor planets of the inner Solar System.
There are millions of asteroids, many thought to be the shattered remnants of planetesimals, bodies within the young Sun's solar nebula that never grew large enough to become planets. The large majority of known asteroids orbit in the asteroid belt between the orbits of Mars and Jupiter or co-orbital with Jupiter (the Jupiter Trojans). However, other orbital families exist with significant populations, including the near-Earth asteroids. Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, S-type, and M-type. These were named after and are generally identified with carbon-rich, stony, and metallic compositions, respectively.
2. سیارکها (Asteroids ، minor planets):
سیارات بسیار کوچکی هستند که از صخره و فلز ساخته شدهاند. سیارکها معمولاً اجسام نامنتظمی هستند و بر گرد خورشید حرکت میکنند. هزاران سیارک در منظومه خورشیدی ما وجود دارند. بسیاری از آنها میان مدار بهرام (مریخ) و مدار هرمز (مشتری) قرار گرفتهاند و گرد خورشید میگردند. دستهای دیگر از آنها در مکانهای دیگر منظومه خورشیدی یافت میشوند.به نظر می رسد علت اینکه اغلب آنها در فاصلهٔ مریخ و مشتری دیده می شوند این است که احتمالاً در مدار بین این دو سیاره، سیارهٔ دیگری نیز وجود داشته است که به علت جاذبهٔ شدید مشتری متلاشی شده است و سیارکها پدید آمده باشند.
به سیارکهایی که بر اثر نیروی گرانش سیارهها در مداری گیر افتاده باشند «سیارک اسیر» میگویند. در این صورت سیاره مزبور به گرد سیاره بزرگتر میگردد. نزدیکترین سیارک به زمین، توتاتیس نام دارد
3. A 150-foot asteroid hurtled toward Earth's backyard, destined Friday Februry 15 (today) to make the closest known flyby for a rock of its size. NASA promised the asteroid would miss Earth by 17,150 miles, avoiding catastrophe. But that's still closer than many communication and weather satellites; scientists insisted these, too, would be spared. Asteroid 2012 DA14, as it's called, is too small to see with the naked eye even at its closest approach around 2:25 p.m. EST, over the Indian Ocean near Sumatra. The best viewing locations, with binoculars and telescopes, are in Asia, Australia and eastern Europe. Even there, all anyone can see is a pinpoint of light as the asteroid zooms by at 17,400 mph. As asteroids go, DA14 is a shrimp. The one that wiped out the dinosaurs 65 million years ago was 6 miles across. But this rock could still do immense damage if it struck, releasing the energy equivalent of 2.4 million tons of TNT and wiping out 750 square miles.
Scientists are certain it won't impact Earth. And chances are extremely remote it will run into any of the satellites orbiting 22,300 miles up. Most of the solar system's asteroids are situated in a belt between the orbits of Mars and Jupiter, and remain stable there for billions of years. Some occasionally pop out, though, into Earth's neighborhood. The flyby provides a rare learning opportunity for scientists eager to keep future asteroids at bay, and a prime-time advertisement for those anxious to step up preventive measures.
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