Q: 老師，我經常看到 comet, asteroid, meteoroid, meteor, meteorite 在英語測驗出現 。它們的區別是什麼啊?
A: May you see many comets and meteors (shooting stars) in the skies,* but never see a meteorite coming toward you.
General size [一般大小比較]: asteroid [小行星] > comet [彗星] > meteoroid [流星體] > meteor/shooting star [流星] > meteorite [隕石]
Usually, an asteroid is always denser than a comet but may not always be bigger.
*sky vs skies: http://tw.knowledge.yahoo.com/question/question?qid=1306062009963, http://english.stackexchange.com/questions/29302/using-skies-instead-of-sky
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補充資料:
1. asteroid: 小行星
Asteroids are small solar bodies orbiting the Sun. Made of rock and metal, they can also contain organic compounds. Asteroids are similar to comets but do not have a visible fuzzy tail like comets do. 一類小天體，個體較小（最大的直徑也只有數百千米），很多形狀不規則。成分不一，有的主要是矽酸鹽，有的主要是金屬，還有少數含相當數量的碳，不擁有彗尾。
2. comet: 彗星
A comet is a relatively small solar system body that orbits the Sun. When close enough to the Sun they display a visible COMA (a fuzzy outline due to solar radiation) and sometimes a tail. 太陽系中的一類小天體，通常繞太陽運行。主要由塵埃和冰，靠近太陽時溫度上升，噴出塵埃和氣體，在太陽光壓的作用下延展至運行的相反方向，形成彗發和彗尾。
3. meteoroid: 流星體
A meteoroid is a small rock or particle of debris in our solar system. They range in size from dust to around 10 meters in diameter. 其實就是太空中的塵埃顆粒和碎石，進入大氣層後因摩擦產生高溫發光，就是流星。
4. meteor: 流星
A meteoroid becomes a meteor or “shooting star” when it starts to fall through Earth’s atmosphere. It heats up by friction with air molecules, emitting light along its trail to form a fireball. 流星體進入大氣層後，與大氣摩擦發熱發光產生的現象。
5. meteorite: 隕石
A meteor that survives falling through the Earth’s atmosphere and colliding with the Earth’s surface is known as a meteorite. 指的是從太空中掉入地球的鐵或岩石. 它在進入地球的大氣層時不會完全燃燒掉.
The largest distinct meteorite discovered was in 1920 in South Africa. It weighed about 50 tons. 最大的隕石於1920年在南非發現。它重約50噸。
6. bolide: 火流星
The term is synonymous with “fireball.” Astronomers often use “bolide” to identify an exceptionally bright fireball, particularly one that explodes (sometimes called a detonating fireball).
Now, you know the difference when you see these words on the TOEFL.
Sources:
http://neat.jpl.nasa.gov/neofaq.html
http://www.pibburns.com/catastro/meteors.htm
http://zhidao.baidu.com/question/179222678.html
http://www.aerospaceguide.net/whatisanasteroid.html
http://www.lifeslittlemysteries.com/886-asteroid-comet-or-meteor.html
http://hubblesite.org/reference_desk/faq/answer.php.id=22&cat=solarsystem
http://wiki.answers.com/Q/What_is_the_difference_between_asteroids_and_comets