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Halley's Comet
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<p>Halley's Comet is the only known <a href="/facts/List_of_periodic_comets/35UuHPYc">short-period</a> <a href="/facts/Comet/w34utQCd">comet</a> that is consistently visible to the <a href="/facts/Naked_eye/ePEIa0lK">naked eye</a> from <a href="/facts/Earth/HLLmDfj0">Earth</a>, appearing every 72–80 years, though with the majority of recorded apparitions (25 of 30) occurring after 75–77 years. It last appeared in the inner parts of the <a href="/facts/Solar_System/QIcLSazQ">Solar System</a> in 1986 and will next appear in mid-2061. Officially designated 1P/Halley, it is also commonly called Comet Halley, or sometimes simply Halley. </p><p>Halley's periodic returns to the <a href="/facts/Inner_Solar_System/QIcLSazQ">inner Solar System</a> have been observed and recorded by astronomers around the world since at least 240 BC, but it was not until 1705 that the English astronomer <a href="/facts/Edmond_Halley/JgjIRcUA">Edmond Halley</a> understood that these appearances were re-appearances of the same comet. As a result of this discovery, the comet is named after Halley. </p><p>During its 1986 visit to the inner Solar System, Halley's Comet became the first comet to be observed in detail by a <a href="/facts/Spacecraft/YQqQaUw0">spacecraft</a>, <i><a href="/facts/Giotto_(spacecraft)/q9moaxxL">Giotto</a></i>, providing the first observational data on the structure of a <a href="/facts/Comet_nucleus/dpJGJlqw">comet nucleus</a> and the mechanism of <a href="/facts/Coma_(cometary)/o6eNWYyL">coma</a> and <a href="/facts/Comet_tail/RbIDJMyC">tail</a> formation. These observations supported several longstanding hypotheses about comet construction, particularly <a href="/facts/Fred_Whipple/wylsIT2S">Fred Whipple</a>'s <a href="/facts/Comet_nucleus/dpJGJlqw">"dirty snowball"</a> model, which correctly predicted that Halley would be composed of a mixture of <a href="/facts/Volatile_(astrogeology)/yYCe49qX">volatile ices</a>—such as <a href="/facts/Water/0lkXnJPK">water</a>, <a href="/facts/Carbon_dioxide/xGzpppEp">carbon dioxide</a>, <a href="/facts/Ammonia/MtLtJFtz">ammonia</a>—and <a href="/facts/Cosmic_dust/Uy6hlL29">dust</a>. The missions also provided data that substantially reformed and reconfigured these ideas; for instance, it is now understood that the surface of Halley is largely composed of dusty, non-volatile materials, and that only a small portion of it is icy. </p>