Editing Halley's Comet (section)

==Structure and composition==
[[File:Comet Halley close up-cropped.jpg|thumb|right|The nucleus of Halley's Comet, imaged by the ''Giotto'' probe on 14{{nbsp}}March 1986. The dark colouration of the nucleus can be observed, as well as the jets of dust and gas erupting from its surface.|alt=A large, black, rock-like structure is visible amid an onrushing cloud of dust. A stream of brilliant white arcs up from the left.]]
The ''Giotto'' and ''[[Vega program|Vega]]'' missions gave planetary scientists their first view of Halley's surface and structure. The [[Comet nucleus|nucleus]] is a conglomerate of [[Volatile (astrogeology)|ices]] and [[dust]], often referred to as a "dirty snowball".<ref name="Delehanty"/> Like all comets, as Halley nears the Sun, its volatile compounds (those with low boiling points, such as [[water]], [[carbon monoxide]], [[carbon dioxide]] and other ices) begin to [[Sublimation (chemistry)|sublimate]] from the surface.<ref name="graw"/> This causes the comet to develop a [[Coma (cometary)|coma]], or atmosphere, at distances up to {{convert|230000|km}} from the nucleus.<ref name="Altwegg1993"/> [[Sublimation (phase transition)|Sublimation]] of this dirty ice releases dust particles, which travel with the gas away from the nucleus. Gas molecules in the coma absorb solar light and then re-radiate it at different wavelengths, a phenomenon known as [[fluorescence]], whereas dust particles [[scattering|scatter]] the solar light. Both processes are responsible for making the coma visible.<ref name="Delehanty"/> As a fraction of the gas molecules in the coma are [[ionization|ionised]] by the solar [[ultraviolet radiation]],<ref name="Delehanty"/> pressure from the [[solar wind]], a stream of charged particles emitted by the Sun, pulls the coma's ions out into a long [[Comet tail|tail]], which may extend more than 100&nbsp;million&nbsp;kilometres into space.<ref name="graw"/><ref name="Biermann1958"/> Changes in the flow of the solar wind can cause [[disconnection event]]s, in which the tail completely breaks off from the nucleus.<ref name="Brandt1987"/>

Despite the vast size of its coma, Halley's nucleus is relatively small: barely {{convert|15|km}} long, {{convert|8|km}} wide and perhaps {{convert|8|km}} thick.<ref name="Keller1987"/><ref name="Reitsema"/> Based on a reanalysis of images taken by the ''Giotto'' and ''Vega'' spacecraft, Lamy et al. determined an [[mean diameter|effective diameter]] of {{convert|11|km}}.<ref name="Lamy2004"/><ref name="Reitsema"/> Its shape has been variously compared to that of a [[peanut]], a [[potato]], or an [[avocado]].<ref name="Mendis1986"/> Its mass is roughly 2.2{{E-sp|14}}&nbsp;kg,<ref name="mass"/> with an average density of about {{convert|0.55|g/cm3|oz/cuin}}.<ref name="situ"/> The low density indicates that it is made of a large number of small pieces, held together very loosely, forming a structure known as a [[rubble pile]].<ref name="density"/> Ground-based observations of coma brightness suggested that Halley's [[rotation period]] was about 7.4 days. Images taken by the various spacecraft, along with observations of the jets and shell, suggested a period of 52&nbsp;hours.<ref name="situ"/><ref name="BriceHoover2004"/> Given the irregular shape of the nucleus, Halley's rotation is likely to be complex.<ref name="graw"/> The flyby images revealed an extremely varied topography, with hills, mountains, ridges, depressions, and at least one crater.<ref name="situ"/>

Halley's day side (the side facing the Sun) is far more active than the night side.<ref name="situ"/> Spacecraft observations showed that the gases ejected from the nucleus were 80% water vapour, 17% carbon monoxide and 3–4% carbon dioxide,<ref name="Woods1986"/> with traces of hydrocarbons<ref name="Chyba1987"/> although more recent sources give a value of 10% for carbon monoxide and also include traces of [[methane]] and [[ammonia]].<ref name="ESA_2006"/> The dust particles were found to be primarily a mixture of carbon–hydrogen–oxygen–nitrogen (CHON) compounds common in the outer Solar System, and silicates, such as are found in terrestrial rocks.<ref name="graw"/> The dust particles ranged in size down to the [[LOQ|limits of detection]] (≈0.001&nbsp;μm).<ref name="post"/> The ratio of [[deuterium]] to [[hydrogen]] in the water released by Halley was initially thought to be similar to that found in Earth's ocean water, suggesting that Halley-type comets may have delivered water to Earth in the distant past. Subsequent observations showed Halley's deuterium ratio to be far higher than that found in Earth's oceans, making such comets unlikely sources for Earth's water.<ref name="graw"/>

''Giotto'' provided the first evidence in support of [[Fred Whipple]]'s "dirty snowball" hypothesis for comet construction; Whipple postulated that comets are icy objects warmed by the Sun as they approach the inner Solar System, causing ices on their surfaces to [[Sublimation (phase transition)|sublime]] (change directly from a solid to a gas), and jets of volatile material to burst outward, creating the coma. ''Giotto'' showed that this model was broadly correct,<ref name="graw"/> though with modifications. Halley's [[albedo]], for instance, is about 4%, meaning that it reflects only 4% of the sunlight hitting it &ndash; about what one would expect for coal.<ref name="Weaver1997"/> Thus, despite astronomers predicting that Halley would have an albedo of about 0.17 (roughly equivalent to bare soil), Halley's Comet is in fact pitch black.<ref name="Belton1982"/> The "dirty ices" on the surface sublime at temperatures between {{convert|170|K|°C}} in sections of higher albedo to {{convert|220|K|°C}} at low albedo; ''[[Vega 1]]'' found Halley's surface temperature to be in the range {{convert|300|–|400|K|°C}}. This suggested that only 10% of Halley's surface was active, and that large portions of it were coated in a layer of dark dust that retained heat.<ref name="post"/> Together, these observations suggested that Halley was in fact predominantly composed of non-[[Volatile (astrogeology)|volatile]] materials, and thus more closely resembled a "snowy dirtball" than a "dirty snowball".<ref name="situ"/><ref name="NASA_explore"/>