Editing Giant-impact hypothesis (section)

== History==
In 1898, [[George Darwin]] made the suggestion that Earth and the Moon were once a single body. Darwin's hypothesis was that a molten Moon had been spun from Earth because of [[centrifugal force]]s, and this became the dominant academic explanation.<ref name="Binder 1974 53–76"/> Using [[Newtonian mechanics]], he calculated that the Moon had orbited much more closely in the past and was drifting away from Earth. This drifting was later confirmed by [[Apollo program|American]] and [[Roscosmos|Soviet]] experiments, using [[Lunar Laser Ranging experiment|laser ranging targets]] placed on the Moon.

Nonetheless, Darwin's calculations could not resolve the mechanics required to trace the Moon back to the surface of Earth. In 1946, [[Reginald Aldworth Daly]] of [[Harvard University]] challenged Darwin's explanation, adjusting it to postulate that the creation of the Moon was caused by an impact rather than centrifugal forces.<ref name=dalypaps/> Little attention was paid to Professor Daly's challenge until a conference on satellites in 1974, during which the idea was reintroduced and later published and discussed in ''[[Icarus (journal)|Icarus]]'' in 1975 by [[William Kenneth Hartmann|William K. Hartmann]] and [[Donald R. Davis (astronomer)|Donald R. Davis]]. Their models suggested that, at the end of the planet formation period, several satellite-sized bodies had formed that could collide with the planets or be captured. They proposed that one of these objects might have collided with Earth, ejecting refractory, volatile-poor dust that could coalesce to form the Moon. This collision could potentially explain the unique geological and geochemical properties of the Moon.<ref name=icarus24_504/>

A similar approach was taken by Canadian astronomer [[Alastair G. W. Cameron]] and American astronomer [[William Ward (astronomer)|William R. Ward]], who suggested that the Moon was formed by the [[tangent]]ial impact upon Earth of a body the size of Mars. It is hypothesized that most of the outer silicates of the colliding body would be vaporized, whereas a metallic core would not. Hence, most of the collisional material sent into orbit would consist of silicates, leaving the coalescing Moon deficient in iron. The more volatile materials that were emitted during the collision probably would escape the Solar System, whereas [[silicate]]s would tend to coalesce.<ref name=alpc7_120/>

Eighteen months prior to an October 1984 conference on lunar origins, Bill Hartmann, Roger Phillips, and Jeff Taylor challenged fellow lunar scientists: "You have eighteen months. Go back to your Apollo data, go back to your computer, and do whatever you have to, but make up your mind. Don't come to our conference unless you have something to say about the Moon's birth." At the 1984 conference at [[Kona, Hawaii]], the giant-impact hypothesis emerged as the most favored hypothesis. 
{{blockquote|Before the conference, there were partisans of the three "traditional" theories, plus a few people who were starting to take the giant impact seriously, and there was a huge apathetic middle who didn't think the debate would ever be resolved. Afterward, there were essentially only two groups: the giant impact camp and the agnostics.<ref name=Dana-Mackenzie>{{cite book |first=Dana | last=Mackenzie |title=The Big Splat, or How Our Moon Came to Be |url=https://archive.org/details/bigsplatorhowour00mack |url-access=registration |date=21 July 2003 |publisher=[[John Wiley & Sons]] |isbn=978-0-471-48073-0 |pages=[https://archive.org/details/bigsplatorhowour00mack/page/166 166]–168 |df=dmy-all |access-date=11 June 2019 |archive-date=17 June 2020 |archive-url=https://web.archive.org/web/20200617181357/https://archive.org/details/bigsplatorhowour00mack |url-status=live }}</ref>}}