Artist's depiction of the enormous collision that is hypothesized to have formed the Moon
THE IDEA
At the time Earth formed 4.5 billion years ago, other smaller planetary bodies were also growing. One of these hit earth late in Earth’s growth process, blowing out rocky debris. A fraction of that debris went into orbit around the Earth and aggregated into the moon.
Why this is a good hypothesis:
The Earth has a large iron core, but the moon does not. This is because Earth’s iron had already drained into the core by the time the giant impact happened. Therefore, the debris blown out of both Earth and the impactor came from their iron-depleted, rocky mantles. The iron core of the impactor melted on impact and merged with the iron core of Earth, according to computer models.
Earth has a mean density of 5.5 grams/cubic centimeter, but the moon has a density of only 3.3 g/cc. The reason is the same, that the moon lacks iron.
The moon has exactly the same oxygen isotope composition as the Earth, whereas Mars rocks and meteorites from other parts of the solar system have different oxygen isotope compositions. This shows that the moon formed form material formed in Earth’s neighborhood.
If a theory about lunar origin calls for an evolutionary process, it has a hard time explaining why other planets do not have similar moons. (Only Pluto has a moon that is an appreciable fraction of its own size.) Our giant impact hypothesis had the advantage of invoking a stochastic catastrophic event that might happen only to one or two planets out of nine.
What were some earlier ideas?
One early theory was that the moon is a sister world that formed in orbit around Earth as the Earth formed. This theory failed because it could not explain why the moon lacks iron.
A second early idea was that the moon formed somewhere else in the solar system where there was little iron, and then was captured into orbit around Earth. This failed when lunar rocks showed the same isotope composition as the Earth.
A third early idea was that early Earth spun so fast that it spun off the moon. This idea would produce a moon similar to Earth’s mantle, but it failed when analysis of the total angular momentum and energy involved indicated that the present Earth-moon system could not form in this way.
Where did the theory come from?
Hartmann and Davis were familiar with the work done in the Soviet Union in the 1960’s, on the aggregation of planets out of countless asteroid-like bodies called planetesimals. Much of this work was pioneered by a Russian astrophysicist named V. S. Safronov.
Picking up on Safronov’s general ideas, Hartmann and Davis ran calculations of the rate of growth of the 2nd-largest, 3rd largest, etc., bodies in the general vicinity of Earth, as the Earth itself was growing. Just as the asteroid belt today has a largest asteroid (Ceres) at a 1000 km diameter, and several smaller bodies in the 300-500 km diameter range, the region of Earth’s orbit would have had several bodies up to about half the size of the growing Earth. Our idea was that in the case of Earth (but not the other planets) the impact happened late enough, and in such a direction relative to Earth’s rotation, that abundant enough middle material was thrown out to make a moon.
How did the theory develop?
After we first presented the theory in 1974 at a conference on satellites, Harvard researcher A. G. W. Cameron rose to say that he and William Ward were also working on the same idea, but coming at it from a different motivation — the study of angular momentum in the system — and that they had concluded the impacting body had to be roughly Mars size (a third or half the size of Earth). Our paper was published in 1975 (Hartmann and Davis, Icarus, 24, 504-505) Cameron and Ward published an abstract on this idea at the Lunar Science conference in 1976, two years after the PSI paper.
Some work was done by Thompson and Stevenson in 1983 about the formation of moonlets in the disk of debris that formed around Earth after the impact. However, in general the theory languished until 1984 when an international meeting was organized in Kona, Hawaii, about the origin of the moon. At that meeting, the giant impact hypothesis emerged as the leading hypothesis and has remained in that role ever
since. Dr. Michael Drake, director of the University of Arizona’s Planetary Science Department, recently described that meeting as perhaps the most successful in the history of planetary science.
A collection of papers from that meeting was published by the Lunar and Planetary Institute (Houston) in the 1986 book, Origin of the Moon, edited by PSI scientist William Hartmann, together with Geoffry Taylor and Roger Phillips. This book remains the prime reference on this subject. In the meantime, researchers such as Willy Benz, Jay Melosh, A. G. W. Cameron, and others have attempted computer models of the giant impact, to determine how much material would go into orbit. Some of these results have been used by Hartmann to make the paintings on this web page, attempting to show how the impact would have looked to a human observer (if humans had been around — they didn’t come along until 4.5 billion years later!)
In the 1990’s, Dr. Robin Canup wrote a Ph.D. dissertation on the moon’s origin and the giant impact hypothesis, which produced new modeling of the aggregation of the debris into moonlets, and eventually, into the moon itself. Dr. Canup is continuing the modeling of the lunar accretion process.
Current status:
In 1997, Dr. Canup’s work received a great deal of publicity by media news sources, some of whom mistakenly thought that the giant impact was a brand new idea. Canup’s early work, presented in July 1997, suggested the debris from an impact might not make a moon, but only a swarm of moonlets. Her later work (fall 1997) led to more “success” in aggregating the debris into a single moon.
Current status:
In 1997, Dr. Canup’s work received a great deal of publicity by media news sources, some of whom mistakenly thought that the giant impact was a brand new idea. Canup’s early work, presented in July 1997, suggested the debris from an impact might not make a moon, but only a swarm of moonlets. Her later work (fall 1997) led to more “success” in aggregating the debris into a single moon.
Thus, the giant impact hypothesis continues to be the leading hypothesis on how the moon formed. Is it right? Can it be disproven by more careful research? Only time will tell, but so far it has stood up to 25 years of scrutiny.
Links and Sources:
Rashid's Blog: Portal for Inquisitive Learners 
To my view, there are so many contrary evidences that would seem to argue against this theory. Here are only a few of such:
1. Where’s the original body that impacted with Earth?
If this “body” was of sufficient mass to impact Earth, strip away much of its crust, and still have enough inertia left to propel this crust material into such an incredibly high orbit as the Moon has today… yet did so without enough force to destroy Earth in the process, then by all the law of physics, it would have been traveling slow enough following the collision (if it hadn’t been before) to be captured in orbit by the Sun. So… where is this phantom body today?
2. If this object struck the Earth a glancing blow, upon which principle this theory absolutely depends otherwise given the obvious size and mass this theoretical object would have to have possessed, then the ejected crust material would have had an elliptical orbit about Earth, if not an extremely elliptical orbit. So then, why does the Moon have an almost entirely circular non-elliptical orbit?
3. (This last is a theory I only arrived at just today and seems perhaps the strongest of the former two)
As anyone can discover my searching online, the Moon revolves around the Earth in the opposite direction of the Earth’s spin. In relation to the North Star (Polaris), the Earth… as also does every other planet in our solar system with the exceptions of Venus and Uranus… rotates counter-clockwise on its axis. Thus, it is standard in this solar system for planets to rotate counter-clockwise on their axes.
We also know that unlike this apparent standard, the Moon revolves around the Earth in the opposite direction: clockwise from West to East.
By the collision theory, the only possible explanation for this is that the object hitting Earth a glancing blow must have done so from the opposite direction of the Earth’s rotation such that the expelled crust material that became the Moon assumed an orbit in the opposite direction. Thus, logically, for the Earth to still be spinning on its axis in a counter-clockwise direction AFTER having been slowed by this enormous object that collided with it anciently with such speed and force to eject a good deal of its crust into such a high orbit… the Earth must have originally been SPINNING COUNTER-CLOCKWISE AT ENORMOUS VELOCITY LIKE A TOP so that after being slowed following the collision, our current rotational speed is about 1000 miles per hour at the equator. Of course, as no other planet in our solar system is spinning like a top, it makes no sense that our planet would have been, either.
Mars, our closest and most Earth-like neighbor, rotates at about 550 mph at the equator whereas our next closest planet in size, Venus, only rotates at about 5 mph at the equator.
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1. It’s incorporated into the earth/moon objects. It never left. In any real sense it did “destroy” the earth. Gravity pulled the pieces together and 4 billion years took care of the rest.
2. Tidal forces from the earth both circularize and enlarge the moon’s orbit. The original body would have been closer and probably more elliptical.
3. The moon does not revolve around in the opposite direction. When looking down from the north pole, the earth’s rotation, the moon’s orbit about the earth, and the earth’s orbit about the sun are all counterclockwise. Because the earth’s rotation is faster than the moon’s period, it appears to orbit clockwise if you’re looking from the earth’s surface. But that is incorrect. Geostationary satellites don’t show any motion from the earth’s surface, but they too are orbiting in a counterclockwise direction.
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