Short answers, pulled from the story.
William K. Hartmann and Donald R. Davis published models suggesting satellite-sized bodies could collide with planets in 1975. The name of the hypothesized protoplanet is derived from the mythical Greek titan Theia, a designation initially proposed by English geochemist Alex N. Halliday in 2000.
Evidence presented in 2008 suggests the collision occurred at approximately 4.48 billion years ago. Astronomers think the collision happened around 4.4 to 4.45 billion years ago, which was about 0.1 billion years after the Solar System began to form.
Modern theories suggest Theia orbited in the L4 or L5 configuration presented by the Earth-Sun system. Princeton University mathematician Edward Belbruno and astrophysicist J. Richard Gott III proposed that Theia coalesced at the Lagrangian point relative to Earth.
Oxygen isotope abundance in lunar rock suggests vigorous mixing of Theia and Earth, indicating a steep impact angle rather than a glancing blow. Computer simulations show an initial impactor velocity below escape velocity at infinity that increased to over 10 kilometers per second at impact.
Estimates based on computer simulations suggest some twenty percent of Theia's original mass ended up as an orbiting ring of debris. About half of this matter coalesced into the Moon while significant portions of mantle material from both bodies were ejected into orbit.