We know a lot. The impactor, known as Theia, would have liquefied any solid rock on the earth and thrown about two moon's worth of stuff into the space around the now combined earth. This stuff impacted and combined to form what we now see as the moon, with the rest falling back to earth. These impacts generate heat which make the early moon molten. Then you know how much the moon weighs, how big it is and its surface area so you can calculate how fast it cooled from the initial heat of creation. One of the main problems with the giant impact idea has been why there is not as much Iron on the moon. The new article shows that if the object hit head on instead of at a glance, most of the iron in the impactor ended up in the earth with more the lighter stuff like silicates making up the moon.
And do we have any idea what that collision would have been like?
Video of a simulation!
A great Vid using using Universe Sandbox!