Possible impact effects
NASA initially estimated the energy that Apophis would have released if it struck Earth as the equivalent of 1,480 megatons of TNT. A later, more refined NASA estimate was 880 megatons.[14] The impacts which created the Barringer Crater or caused the Tunguska event are estimated to be in the 3-10 megaton range[15] The 1883 eruption of Krakatoa was the equivalent of roughly 200 megatons.
Path of risk where 99942 Apophis may impact Earth in 2036.
The exact effects of any impact would vary based on the asteroid's composition, and the location and angle of impact. Any impact would be extremely detrimental to an area of thousands of square kilometres, but would be unlikely to have long-lasting global effects, such as the initiation of an impact winter.
The B612 Foundation made estimates of Apophis path if a 2036 Earth impact were to occur as part of an effort to develop viable deflection strategies.[16] The result is a narrow corridor a few miles wide, called the path of risk, and it includes most of southern Russia, across the north Pacific (relatively close to the coastlines of California and Mexico), then right between Nicaragua and Costa Rica, crossing northern Colombia and Venezuela, ending in the Atlantic, just before reaching Africa.[17][18] Using the computer simulation tool NEOSim, it was estimated that the hypothetical impact of Apophis in countries such as Colombia and Venezuela, which are in the path of risk, would have had more than 10 million casualties.[19] An impact several thousand miles off the West Coast of the US would produce a devastating tsunami.[20]
http://en.wikipedia.org/wiki/99942_Apophis#Possible_impact_effects
If it hit the moon, it would be a grand light show and there might be a few meteors that burned up in our atmosphere.
