Both a comet and ball lighting have been suggested. This means the real answer is still unknown.
(FOX) The timing of the fireballs suggests they might have been debris from Comet 73P/Schwassmann - Wachmann 3, said physicist Stephen Hughes at the Queensland University of Technology in Brisbane.
The green ball the farmer saw roll down the slope was almost certainly not a meteorite, Hughes said. No perfectly round meteorite a foot wide has ever been found, and if it were one, it wouldn't be rolling slowly down a hill.
Instead, Hughes suggests it might have been ball lightning, mysterious glowing orbs of light usually seen during thunderstorms. The green fireball might have provided an electrical connection between the ground and the ionized layer of atmosphere known as the ionosphere, providing the energy needed for ball lightning.