 Despite published predictions , astronomers had not expected to see the fireballs from the impacts and did not have any idea in advance how visible the other atmospheric effects of the impacts would be from Earth . Observers soon saw a huge dark spot after the first impact . The spot was visible even in very small telescopes , and was about 6 @,@ 000 km ( 3 @,@ 700 mi ) ( one Earth radius ) across . This and subsequent dark spots were thought to have been caused by debris from the impacts , and were markedly asymmetric , forming crescent shapes in front of the direction of impact . 
