Original: 99942 Apophis ( , previously known by its provisional designation ' ) is a near-Earth asteroid that caused a brief period of concern in December 2004 because initial observations indicated a small probability ( up to 2.7 % ) that it would strike the Earth in 2029 .
Simplified: ( 99942 ) Apophis is a near-Earth asteroid that worried people for a short time in December 2004 . This was because observations seemed to show that it would hit the Earth in 2029 .