text: However , a possibility remained that during the 2029 close encounter with Earth , Apophis would pass through a gravitational keyhole , a precise region in space no more than about a half-mile wide , that would set up a future impact on April 13 , 2036 .
simplified: During 2029 , Apophis would go through a `` gravitational keyhole '' , an area in space no more than about 400 meters across . This would make it hit the Earth on April 13 , 2036 .