Definitions: ground reaction force
: A force exerted by the ground in response to the forces a body exerts on it.
: a small region of space where a planet's gravity would alter the orbit of a passing asteroid such that the asteroid would collide with that planet on a future orbital pass.
: 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.
Additional observations provided improved predictions that eliminated the possibility of an impact on Earth or the Moon in 2029.
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.
This possibility kept the asteroid at Level 1 on the
Torino impact hazard scale
until August 2006, when the probability that Apophis will pass through the keyhole was determined to be very small.
Apophis broke the record for the highest level on the Torino Scale, being, for only a short time, a level 4, before it was lowered.
Its diameter is approximately 270 meters (885 ft).
As of October 7, 2009, the probability of an April 13, 2036 impact is considered to be 1 in 250,000.
Of objects not recently observed, there are 7 asteroids with a more notable
Palermo Technical Impact Hazard Scale
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