99942 Apophis

99942 Apophis (provisional designation 2004 MN₄) is a near-Earth asteroid and a potentially hazardous object, 450 metres (1,480 ft) by 170 metres (560 ft) in size, that caused a brief period of concern in December 2004 when initial observations indicated a probability of 0.027 (2.7%) that it would hit Earth on Friday, April 13, 2029. Additional observations provided improved predictions that eliminated the possibility of an impact on Earth in 2029. A small possibility nevertheless remained that, during its 2029 close encounter with Earth, Apophis would pass through a gravitational keyhole estimated to be 800 metres in diameter, which would have set up a future impact exactly seven years later on Easter Sunday, April 13, 2036. This possibility kept it at Level 1 on the 0 to 10 Torino impact hazard scale until August 2006, when the probability that Apophis would pass through the keyhole was determined to be very small and Apophis's rating on the Torino scale was lowered to Level 0. By 2008, the keyhole had been determined to be less than 1 km wide. During the short time when it had been of greatest concern, Apophis set the record for highest rating ever on the Torino scale, reaching Level 4 on December 27, 2004.

The discovery of Apophis in 2004 is rather surprising, because it is estimated that an asteroid as big or bigger coming so close to Earth happens only once in 800 years on average. Such an asteroid is expected to actually hit Earth once in about 80,000 years.

Preliminary observations by Goldstone radar in January 2013 effectively ruled out the possibility of an Earth impact by Apophis in 2036 (probability less than 1 in a million). In February 2013 the estimated probability of an impact in 2036 was reduced to 7×10⁻⁹. It is now known that in 2036, Apophis will approach the Earth at a third the distance of the Sun in both March and December, about the distance of the planet Venus when it overtakes Earth every 1.6 years. Simulations in 2013 showed that the Yarkovsky effect might cause Apophis to hit a "keyhole" in 2029 so that it will come close to Earth in 2051, and then could hit another keyhole and hit Earth in 2068. But the chance of the Yarkovsky effect having exactly the right value for this was estimated as 2 in a million. Radar observations in March 2021 helped to refine the orbit, and in March 2021 the Jet Propulsion Laboratory announced that Apophis has no chance of impacting Earth in the next 100 years. The uncertainty in the 2029 approach distance has been reduced from hundreds of kilometres to now just a couple of kilometres, greatly enhancing predictions of future approaches. Entering March 2021, six asteroids each had a more notable cumulative Palermo scale rating than Apophis, and none of those has a Torino level above 0. However, Apophis will continue to be a threat possibly for thousands of years until it is removed from being a potentially hazardous object, for instance by passing close to Venus or Mars.