99942 Apophis

99942 Apophis (provisional designation 2004 MN₄) is a near-Earth asteroid and a potentially hazardous object with a diameter of 370 metres (1,210 feet) that caused a brief period of concern in December 2004 when initial observations indicated a probability up to 2.7% that it would hit Earth on April 13, 2029. Additional observations provided improved predictions that eliminated the possibility of an impact on Earth in 2029. Until 2006, a small possibility nevertheless remained that, during its 2029 close encounter with Earth, Apophis would pass through a gravitational keyhole of no more than about 800 kilometres (500 mi) in diameter, which would have set up a future impact exactly seven years later on April 13, 2036. This possibility kept it at Level 1 on the 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 zero. 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.

Preliminary observations by Goldstone radar in January 2013 effectively ruled out the possibility of an Earth impact by Apophis in 2036. By May 6, 2013 (April 15, 2013, observation arc), the possibility of an impact on April 13, 2036 had been eliminated altogether. In 2036, Apophis will approach the Earth at a third the distance of the Sun in both March and December, but this is about the distance of the planet Venus when it overtakes Earth every 1.6 years. On April 12, 2068, the nominal trajectory has Apophis 1.87 AU (280 million km) from Earth. Entering March 2021, six asteroids each had a more notable cumulative Palermo Technical Impact Hazard Scale rating than Apophis, and none of those has a Torino level above 0. On average, an asteroid the size of Apophis (370 metres) is expected to impact Earth once in about 80,000 years. Observations in 2020 by the Subaru telescope confirmed David Vokrouhlický's 2015 Yarkovsky effect predictions. The Goldstone radar observed Apophis March 3–11, 2021, helping to refine the orbit again, and on March 25, 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 kilometers to now just a couple of kilometers, greatly enhancing predictions of future approaches.