A near-Earth object (NEO) is any small Solar System body orbiting the Sun whose closest approach to the Sun (perihelion) is less than 1.3 times the Earth–Sun distance. This definition applies to the object's orbit around the Sun, rather than its current position, thus an object with such an orbit is considered an NEO even at times when it is far from making a close approach of Earth. If an NEO's orbit crosses the Earth's orbit, and the object is larger than 140 meters (460 ft) across, it is considered a potentially hazardous object (PHO). Most known PHOs and NEOs are asteroids, but about 0.35% are comets.
The Torino scale is a method for categorizing the impact hazard associated with near-Earth objects (NEOs) such as asteroids and comets. It is intended as a communication tool for astronomers and the public to assess the seriousness of collision predictions, by combining probability statistics and known kinetic damage potentials into a single threat value. The Palermo Technical Impact Hazard Scale is a similar, but more complex scale.
The Palermo Technical Impact Hazard Scale is a logarithmic scale used by astronomers to rate the potential hazard of impact of a near-Earth object (NEO). It combines two types of data—probability of impact and estimated kinetic yield—into a single "hazard" value. A rating of 0 means the hazard is equivalent to the background hazard. A rating of +2 would indicate the hazard is 100 times as great as a random background event. Scale values less than −2 reflect events for which there are no likely consequences, while Palermo Scale values between −2 and 0 indicate situations that merit careful monitoring. A similar but less complex scale is the Torino Scale, which is used for simpler descriptions in the non-scientific media.
(29075) 1950 DA is a risk-listed asteroid, classified as a near-Earth object and potentially hazardous asteroid of the Apollo group, approximately 1.3 kilometers in diameter. It once had the highest known probability of impacting Earth. In 2002, it had the highest Palermo rating with a value of 0.17 and a probability of 1 in 306 (0.33%) for a possible collision in 2880. Since that time, the estimated risk has been updated several times. In December 2015, the odds of an Earth impact were revised to 1 in 8,300 (0.012%) with a Palermo rating of −1.42. As of June 2024, it is listed on the Sentry Risk Table with the highest cumulative Palermo rating of -0.93. 1950 DA is not assigned a Torino scale rating, because the 2880 date is over 100 years in the future. As of 24 April 2024, the odds of an Earth impact reached 1 in 2,600 (0.038%).
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.
Sentry is an automated impact prediction system started in 2002 and operated by the Center for Near Earth Object Studies (CNEOS) at NASA's Jet Propulsion Laboratory. It continually monitors the most up-to-date asteroid catalog for possibilities of future impact with Earth over the next 100+ years. Whenever a potential impact is detected, it will be analyzed and the results immediately published by CNEOS. However, alerts do not imply certainty about future impacts, as the small amounts of optical data that can trigger an alert are not enough to conclusively identify an impact years in the future. In contrast, eliminating an entry on the risk page is a negative prediction.
(292220) 2006 SU49, provisional designation 2006 SU49, is a sub-kilometer asteroid, classified as near-Earth object and potentially hazardous asteroid of the Apollo group that had a small chance of impacting Earth in 2029.
(417634) 2006 XG1 provisional designation 2006 XG1, is a sub-kilometer asteroid, classified as near-Earth object and potentially hazardous asteroid of the Apollo group, that had a low but non-zero probability of impacting Earth on 31 October 2041. The asteroid was discovered on 20 September 2006, by astronomers of the Catalina Sky Survey, using a dedicated 0.68-meter telescope at Mount Lemmon Observatory in Arizona, United States.
2007 WD5 is an Apollo asteroid some 50 m (160 ft) in diameter and a Mars-crosser asteroid first observed on 20 November 2007, by Andrea Boattini of the Catalina Sky Survey. Early observations of 2007 WD5 caused excitement amongst the scientific community when it was estimated as having as high as a 1 in 25 chance of colliding with Mars on 30 January 2008. However, by 9 January 2008, additional observations allowed NASA's Near Earth Object Program (NEOP) to reduce the uncertainty region resulting in only a 1-in-10,000 chance of impact. 2007 WD5 most likely passed Mars at a distance of 6.5 Mars radii. Due to this relatively small distance and the uncertainty level of the prior observations, the gravitational effects of Mars on its trajectory are unknown and, according to Steven Chesley of NASA's Jet Propulsion Laboratory Near-Earth Object program, 2007 WD5 is currently considered 'lost' (see lost asteroids).
A potentially hazardous object (PHO) is a near-Earth object – either an asteroid or a comet – with an orbit that can make close approaches to the Earth and which is large enough to cause significant regional damage in the event of impact. They are conventionally defined as having a minimum orbit intersection distance with Earth of less than 0.05 astronomical units and an absolute magnitude of 22 or brighter, the latter of which roughly corresponds to a size larger than 140 meters. More than 99% of the known potentially hazardous objects are no impact threat over the next 100 years. As of September 2022, just 17 of the known potentially hazardous objects listed on the Sentry Risk Table could not be excluded as potential threats over the next hundred years. Over hundreds if not thousands of years though, the orbits of some "potentially hazardous" asteroids can evolve to live up to their namesake.
2016 EU85 is an asteroid, classified as near-Earth object and potentially hazardous asteroid of the Apollo group, approximately 400 meters in diameter. It was first observed on 10 March 2016, by the Pan-STARRS survey at Haleakala Observatory, Hawaii, United States.
2010 GZ60 was originally estimated by JPL to be a near-Earth asteroid approximately 2 kilometers (1.2 miles) in diameter. But is now known to be an asteroid from the inner region of the asteroid belt that does not get closer than 1.5 AU (220 million km) to Earth.
2018 VP1 is an Apollo near-Earth asteroid roughly 2 meters (7 feet) in diameter. The asteroid had a 0.41% chance (1 in 240) of impacting Earth on 2 November 2020 01:12 UT. It was discovered on 3 November 2018 when it was about 0.003 AU (450,000 km; 280,000 mi) from Earth and had a solar elongation of 165 degrees. The asteroid has a short 12.9 day observation arc. It was last observed on 16 November 2018 by the European Southern Observatory Very Large Telescope at apparent magnitude 26 pushing the telescope close to the limiting magnitude.
2006 QQ23 is a sub-kilometre asteroid, classified as a near-Earth object of the Aten group that is potentially hazardous only as the orbit evolves over millennia. It was first observed on 21 August 2006 by the Siding Spring Survey. On 10 August 2019, the object safely passed 7.4 million kilometres (4.6 million miles) from Earth. With a 12 year observation arc it has a well determined orbit and is not a threat for the foreseeable future.
2015 ME131 (also written 2015 ME131) was a lost asteroid and a Near-Earth object (NEO). It is an Atira asteroid, which is by far the smallest group of near-Earth objects. This makes it an interior-Earth object (IEO), meaning that it has an orbit entirely confined within Earth's orbit. It was recovered on September 15, 2020 as 2020 RX8 which has extended the observation arc from 1.8 days to 5 years. It was removed from the Sentry Risk Table on February 15, 2021 after the two orbits were linked together. It was first observed on June 23, 2015, when the asteroid was more than 1 AU from Earth and had a solar elongation of 68 degrees.
(696513) 2016 NL56 (provisional designation 2016 NL56) is a near-Earth object (NEO) and a potentially hazardous object (PHA), meaning that it has an orbit that can make close approaches to the Earth and large enough to cause significant regional damage in the event of impact. It is an Apollo asteroid, meaning that it is an Earth-crossing asteroid that has an orbit larger than the orbit of the Earth. It was first observed on 12 July 2016, when the asteroid was more than 1 AU from Earth and had a solar elongation of 163 degrees.
2017 MZ8 (also written 2017 MZ8) is a near-Earth object and a potentially hazardous asteroid, meaning that it has an orbit that can make close approaches to the Earth and large enough to cause significant regional damage in the event of impact. It is an Apollo asteroid, meaning that it is an Earth-crossing asteroid that has an orbit larger than the orbit of the Earth. It was first observed on 22 June 2017, when the asteroid was about 1 AU from Earth and had a solar elongation of 131 degrees.
2017 DB120 (also written 2017 DB120) is a near-Earth object, meaning that it has an orbit which brings it into proximity with Earth. It is an Amor asteroid, meaning that its orbit does not cross Earth's orbit, but its perihelion is close to, but greater than, the aphelion of Earth. It was first observed on February 25, 2017, when the asteroid was less than 1 AU from Earth and had a solar elongation of 169°.
2017 SG33 (also written 2017 SG33) is a near-Earth object, meaning that it has an orbit which brings it into proximity with Earth. It is an Amor asteroid, meaning that its orbit does not cross Earth's orbit, but its perihelion is close to, but greater than, the aphelion of Earth. It was first observed on 25 September 2017, when the asteroid was less than 1 AU from Earth and had a solar elongation of 169°.
2020 VV is an Apollo near-Earth asteroid roughly 12 meters in diameter. On 20 November 2020, the asteroid had a 4.4% chance of impacting Earth on 12 October 2033 11:43 UT. As of mid-December 2020, the asteroid has a modest 61 day observation arc. The nominal Earth approach is on 17 October 2033 at a distance of 0.009 AU, but the line of variations (LOV) is only known with an accuracy of ±22 hours. The line of variations allows the asteroid to come as close as 0.006 AU or pass as far away as 0.01 AU. With a diameter range of 10–22 meters the asteroid could be as large as the Chelyabinsk meteor.
