2024 YR4

2024 YR4 is an asteroid with an estimated diameter of 53 to 67 metres (174 to 220 ft) that is classified as an Apollo-type (Earth-crossing) near-Earth object. From 27 January to 20 February 2025, it had an impact rating of 3 on the Torino scale, with an estimated probability greater than 1% that it would impact Earth on 22 December 2032. The estimated impact probability peaked at 3.1% on 18 February, but by 23 February additional observations had reduced the asteroid's Torino rating to 0. By 2 April all three risk-lists had removed the year 2032 as an Earth-impact risk. As of April 2025, the asteroid retains roughly a 4% chance of impacting the Moon on 22 December 2032 around 15:19 UTC.
The asteroid was discovered by the Chilean station of the Asteroid Terrestrial-impact Last Alert System (ATLAS) at Río Hurtado on 27 December 2024. When additional observations indicated an impact probability of greater than 1%, the first step in planetary defense responses was triggered, prompting additional data gathering using several major telescopes and leading United Nations–endorsed space agencies to begin planning asteroid threat mitigation.
The asteroid made a close approach to Earth at a distance of 828,800 kilometres (515,000 miles; 2.156 lunar distances) on 25 December 2024, two days before its discovery, and will be moving away from the Sun until November 2026. Its next close approach will take place on 17 December 2028. Analysis of spectral and photometric time series suggests that 2024 YR4 is a stony S-type (most likely), L-type or K-type asteroid, with a rotation period of approximately 19.5 minutes. A number of known asteroids, including other virtual impactors, follow orbits somewhat consistent with that of 2024 YR4.
The asteroid's provisional designation as a minor planet, "2024 YR4", was assigned by the Minor Planet Center when its discovery was announced on 27 December 2024. "Y", the first letter after the discovery year, indicates that the asteroid was discovered in the second half-month of December (16 to 31 December), and "R4" indicates that it was the 117th provisional designation to be assigned in that half-month.
Measurements of 2024 YR4's mid-infrared thermal emission by the James Webb Space Telescope (JWST) from 26 March 2025 indicate that it has a diameter of 60 m (200 ft), with an uncertainty of 7 m (23 ft).
Previous estimates for 2024 YR4's diameter were based on its brightness (absolute magnitude) using a range of plausible values for its surface reflectivity (geometric albedo). If 2024 YR4 reflects between 5% and 25% of visible light, as do the vast majority of asteroids with a measured albedo, then its diameter is between 40 and 90 m (130 and 300 ft). An estimate by NASA placed its diameter at 55 m (180 ft) for an assumed geometric albedo of 0.154. These estimates made 2024 YR4 around the same size as the asteroid that caused the 1908 Tunguska event or the iron–nickel asteroid that created the Meteor Crater in Arizona 50,000 years ago. 2024 YR4 is significantly smaller than Dimorphos, the impact target of NASA's Double Asteroid Redirection Test (DART) in 2022.
The mass and density of 2024 YR4 have not been measured, but the mass can be loosely estimated with an assumed density and assumed diameter. Assuming a density of 2.6 g/cm (1.5 oz/cu in), which is within the density range for stony asteroids such as 243 Ida, with an assumed diameter of 55 m (180 ft), the Sentry risk table estimates a mass of 2.2×10 kg (4.9×10 lb). Both the assumed density and the inferred diameter contribute large uncertainties to the mass estimate, with the latter dominant as the volume of the asteroid depends on the cube of the diameter.
Preliminary spectroscopic analysis from the Gran Telescopio Canarias and Lowell Discovery Telescope suggests that 2024 YR4 is either an S-type asteroid (17% of the asteroid population), an L-type asteroid, or a K-type asteroid, all of which point to a stony composition. Spectroscopy by the Gemini South telescope in February 2025 suggest either an R or Sa spectral type for 2024 YR4. JWST measurements of 2024 YR4's thermal emission suggest "a rockier surface than commonly inferred".
Photometric observations by the Very Large Telescope (VLT) and the La Silla Observatory's 1.54-metre (5.1 ft) telescope indicate 2024 YR4 has a rotation period near 19.5 minutes. Observations by the Gemini South telescope from February 2025 found similar results for 2024 YR4's rotation period. This is a relatively fast rotation period for an asteroid, although it is not fast enough to rule out a rubble pile structure for 2024 YR4. The VLT has also observed 2024 YR4 at multiple phase angles from 5° to 35°, which would allow for the construction of a phase curve which can constrain the asteroid's surface properties.
The brightness of 2024 YR4 varies by 0.42 magnitudes as it rotates, indicating it has an elongated shape with its longest equatorial length being at least 1.4 times that of its shortest equatorial length. Gemini South telescope measurements of 2024 YR4's rotational light curve at various phase angles show that the asteroid has a retrograde rotation and a highly flattened shape with an equatorial diameter roughly 3 times as long as its polar diameter.
As an Apollo-type near-Earth object, 2024 YR4 orbits the Sun on an elliptical orbit that crosses Earth's orbit. Since its close approach in December 2024, the asteroid has an orbital period of about 3.99 years and an orbital inclination of 3.41 degrees with respect to Earth's orbit (the ecliptic). The period, considered as an osculating element, dips slightly at the approach in December 2028 and then slowly rises to around 4.01 years by 2031. Its orbit will be strongly perturbed at the close encounter of 2032. Astronomers Carlos and Raúl de la Fuente Marcos have proposed that 2024 YR4 could be related to a group of near-Earth asteroids on similar orbits that also have virtual impactors: 2017 UW5, 2018 GG4, 2019 SC, and 2020 MQ61. The 2015 Porangaba meteor orbit has a 5% probability of matching that of 2024 YR4.
Due to the Yarkovsky effect, 2024 YR4's retrograde rotation causes its orbit to shrink over time. This indicates 2024 YR4 originated farther out in the Solar System, specifically the central main asteroid belt. 2024 YR4's stony composition supports the possibility of an origin from the central main belt, since S-type and C-type asteroids are the most abundant spectral types in that region. The inward migration of 2024 YR4 from the main belt to near-Earth space was likely chaotic since the asteroid would have to cross multiple orbital resonances, such as the 3:1 mean-motion resonance with Jupiter's orbital period at 2.5 AU and the ν6 secular resonance with Saturn's orbital precession at 2.2 AU.
The asteroid reached perihelion (its closest approach to the Sun) on 22 November 2024, and made a close approach to Earth on 25 December 2024, two days before its discovery. During this encounter, 2024 YR4 passed 828,800 km (515,000 mi; 2.156 lunar distances) from Earth and then 488,300 km (303,400 mi; 1.270 LD) from the Moon. The asteroid will make its next close approach to Earth on 17 December 2028, when it will pass 8,010,000 ± 60,000 km (4,977,000 ± 37,000 mi; 20.84 ± 0.16 LD) from Earth. The 2028 encounter will provide astronomers the opportunity to perform additional observations and extend the observation arc by four years. This will significantly improve calculations of 2024 YR4's orbit in preparation for its subsequent close approach on 22 December 2032.
Since the 2032 close approach is not yet well constrained enough to rule out a Moon impact, the resulting perturbation by the Earth–Moon system is highly uncertain, and all close approaches after 2032 are therefore not well constrained either. The 1 March 2025 position of the asteroid is known with a 3-sigma uncertainty in the asteroid's position of ± 600 km. By mid-2034, the uncertainty in the position propagates to about 500 million km (310 million mi), or about three times the distance between Earth and the Sun. Since 2024 YR4 is expected to pass very close to the Moon in 2032, the post-2032 uncertainty will be even greater due to its trajectory being affected by gravitational focusing. The possible trajectories become more divergent with time and the greatest risk of an Earth impact is in December 2047. By December 2047, the uncertainty in the asteroid's position along its orbit is 9 billion km (60 AU) and wraps around the asteroid's orbit.
On 22 December 2032, 2024 YR4 will come closest to Earth sometime between 07:00 and 10:30 UTC, approaching from the direction of Sagittarius. The nominal (best-fit) closest approach to Earth on 22 December 2032 is at 08:46 UTC (with an uncertainty of about 1.7 hours in the closest-approach time), at a distance of 260,000 km (160,000 mi), with a 3-sigma uncertainty of 83,000 km (52,000 mi). The nominal closest approach to the Moon occurs at 15:18 UTC, with a nominal distance of about 3,100 km (1,900 mi).
Due to 2024 YR4's size and previously greater-than-1% impact probability, it reached a rating of 3 on the Torino scale on 27 January 2025, which prompted the International Asteroid Warning Network to issue a notice on 29 January 2025. This was the second-highest Torino rating ever reached by an asteroid, behind 99942 Apophis, which briefly reached a rating of 4 in late 2004. NASA's Sentry gave 2024 YR4 a rating on the Palermo scale as high as −0.18 on 18 February 2025, when it had a 55-day observation arc and a 3.1% chance of impacting the Earth in 2032. This gave 2024 YR4 a corresponding impact hazard of 66% of the background hazard level, given the asteroid's relatively small size of 60 metres (200 ft). The asteroid approached but never exceeded the background risk of a random asteroid of the same size impacting Earth by 2032, which by definition corresponds to a Palermo rating of 0. On 18 February 2025, the European Space Agency's Aegis listed a 2.8% chance of an Earth impact in 2032. On 23 February 2025 (with a 60-day observation arc), 2024 YR4 was reduced to a Torino rating of 0.
On 2 April 2025, with a 91-day observation arc, NASA removed the chance of a 2032 Earth impact. The European Space Agency removed the chance of a 2032 impact on 8 March 2025. NEODyS removed the chance of a 2032 impact on 1 March 2025.
A risk corridor of 2024 YR4's possible impact locations in 2032, estimated from existing observations in late January 2025, begins from the eastern equatorial Pacific Ocean, runs through northern South America, the equatorial Atlantic Ocean, Nigeria, central Africa, the north of eastern Africa, the southwest corner of the Arabian Peninsula, the northwestern Indian Ocean, India, and ends in Bangladesh. Using NASA's estimated diameter, mass, and density for 2024 YR4, the asteroid would release energy equivalent to 7.7 megatonnes of TNT (32.2 petajoules) if it were to impact Earth at its predicted velocity at atmospheric entry of 17.20 km/s (10.69 mi/s), equivalent to about 500 times the energy released by Little Boy (the atomic bomb dropped on Hiroshima), two and a half times of Grapple Y, 50% of Castle Bravo, or 15% of Tsar Bomba.
Due to its stony composition, this would more likely produce a meteor air burst than an impact crater (for an impact on land) or tsunami (for an oceanic impact). It could cause damage as far as 50 km (30 mi) from the impact site. Despite its potential to cause damage if it were to impact, 2024 YR4 is not categorized as a potentially hazardous object (PHO) because it has an absolute magnitude dimmer than 22, which usually means that such an asteroid is less than 140 m (460 ft) in diameter and its potential damage therefore would be localized.
2024 YR4 has around a 4% chance of impacting a 70% waning gibbous moon on 22 December 2032 around 15:17 to 15:21 UTC. The nominal approach to the Moon is near the impact scenario at around 15:18 UTC ± 1.5 hours at a distance of 3,100 km (1,900 mi; 0.0081 LD), or about 1,400 km above the 1,737 km radius of the Moon, with a 3-sigma uncertainty of 83,000 km (52,000 mi).
The impact could create an impact crater with a diameter of 500 to 2,000 metres (0.31 to 1.24 mi) wide on the lunar surface, releasing about 5.2 megatonnes of TNT (21.8 petajoules) of energy if it were to impact the Moon at an estimated velocity of 13.9 km/s (8.6 mi/s), an explosion about 340 times more powerful than the Hiroshima bomb. The impact corridor is a line that extends through the southern parts of Mare Humorum and Mare Nubium.
Michael Busch of the SETI Institute notes that an explosion on the Moon "would be very obvious to any spacecraft observing from lunar orbit" but may not be as visible to the unaided eye from Earth due to the Moon's brightness. However, other astronomers believe the impact could be visible from Earth. Gareth Collins suggested that "the impact flash of vaporized rock would be visible from Earth, even in the daytime", while Daniel Bamberger of the Northolt Branch Observatories in London stated that the impact "could be brighter than the full moon" making it clearly visible to the naked eye.
Additional observations of 2024 YR4 are necessary to reduce uncertainties in its trajectory. Because the asteroid is now moving away from Earth, it is becoming fainter, necessitating the use of larger-aperture telescopes such as the 10-meter Keck telescope and the Very Large Telescope. As of 14 March 2025 the asteroid had reached apparent magnitude 26, which is 63 million times fainter than what can be observed with the naked eye. As of the last ground−based observation on 23 March 2025 by Paranal Observatory, the asteroid was 1.8 AU from the Sun which is just beyond the orbit of Mars. The asteroid was not observed between 11–13 January and 8–15 February 2025 due to interference from moonlight. After mid-February, a 2-meter telescope or better was required. After 4 March 2025, a 4-meter or better class telescope became required. After 1 April, an 8-meter or larger was required. Space-based infrared telescopes such as the JWST can observe 2024 YR4 at farther distances until 20 May. The JWST observed 2024 YR4 on 8 March, when the position of the asteroid first became compatible with the pointing restrictions of the telescope, then 26 March, and will observe between 20 April and 16 May 2025. JWST has used its NIRCam and Mid-Infrared Instrument to measure 2024 YR4's position, infrared thermal emission, and size. NEOWISE burned up in Earth's atmosphere in November 2024, and therefore was not available to take infrared measurements of 2024 YR4's size.
The orbital uncertainty of 2024 YR4 may be further reduced with precovery observations, in which the asteroid would be detected in archival telescope images taken before its discovery. The earliest known precovery observation of 2024 YR4 was by ATLAS on 25 December 2024, but this is just two days before its discovery and the measured position of the asteroid in that observation is also more uncertain than in later observations, due to the rapid motion of the asteroid and a longer exposure than would have been optimal for observations of such a fast-moving asteroid. The asteroid passed within 12 million km of Earth in September 2016 and within 20 million km of Earth in October 2020. A search through 2016 Subaru Telescope archival images did not find 2024 YR4 in a region where it might have been.
Astronomers of the Catalina Sky Survey have inspected a set of images from Mount Lemmon Survey, including images containing the virtual impactor's predicted location; however, no candidates were found. Astronomers of the Pan-STARRS survey identified a few images in 2012, 2016 and 2020, again with no candidates found, alongside images from 2012 and 2020 which did not have a sufficiently deep limiting magnitude to detect 2024 YR4 at its predicted magnitude on those dates. Paolo Tanga checked for possible detections by the Gaia spacecraft, but concluded that 2024 YR4 never came within the spacecraft's field of view. James Bauer checked the NEOWISE data, Deborah Woods checked Space Surveillance Telescope data, and Julien de Wit searched data from TESS and other exoplanet surveys; none of these searches found detections of 2024 YR4.
A positive occultation detection would make possible measurements of the size and shape of the asteroid and more precise measurements of its position. As of 11 February 2025, no positive stellar occultation has been reported. A 6 February occultation had its path very close to the Connecticut–Rhode Island border, and no occultation results have been reported so far. An 8 February occultation passed Xiamen, China; Chenyang Guo reported negative results from two locations. The uncertainty range for the path of both occultations on Earth was a few kilometers wide, and while Fresnel diffraction broadens the penumbral to slightly more than twice the diameter of the asteroid—to 100 and 140 m (330 and 460 ft)—an uncertainty of a few kilometers is still too wide compared to this penumbra to efficiently place movable observing stations across the path.
Observations of the asteroid when it passes near Earth again in 2028 will enable the calculation of a very precise orbit and a much refined estimation of the Moon impact likelihood in 2032 as it will extend the observation arc by four years. The asteroid will be too faint for observation until June 2028. It will be about magnitude 25 when it comes to opposition on 19 July 2028 at an Earth distance of 0.78 AU (117 million km) but it will continue to get closer until December 17 when it will pass about 20.8 LD (8.0 million km; 5.0 million mi) from Earth.
Had the observations up to 2028 not ruled out a 2032 Earth impact with a 5-sigma confidence, then it might have been useful to send an asteroid deflection mission similar to DART to 2024 YR4 to avert its impact. However, mounting such a mission with less than eight years to design and construct a spacecraft would be challenging. A mission could be prepared before the 2028 close encounter so that it would be ready to launch if it is determined that an impact is likely. Alternatively, if deflection is unfeasible and the predicted site of impact is on or close to a continent, it could be evacuated.