2024 YR4

is an asteroid with an estimated diameter of 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, reflecting its size and 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 2025. By 23 February, additional observations effectively ruled out impacting Earth in 2032 and lowered its Torino rating to 0. Based on the James Webb Space Telescope observation on 11 May 2025, there is a roughly 4% chance of impacting the Moon on 22 December 2032 around 15:19 UTC, and the asteroid is expected to pass km from the surface of the Moon.

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 increased its impact probability to 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 on 25 December 2024, two days before its discovery, and it 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 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 .

Provisional designation

The asteroid's provisional designation as a minor planet, "", 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 "" indicates that it was the 117th provisional designation to be assigned in that half-month.

Physical characteristics

Size and mass

Measurements of 's mid-infrared thermal emission by the James Webb Space Telescope (JWST) on 26 March 2025 indicate that it has a diameter of , with an uncertainty of . This makes 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. is significantly smaller than Dimorphos, the impact target of NASA's Double Asteroid Redirection Test (DART) in 2022.

Prior to this measurement, more uncertain estimates for 's diameter were based on its brightness ( absolute magnitude) and using a range of plausible values for its surface reflectivity (geometric albedo). If reflects between 5% and 25% of visible light, as do the vast majority of asteroids with a measured albedo, then its diameter had to be between . An estimate by NASA for instance placed its diameter at by assuming a geometric albedo of 0.154.

The mass and density of have not been measured, but the mass can be loosely estimated with an assumed density and the estimated diameter. Assuming a density of , which is within the density range for stony asteroids such as 243 Ida, and the then nominal diameter of , the Sentry risk table estimated a mass of . Rescaling the diameter to the now better measured value of increases the estimated mass to . Both the assumed density and the inferred diameter contribute large uncertainties to the mass estimated.

Composition, rotation, and shape

Preliminary spectroscopic analysis from the Gran Telescopio Canarias and Lowell Discovery Telescope suggests that 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. Spectro-photometry by the Gemini South telescope in February 2025 suggest either an R or Sa spectral type for . JWST measurements of 's thermal emission suggest "a rockier surface than commonly inferred."

Photometric observations by the Very Large Telescope (VLT) and the La Silla Observatory's telescope indicate has a rotation period near 19.5 minutes. Observations by the Gemini South telescope from February 2025 found similar results for '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 . The VLT has also observed 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 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 '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.

Orbit

As an Apollo-type near-Earth object, 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 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 by 2031. Its orbit will be strongly perturbed at the close encounter of 2032, and the orbital period is expected to be reduced to approximately by April 2033. Astronomers Carlos and Raúl de la Fuente Marcos have proposed that could be related to a group of near-Earth asteroids on similar orbits that also have virtual impactors: , , 2019 SC, and . The 2015 Porangaba meteor orbit has a 5% probability of matching that of .

Due to the Yarkovsky effect, 's retrograde rotation causes its orbit to shrink over time. This indicates originated farther out in the Solar System, specifically the central main asteroid belt. '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 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, passed 828,800 km (; ) from Earth and then from the Moon. The asteroid will make its next close approach to Earth on 17 December 2028, when it will pass 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 '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 By mid-2034, the uncertainty in the position propagates to about , or about 1/3 the distance between Earth and the Sun. Since 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 and wraps around the asteroid's orbit.

File:2024 YR4 orbit.jpg, Diagram of 's orbit
File:Animation of 2024 YR4 around Sun - 2032 close approach.gif, Animation of 's trajectory from 2028 to 2036 (with 2036 being highly uncertain)

File:Animation of 2024 YR4 around Earth - 2032 close approach.gif, Animation of around the Earth showing its 2032 close approach

2032 close approach

On 22 December 2032, 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 08:36 UTC (with an uncertainty of about 1.5 hours in the closest-approach time), at a distance of , with a 3-sigma uncertainty of . The nominal closest approach to the Moon occurs at 15:10 UTC, with a nominal distance of about .

Due to '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 the larger 99942 Apophis which briefly reached a rating of 4 in late 2004. NASA's Sentry gave a rating on the Palermo scale as high as 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 a corresponding impact hazard of 66% of the background hazard level, given the asteroid's relatively small size of . 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), 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.

Potential impact effects

As of late January 2025, the risk corridor of 's possible impact locations in 2032, estimated from the existing observations, began from the eastern equatorial Pacific Ocean, ran 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 ended in Bangladesh. Using NASA's estimated diameter, mass, and density for , the asteroid would have released energy equivalent to if it had been to impact Earth at its predicted velocity at atmospheric entry of , 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 have more likely produced a meteor air burst than an impact crater (for an impact on land) or tsunami (for an oceanic impact). It could have caused damage as far as from the impact site. Despite its potential to cause damage if it were to impact, 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 in diameter and its potential damage therefore would be localized.

Possible impact on the Moon

Using observations through 11 May 2025, has around a 4% chance of impacting a 70% waning gibbous moon on 22 December 2032 around 15:17 to 15:21 UTC. Observations by the James Webb Space Telescope on 11 May 2025 reduced the Earth approach uncertainty region by 20% and increased the odds of a Moon impact from 3.8% to 4.3%. The nominal approach to the Moon is near the impact scenario at around 15:10 UTC ± 1.3 hours at a distance of from the center of the Moon, or about 9,000 km above the 1,737 km radius of the Moon, with a 3-sigma uncertainty of .

The impact could create a crater with a diameter of on the lunar surface, releasing the equivalent of in energy if it were to impact the Moon at an estimated velocity of , 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.

Observation opportunities

2025

Additional observations of reduced the uncertainties in its trajectory. Because the asteroid was already moving away from Earth when it was discovered, it was 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 11–13January and 8–15February 2025 due to interference from moonlight. After mid-February, a 2-meter telescope or better was required. After 4March 2025, a 4-meter or better class telescope became required. After 1April, an 8-meter or larger was required. Space-based infrared telescopes like the JWST were able to observe at farther distances until mid-May. The JWST observed on 8March, when the position of the asteroid first became compatible with the pointing restrictions of the telescope, then 26 March, and observed it a final time on 11May 2025, those observations being that last of the asteroid before its upcoming 2028 approach. JWST has used its NIRCam and Mid-Infrared Instrument to measure 's position, infrared thermal emission, and size.

Precovery

The orbital uncertainty of would be further reduced by precovery observations, in which the asteroid would be detected in archival telescope images taken before its discovery. The earliest known precovery observation of was by ATLAS on 25December 2024, but this is just two days before its discovery and the measured position of the asteroid in that observation is 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 12million km of Earth in September 2016 and within 20million km of Earth in October 2020. A search through 2016 Subaru Telescope archival images did not find in part of the sky region where it should have been.

Astronomers of the Catalina Sky Survey inspected a set of images from Mount Lemmon Survey, including images containing the virtual impactor's predicted location, and similarly found no candidates. 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 never came within the spacecraft's field of view. James Bauer checked the NEOWISE data, Deborah Woods checked Space Surveillance Telescope data, and Julien deWit searched data from TESS and other exoplanet surveys; none of these searches found detections of .

Stellar occultation

A positive occultation detection would make possible measurements of the size and shape of the asteroid and more precise measurements of its position. no positive stellar occultation had been reported. A 6February occultation had its path very close to the Connecticut–Rhode Island border, and no occultation results have been reported so far. An 8February 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 penumbra to slightly more than twice the diameter of the asteroid—to —an uncertainty of a few kilometers is still too wide compared to this penumbra to efficiently place movable observing stations across the path.

2028

Observations of the asteroid when it passes near Earth again in 2028 will extend the observation arc by four years. This will enable the calculation of a very precise orbit and a much refined estimation of the Moon impact likelihood in 2032. The asteroid will be too faint for observation until June 2028. It will be about magnitude 25 when it comes to opposition on 19July 2028 at an Earth distance of and it will continue to get closer until December 17 when it will pass about from Earth.

Defense

Had the observations not ruled out a 2032 Earth impact with a 5-sigma confidence, an asteroid deflection mission similar to DART might have been sent to to avert its impact. However, mounting such a mission with less than eight years to design and construct a spacecraft would have been 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.

See also

* Asteroid impact avoidance

Notes

References

External links

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2024 YR4 Overview
NASA, 7February 2025

Close approach fact sheet for asteroid 2024 YR4
ESA's NEO Coordination Centre

Images of 2024 YR4
taken by ATLAS in January 2025, at '' b612.ai''

Asteroidticker
comparative JPL data on NEOs in imperial units

Light curve of
by Petr Pravec, Ondrejov Asteroid Photometry Project

3D model
of 2024 YR4 on sketchfab

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