2024 YR4 is an asteroid that is classified as an Apollo-type (Earth-crossing) near-Earth object, with an estimated diameter of 40 to 90 metres (130 to 300 ft).

[2][1] When additional observations suggested it had an impact probability of over 1%, it triggered the first step in planetary-defense responses, which prompted several major telescopes to gather data about the asteroid and led United Nations–endorsed space agencies to begin planning asteroid threat mitigation.

[18] Preliminary 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.

[20] The diameter of 2024 YR4 has not been measured, but it can be estimated from its brightness (absolute magnitude) using a range of plausible values for its surface reflectivity (geometric albedo).

[21] 2024 YR4 is significantly smaller than Dimorphos, the impact target of NASA's Double Asteroid Redirection Test (DART) in 2022.

[3] 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, 2018GG4, 2019 SC, and 2020 MQ61.

This is expected to greatly improve calculations of 2024 YR4's orbit in preparation for its subsequent close approach on 22 December 2032.

[25] If 2024 YR4 passes very close to Earth or the Moon in 2032, the uncertainty will be even greater due to its trajectory being affected by gravitational focusing.

[3] Due to 2024 YR4's size and previously greater-than-1% impact probability, it peaked at Torino scale rating level 3, which prompted the International Asteroid Warning Network to issue a notice on 29 January 2025.

[24][16] Using NASA's estimated diameter, mass, and density for 2024 YR4, the asteroid would release energy equivalent to 7.8 megatonnes of TNT (32.6 PJ) if it were to impact Earth at its predicted velocity at atmospheric entry of 17.32 km/s (10.76 mi/s),[7] equivalent to about 500 of the "Little Boy" bomb dropped on Hiroshima, 2 and a half of Grapple Y, 51% of Castle Bravo, or 15.4% of Tsar Bomba.

The effects of the collision could create an impact crater 500 to 2,000 meters (0.3–1.2 miles) wide on the lunar surface, releasing about 5.2 megatonnes of TNT (21.8 PJ) of energy if it were to impact the Moon at a velocity estimated to be 13.9 km/s (8.7 mi/s), an explosion about 300 times more powerful than the Hiroshima bomb.

The potential impact zone extends from just south of Mare Crisium, a solidified lava plain, to Tycho, an ancient crater, all located on the visible side of the Moon.

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.

[34][17] The JWST is scheduled to observe 2024 YR4 between 1 and 24 March, when the position of the asteroid first becomes compatible with the pointing restrictions of the telescope, and again between 20 April and 20 May 2025.

JWST will use its NIRCam and Mid-Infrared Instrument, which will provide measurements of both 2024 YR4's position and its infrared thermal emission, from the latter of which the asteroid's size and albedo can be better estimated.

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.

[24] According to Sam Deen,[39] including this negative precovery data in the orbit calculation decreases the likelihood that the asteroid is somewhere along 60–80% of the non-impacting trajectories calculated from the observation arc to late January, which he estimated raised 2024 YR4's impact probability to 3% at that time; also including the single poorer-quality 25 December 2024 precovery observation increased the probability to 6%.

Images of the region taken in 2016 by Palomar Observatory, not made public as of January 2025[update], may provide further information.

An 8 February occultation passed Xiamen, China; Chenyang Guo reported negative results from two locations.

[41] 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 on the path.