2021 PH27
is a kilometer-sized Atira-type near-Earth asteroid orbiting very close to the Sun. It was discovered by Scott Sheppard using the Dark Energy Camera at Cerro Tololo Inter-American Observatory on 13 August 2021. has the smallest semi-major axis and shortest orbital period among all known asteroids as of 2026, with a velocity at perihelion of. It also has the largest relativistic perihelion shift of any known object orbiting the Sun, 1.6 times that of Mercury. shares an identical orbit and color as, which has led astronomers to believe that the two asteroids split apart from a parent body over 10,500 years ago.
Discovery
was discovered by astronomer Scott Sheppard using the Dark Energy Survey's DECam imager at Cerro Tololo Observatory in Chile on 13 August 2021, two days after the asteroid had reached aphelion The observations were conducted at twilight to search for undiscovered minor planets situated at low elongations from the Sun. The object was discovered at apparent magnitude 19, with a solar elongation of 37 degrees when it was on the far side of the Sun at an Earth distance of. It was then reported to the Minor Planet Center's Near-Earth Object Confirmation Page under the temporary designation v13aug1. Over five days, follow-up observations were conducted by various observatories including Las Campanas, Las Cumbres,,, and, SONEAR, and iTelescope. The object was then provisionally designated by the Minor Planet Center and announced on 21 August 2021. Even in April 2021, the asteroid was never more than 45 degrees from the Sun.Precovery observations of were found in archival Dark Energy Survey images from 16 July 2017. These observations were published by the Minor Planet Center on 10 October 2021.
Orbit and classification
orbits the Sun at a distance of 0.13–0.79 AU once every 4 months. Its orbit has an eccentricity of 0.71 and an inclination of 32 degrees with respect to the ecliptic. It is classified as a near-Earth object due its perihelion distance being less than 1.3 AU. It also falls under the NEO category of Atira asteroids, whose orbits are confined entirely within Earth's orbit at 1 AU from the Sun. Its orbit crosses the paths of Mercury and Venus, with nominal minimum orbit intersection distances of 0.11 AU and 0.015 AU, respectively., holds the record for the smallest semi-major axis and shortest orbital period of any known asteroid, supplanting and 594913 ꞌAylóꞌchaxnim. For comparison, Mercury has a semi-major axis of 0.39 AU and an orbital period of 88 days. Being so close to the Sun, at perihelion the asteroid is moving at. The relativistic perihelion shift of this object is 1.6 times that of Mercury, which is 42.9 arcseconds per century.
With an observation arc over 4 years, the orbit quality of is well secured, with an uncertainty parameter of 2. It currently comes closer to Venus than to any of the other planets. Deep close encounters with Venus control its long-term orbital evolution. As with many other Atira asteroids, it is subjected to the von Zeipel-Lidov-Kozai secular resonance.
| Nominal distance | Minimum distance | Maximum distance |
Physical characteristics
Observations of in different light filters show that it has a neutral to slightly reddish color with a spectral type of X. The compositions of X-type asteroids are ambiguous and may correspond to spectral types of either E, M, or P.is estimated to have a diameter of, based on its absolute magnitude of 17.7 and an assumed X-type asteroid geometric albedo of. The brightness of periodically fluctuates with an amplitude of about 0.1 magnitudes, which could indicate an elongated shape. Observations from February to April 2022 have shown that rotates with a period of 3.49 hours. Due to the asteroid's close proximity to the Sun, the YORP effect can double its rotation rate in thousand years.
At perihelion, the Sun heats the surface of up to temperatures of over, before cooling down as the surface rotates away from the Sun. Due to the resulting temperature differences between 's dayside and nightside areas, thermal shock is theorized to have caused numerous surface fractures on the asteroid. Despite its extreme thermal environment, does not show evidence of dust ejection nor blue tinting of its surface by heated refractory organics.