2021 PH27

 is a near-Earth asteroid of the Atira group. It was discovered by Scott Sheppard using the Dark Energy Survey's DECam imager at NOIRLab's Cerro Tololo Inter-American Observatory on 13 August 2021. has the smallest semi-major axis and shortest orbital period among all known asteroids, with a velocity at perihelion of 106 km/s. It also has the largest relativistic perihelion shift of any object orbiting the Sun, 1.6 times that of Mercury. With an absolute magnitude of 17.7, the asteroid is estimated to be larger than 1 km in diameter.

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 (its furthest distance from the Sun.) 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 1.3 AU. 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 (114 days; semi-major axis of 0.46 AU). 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 (NEO) 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 (0.46 AU) and shortest orbital period (114 days) of any known asteroid, supplanting  and 594913 ꞌAylóꞌchaxnim (0.56 AU, 151 days). 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 106 km/s. 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 3. Nonetheless, additional observations are necessary to constrain uncertainties in its orbit by the time the asteroid approaches perihelion and enters conjunction with the Sun in October 2021, during which it will become unobservable at solar elongations less than 20 degrees. 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.

Temperature
At perihelion is heated up to temperatures of over 1000 K. Due to the resulting temperature differences between 's lit-up and shadowed areas, thermal shock is theorized to have caused numerous surface fractures on the asteroid.

Numbering and naming
As of 2023, this minor planet has neither been numbered nor named by the Minor Planet Center.