2021 PH27

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 (DECam) at Cerro Tololo Inter-American Observatory on 13 August 2021. 2021 PH₂₇ has the smallest semi-major axis and shortest orbital period among all known asteroids as of 2026^[update], with a velocity at perihelion of 106 km/s (240,000 mph). It also has the largest relativistic perihelion shift of any known object orbiting the Sun, 1.6 times that of Mercury. 2021 PH₂₇ shares an identical orbit and color as 2025 GN1, which has led astronomers to believe that the two asteroids split apart from a parent body over 10,500 years ago.

2021 PH₂₇ 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 (190 million km). 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 (304), Las Cumbres (K91), (W85), (W87), and (Q63), SONEAR (Y00), and iTelescope (Q62). The object was then provisionally designated 2021 PH₂₇ 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 2021 PH₂₇ 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.

2021 PH₂₇ 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.

As of 2021^[update], 2021 PH₂₇ holds the record for the smallest semi-major axis (0.46 AU) and shortest orbital period (114 days) of any known asteroid, supplanting 2019 LF6 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 (240,000 mph). 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 2021 PH₂₇ 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.

Observations of 2021 PH₂₇ 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 (enstatite composition), M (metallic composition), or P (organic-rich silicate composition).

2021 PH₂₇ is estimated to have a diameter of 1.2 ± 0.5 km (0.75 ± 0.31 mi), based on its absolute magnitude of 17.7 and an assumed X-type asteroid geometric albedo of 0.098±0.081. The brightness of 2021 PH₂₇ 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 2021 PH₂₇ 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 150±50 thousand years.