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.[12]
Orbit and classification
Oblique view of 2021 PH27's orbit compared to the inner planets
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.[4] 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.[13] Its orbit crosses the paths of Mercury and Venus, with nominal minimum orbit intersection distances of 0.11 AU and 0.015 AU, respectively.[3]
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.[14] Being so close to the Sun, at perihelion the asteroid is moving at 106 km/s (240,000 mph).[5] The relativistic perihelion shift of this object is 1.6 times that of Mercury, which is 42.9 arcseconds per century.[8]
With an observation arc over 4 years, the orbit quality of 2021 PH27 is well secured, with an uncertainty parameter of 3.[4] 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.[3] Deep close encounters with Venus control its long-term orbital evolution.[8] As with many other Atira asteroids, it is subjected to the von Zeipel-Lidov-Kozaisecular resonance.[8]
At perihelion 2021 PH27 is heated up to temperatures of over 1000 K. Due to the resulting temperature differences between 2021 PH27's lit-up and shadowed areas, thermal shock is theorized to have caused numerous surface fractures on the asteroid.[15]
2019 AQ3, another low semi-major axis Atira asteroid
594913 ꞌAylóꞌchaxnim, the first asteroid discovered that always stays within Venus' orbit
Notes
^For example on 2023-May-03, 2021 PH27 can get as bright as apparent magnitude 13.8 when near perihelion on the far side of the Sun and in the full phase. But during perihelion it is also in solar conjunction and not observable. When on the near side in solar conjunction, it can get fainter than magnitude 30.
^Greenstreet, Sarah; Ngo, Henry; Gladman, Brett (January 2012). "The orbital distribution of Near-Earth Objects inside Earth's orbit"(PDF). Icarus. 217 (1): 355–366. Bibcode:2012Icar..217..355G. doi:10.1016/j.icarus.2011.11.010. hdl:2429/37251. Archived from the original(PDF) on 29 May 2019. Atira-class asteroids form part of what has been called Interior-Earth Objects (IEOs) (Michel et al., 2000), because their orbits lie completely interior to Earth's orbit...The NEO 2003 CP20 was discovered by the LINEAR survey and upon being tracked to a high-quality orbit was named 163693 Atira. Following historical precedent, we thus adopt the name Atira for this class of NEO, after its first named member (Schmadel, 2009).
^Williams, David R. (19 August 2021). "Mercury Fact Sheet". NASA Space Science Data Coordinated Archive. NASA. Retrieved 21 August 2021.
