2021 PH27

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. 2021 PH27 has the smallest semi-major axis and shortest orbital period among all known asteroids as of 2021,[7] with a velocity at perihelion of 106 km/s (240,000 mph).[5] It also has the largest value of the relativistic perihelion shift, 1.6 times that of Mercury.[8] With an absolute magnitude of 17.7, the asteroid is estimated to be larger than 1 kilometer (0.6 miles) in diameter.[3]

2021 PH27
Illustration showing the asteroid 2021 PH27 inside Mercury's orbit
Discovery [1]
Discovered byS. S. Sheppard
Discovery siteCerro Tololo Obs.
Discovery date13 August 2021
Designations
2021 PH27
v13aug1 [2]
Atira · NEO [3][4]
Orbital characteristics [4]
Epoch 1 July 2021 (JD 2459396.5)
Uncertainty parameter 3
Observation arc4.15 yr (1,515 days)
Earliest precovery date16 July 2017
Aphelion0.7903 AU
Perihelion0.1331 AU
0.4617 AU
Eccentricity0.7116
0.31 yr (114.60 days)
49.496°
3° 8m 28.602s / day
Inclination31.929°
39.411°
7 October 2021
@ 106 km/s[5]
8.575°
Earth MOID0.2251 AU
Mercury MOID0.1123 AU[3]
Venus MOID0.0147 AU[3]
Physical characteristics
Mean diameter
>1 km
19.3 (discovery)[1]
14 (unobservable)[6][lower-alpha 1]
17.71±0.235[4]

    Discovery

    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,[1] 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.[9] 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).[1] It was then reported to the Minor Planet Center's Near-Earth Object Confirmation Page under the temporary designation v13aug1.[10] 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.[1] Even in April 2021, the asteroid was never more than 45 degrees from the Sun.[11]

    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, 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-Kozai secular resonance.[8]

    Venus Approach 2022-Oct-26[4]
    (as known with a 4.15 yr observation arc)
    Nominal
    distance (AU)    		 Minimum
    distance (AU)    		 Maximum
    distance (AU)
    0.01618 AU (2.420 million km)0.01617 AU (2.419 million km)0.01619 AU (2.422 million km)

    Physical characteristics

    Temperature

    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]

    Numbering and naming

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

    See also
    • Vulcanoids, a hypothetical population of asteroids within Mercury's orbit
    • 2019 LF6, low semi-major axis Atira asteroid
    • 2019 AQ3, another low semi-major axis Atira asteroid
    • 594913 ꞌAylóꞌchaxnim, the first asteroid discovered that always stays within Venus' orbit

    Notes
    1. 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.

    References
    1. "MPEC 2021-Q41 : 2021 PH27". Minor Planet Electronic Circular. Minor Planet Center. 21 August 2021. Retrieved 21 August 2021.
    2. "2021 PH27". NEO Exchange. Las Cumbres Observatory. 21 August 2021. Retrieved 21 August 2021.
    3. "2021 PH27". Minor Planet Center. International Astronomical Union. Retrieved 21 August 2021.
    4. "JPL Small-Body Database Browser: 2021 PH27" (2021-08-30 last obs.). Jet Propulsion Laboratory. Archived from the original on 27 August 2021. Retrieved 21 August 2021.
    5. "Horizons Batch for (2021 PH27) at perihelion on 7 October 2021". JPL Horizons (Perihelion occurs when deldot flips from negative to positive. VmagSn is velocity with respect to Sun.). Jet Propulsion Laboratory. Retrieved 26 August 2021.
    6. "Horizons Batch for (2021 PH27) at apmag 13.8". JPL Horizons. Jet Propulsion Laboratory. Retrieved 1 September 2021.
    7. "JPL Small-Body Database Search Engine" (Q < 0.983 (au)). Jet Propulsion Laboratory. Retrieved 21 August 2021.
    8. de la Fuente Marcos, Carlos; de la Fuente Marcos, Raúl (8 September 2021). "Roaming the Relativistic Realm: Short-term Dynamical Evolution of Atira 2021 PH27". Research Notes of the American Astronomical Society. 5 (9): 205. Bibcode:2021RNAAS...5..205D. doi:10.3847/2515-5172/ac242e.
    9. Tholen, David (21 August 2021). "Re: v13aug1 - new shortest-period asteroid?!?". Minor Planets Mailing List. groups.io. Retrieved 21 August 2021.
    10. ""Pseudo-MPEC" for v13aug1". Project Pluto. 21 August 2021. Archived from the original on 21 August 2021. Retrieved 21 August 2021.
    11. "Horizons Batch for (2021 PH27) Solar Elongation on 13 April 2021". JPL Horizons. Jet Propulsion Laboratory. Retrieved 28 August 2021.
    12. "MPEC 2021-T202 : DAILY ORBIT UPDATE (2021 October 10)". Minor Planet Electronic Circular. Minor Planet Center. 10 October 2021. Retrieved 10 October 2021.
    13. 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).
    14. Williams, David R. (19 August 2021). "Mercury Fact Sheet". NASA Space Science Data Coordinated Archive. NASA. Retrieved 21 August 2021.
    15. Müller, Thomas; Santana-Ros, Toni; Micheli, Marco; Pantin, Eric (23 September 2022). "No one gets closer to the Sun: Thermophysical properties of Atira object 2021 PH27". EPSC Abstracts. Europlanet Science Conference 2022. 16. doi:10.5194/epsc2022-524. Retrieved 10 February 2023.
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