C/2021 O3 (PanSTARRS)

For other comets of the same name, see List of Pan-STARRS discoveries.

C/2021 O3 (PanSTARRS) is a hyperbolic Oort cloud comet discovered on 26 July 2021 by the Pan-STARRS sky survey. It came to perihelion on 21 April 2022 at 0.287 AU (42.9 million km) from the Sun.

C/2021 O3 (PanSTARRS)

Discovery[1]
Discovered by	Robert Weryk
Discovery site	Pan-STARRS
Discovery date	26 July 2021
Orbital characteristics[2][3]
Epoch	30 October 2021 (JD 2459517.5)
Observation arc	351 days
Number of observations	760
Orbit type	Oort cloud
Perihelion	0.287 AU
Eccentricity	1.00015
Inclination	56.75°
Longitude of ascending node	189.03°
Argument of periapsis	299.98°
Mean anomaly	–0.002°
Last perihelion	21 April 2022
TJupiter	0.446
Earth MOID	0.063 AU
Jupiter MOID	2.165 AU
Physical characteristics[4]
Mean radius	~1.0–1.7 km (0.62–1.06 mi)
Geometric albedo	0.04 (assumed)
Comet total magnitude (M1)	10.6
Apparent magnitude	7.6 (2022 apparition)[5]

Observational history

The comet was expected to reach apparent magnitude 5.0 by late April 2022, while being only 15 degrees from the Sun.^[1][6] While near perihelion the comet was dimmer than expectations, only reaching magnitude 7.6 at its peak brightness on 19 April 2022.^[5] It was faintly visible in STEREO/SECCHI COR2-A on 27 April 2022.^[7] Observations by Lowell Discovery Telescope on 29 April in the twilight detected a diffuse glow with a magnitude of 9.0 where the comet was expected to be, indicating that its nucleus disintegrated during perihelion,^[8] however follow-up studies in 2025 revealed that this is unlikely and the comet remains intact post-perihelion.^[4] C/2021 O3 made its closest approach to Earth on 8 May 2022 at a distance of 0.60 AU (90 million km).^[9]

The comet was recovered by multiple observatories after perihelion at magnitudes not too different from those observed pre-perihelion.^[10] Calculations carried out using the pre- and post-perihelion orbits indicate that although the comet is probably dynamically old, it may also be a fragment of a dynamically new comet that was released during the first perihelion passage of its parent comet.^[10][11]

Orbit

Orbit through inner Solar System

With a short observation arc of 7 days, the Minor Planet Center used an assumed eccentricity of 1.0 for the orbit solution.^[12] Due to statistics of small numbers, with a short 10 day arc JPL had an eccentricity of 0.99595±0.00444 which could be as high as 1.00039 or as low as 0.99151.^[13] With an observation arc of 53 days, JPL Horizons shows both an inbound and outbound eccentricity greater than 1.^[2]

C/2021 O3 likely took millions of years to arrive from the outer Oort cloud and, had it survived, may have been fated to be ejected from the Solar System.^[2] This is also the most likely scenario when considering the post-perihelion orbit determination of the surviving object.^[10]