C/2021 T4 (Lemmon)

For other comets discovered by the Mount Lemmon Survey, see Comet Lemmon.

C/2021 T4 (Lemmon) is a non-periodic comet discovered by the Mount Lemmon Survey on 7 October 2021.^[2] This passage through the planetary region of the Solar System will reduce the orbital period from millions of years to thousands of years.^[4]

C/2021 T4 (Lemmon)

Comet C/2021 T4 (Lemmon) photographed by the Zwicky Transient Facility on 21 December 2022
Discovery[1][2]
Discovered by	E. Bryssinck
Discovery site	Mount Lemmon Survey
Discovery date	7 October 2021
Designations
Alternative designations	CK21T040, C6131F2
Orbital characteristics[3]
Epoch	14 April 2023 (JD 2460048.5)
Observation arc	3.63 years
Earliest precovery date	7 August 2021
Number of observations	2,107
Aphelion	≈44,000 AU (inbound)[4] ≈2,200 AU (outbound)
Perihelion	1.483 AU
Semi-major axis	~18,130 AU
Eccentricity	0.99992
Orbital period	millions of years (inbound)[4] ≈36,000 years (outbound)
Inclination	160.78°
Longitude of ascending node	257.88°
Argument of periapsis	329.81°
Mean anomaly	359.99°
Last perihelion	31 July 2023
TJupiter	–1.426
Earth MOID	0.498 AU
Jupiter MOID	0.869 AU
Physical characteristics
Comet total magnitude (M1)	12.4
Apparent magnitude	8.0 (2023 apparition)

It has been south of the celestial equator since October 2022. On 13 June, it was 1.5 degrees from the star Beta Ceti. Closest approach to Earth was on 20 July 2023 at a distance of 0.54 AU (81 million km).^[5] The next day it reached its southernmost declination, at -56 degrees. On 25 July, it passed next to the globular cluster NGC 6397.^[6] It reached perihelion on 31 July 2023 at a distance of 1.48 AU (221 million km). The comet brightened to around apparent magnitude 8 at its peak.^[7]