C/2021 T4 (Lemmon)

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

C/2021 T4 (Lemmon)

The comet on 21 December 2022, by ZTF
Discovery
Discovered by	E. Bryssinck
Discovery site	Mount Lemmon Observatory
Discovery date	7 October 2021
Designations
Alternative designations	CK21T040 C6131F2
Orbital characteristics
Observation arc	2.91 years (1,064 days)
Number of observations	2,107
Aphelion	≈44,000 AU (inbound) ≈2,200 AU (outbound)
Perihelion	1.4833 AU
Semi-major axis	48,585.76 AU
Eccentricity	0.99997
Orbital period	millions of years (inbound) ≈36,000 years (outbound)
Inclination	160.78°
Longitude of ascending node	257.88°
Argument of periapsis	329.81°
Last perihelion	31 July 2023
Earth MOID	0.4975 AU (74.42×10^6 km)
Jupiter MOID	0.8699 AU (130.14×10^6 km)
Comet total magnitude (M1)	6.9

C/2021 T4 (Lemmon) is a long period comet discovered by the Mount Lemmon Observatory on 7 October 2021. This passage through the planetary region of the Solar System will reduce the orbital period from millions of years to thousands of years.

It has been south of the celestial equator since October 2022. On 13 June it was 1.5 degrees from magnitude 2 Beta Ceti. Closest approach to Earth was on 20 July 2023 at a distance of 0.54 AU (81 million km). The next day it reached its southernmost declination, at -56 degrees. On 25 July it passed next to the globular cluster NGC 6397. It reached perihelion on 31 July 2023 at a solar distance of 1.48 AU. The comet brightened to around apparent magnitude 8.