C/2021 T4 (Lemmon)

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

C/2021 T4 (Lemmon) is a long period comet discovered by the Mount Lemmon Observatory on 7 October 2021.^[1] This passage through the planetary region of the Solar System will reduce the orbital period from millions of years to thousands of years.^[3]

C/2021 T4 (Lemmon)

The comet on 21 December 2022, by ZTF
Discovery[1]
Discovered by	E. Bryssinck
Discovery site	Mount Lemmon Observatory
Discovery date	7 October 2021
Designations
Alternative designations	CK21T040 C6131F2
Orbital characteristics[2]
Observation arc	2.91 years (1,064 days)
Number of observations	2,107
Aphelion	≈44,000 AU (inbound)[3] ≈2,200 AU (outbound)
Perihelion	1.4833 AU
Semi-major axis	48,585.76 AU
Eccentricity	0.99997
Orbital period	millions of years (inbound)[3] ≈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

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).^[4] The next day it reached its southernmost declination, at -56 degrees. On 25 July it passed next to the globular cluster NGC 6397.^[5] It reached perihelion on 31 July 2023 at a solar distance of 1.48 AU. The comet brightened to around apparent magnitude 8.^[6]