| C/2021 T4 (Lemmon) | |
| Discoverer: | E. Bryssinck |
| Discovery Site: | Mount Lemmon Observatory |
| Discovery Date: | 7 October 2021 |
| Designations: | CK21T040 C6131F2 |
| Observation Arc: | 2.91 years (1,064 days) |
| Obs: | 2,107 |
| Aphelion: | ≈44,000 AU (inbound) ≈2,200 AU (outbound) |
| Semimajor: | 48,585.76 AU |
| Perihelion: | 1.4833 AU |
| Eccentricity: | 0.99997 |
| Period: | millions of years (inbound) ≈36,000 years (outbound) |
| Inclination: | 160.78° |
| Asc Node: | 257.88° |
| Arg Peri: | 329.81° |
| M1: | 6.9 |
| Last P: | 31 July 2023 |
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 0.54abbr=unitNaNabbr=unit.[1] The next day it reached its southernmost declination, at -56 degrees. On 25 July it passed next to the globular cluster NGC 6397.[2] It reached perihelion on 31 July 2023 at a solar distance of 1.48 AU. The comet brightened to around apparent magnitude 8.[3]
