| C/2021 T4 (Lemmon) | |
| Designations: | CK21T040, C6131F2 |
| Discovery Date: | 7 October 2021 |
| Observation Arc: | 1.7 years |
| Next P: | 31 July 2023 |
| Perihelion: | 1.4823 AU |
| Inclination: | 160.76° |
| Eccentricity: | 0.99970 |
| Jupiter Moid: | 0.869AU |
| Earth Moid: | 0.497AU |
| Arg Peri: | 329.78° |
| Asc Node: | 257.79° |
| Period: | millions of years (inbound) ≈36,000 years (outbound) |
| Aphelion: | ≈44,000 AU (inbound) ≈2,200 AU (outbound) |
| M1: | 6.9 |
C/2021 T4 (Lemmon) is an inbound 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]
