C/2021 A1 (Leonard) is an inbound long period comet discovered by G. J. Leonard at the Mount Lemmon Observatory on 3 January 2021 (a year before perihelion) when the comet was 5 AU (750 million km) from the Sun. Jupiter's orbit (5 AU from the Sun) is at the frost line where methanol (CH3OH) and water start sublimation. This is the first comet discovered in 2021 and has a retrograde orbit. On 12 December 2021 the comet will be 0.233 AU (34.9 million km) from Earth and on 18 December 2021 will be 0.028 AU (4.2 million km) from Venus. It will make its closest approach to the Sun on 3 January 2022. It may reach naked eye visibility in December 2021.[6] At an apparent magnitude of 4, it should be a good binocular comet. On 10 October the comet showed a short but dense dust tail. As of mid-November the comet has a total magnitude (coma+nucleus) of around 9.
On the morning of 6 December 2021 the comet will be about 5 degrees from the star Arcturus. On 14 December 2021 the comet will be 14.7 degrees from the Sun and will quickly become better seen from the southern hemisphere. The forward scattering of light could cause the comet to brighten to as much as magnitude 1.
C/2021 A1 has been inside of the orbit of Neptune since May 2009. Using an epoch of 1950 which is well before the comet entered the planetary region of the Solar System, a barycentric orbit solution suggests the comet had roughly a 80 thousand year orbital period. Therefore the comet had spent the last 40 thousand years inbound from approximately 3,700 AU (550 billion km). After perihelion the comet will be ejected from the Solar System. The barycentric orbit will remain hyperbolic after September 2022.
