Follow C/2021 A1 Leonard, which may reach naked-eye brightness this month

The possibility of a naked-eye comet is an exciting prospect. In 2020 we had C/2020 F3 Neowise, which gave us a major comet boost. At the start of 2021, predictions that comet C/2021 A1 Leonard might perform a similar trick produced a lot of excitement.

As time passed, A1 Leonard looked like it would fall short. Initially expected to be around the mag. +6.8 mark on 1 December, revised estimates now have it at around mag. +9.2. It won’t be the easiest of comets to locate either, appearing low in the morning sky. Following the latest brightness curve, A1 Leonard will only reach a peak magnitude of around mag. +6.8 on the 13th, which places it below naked-eye threshold and within binocular range.

However, there’s still hope for cometary redemption due to an effect known as forward-scattering enhancement. This occurs when the particles associated with the comet reflect sunlight towards the observer, so increasing the apparent brightness of the comet. The Sun-comet-Earth geometry needs to be correct for this to happen, but during December this is the case. The effect is predicted to kick in from 9–20 December, giving A1 Leonard an extra brightness boost that may tip it above the naked-eye threshold, maybe elevating it to fourth magnitude around the 13th.