Comet C/1861 G1 (Thatcher) is a long-period comet with roughly a 422-year orbit that is expected to return around 2283. It was discovered by A. E. Thatcher. It is responsible for the April Lyrid meteor shower. Carl Wilhelm Baeker also independently found this comet. The comet passed about from the Earth on 1861-May-05 and last came to perihelion (closest approach to the Sun) on 1861-Jun-03.
C/1861 G1 is listed as a long-period "non-periodic comet" because it has not yet been observed at two perihelion passages. When it is seen to come back around 2283, it should receive the P/ designation.
The comet is the parent body of the April Lyrids meteor shower.