A lost periodic comet, discovered by C. J. Messier on 1770 June 14. On July 1 it passed 0.015 au (2.2 million km) from Earth, the closest approach of a comet on record, appearing of 2nd magnitude with a coma over 2° wide. Calculations in 1779 by the Swedish astronomer Anders Johan Lexell (1740–84), after whom the comet is named, indicated that it was in an elliptical orbit of period 5.6 years. The comet’s orbit has been considerably modified by close approaches to Jupiter. A close passage to Jupiter in 1779 increased its perihelion distance and may even have placed it into a hyperbolic orbit, effectively ejecting it from the Solar System. It has not been seen since.

http://cometography.com/pcomets/1770l1.html Information page at Cometography website.