120 Lachesis is a large main-belt asteroid. It was discovered by French astronomer Alphonse Borrelly on April 10, 1872, and independently by German-American astronomer Christian Heinrich Friedrich Peters on April 11, 1872, then named after Lachesis, one of the Moirai, or Fates, in Greek mythology. A Lachesean occultation of a star occurred in 1999 and was confirmed visually by five observers and once photoelectrically, with the chords yielding an estimated elliptical cross-section of .
This body is orbiting the Sun with a period of 5.50 years and an eccentricity (ovalness) of 0.05. The orbital plane is inclined by 7ð to the plane of the ecliptic. Photometric observations of this asteroid were made in early 2009 at the Organ Mesa Observatory in Las Cruces, New Mexico. The resulting light curve shows a synodic rotation period of 46.551 ñ 0.002 hours with a brightness variation of 0.14 ñ 0.02 in magnitude. It is a very slow rotator with the longest rotation period of an asteroid more than 150 km in diameter. As a primitive C-type asteroid it is probably composed of carbonaceous material.