(20729) 1999 XS143, provisional designation 1999 XS143, is a Jupiter trojan from the Greek camp, approximately 51 kilometers (32 miles) in diameter. It was discovered on 15 December 1999, by American astronomer Charles Juels at the Fountain Hills Observatory in Arizona. The dark Jovian asteroid has a short rotation period of 5.72 hours and belongs to the . It has not been named since its numbering in January 2001.