5751 Zao, provisional designation 1992 AC, is an Amor asteroid discovered by Japanese astronomer M. Koishikawa at the Ayashi Station of the Sendai Astronomical Observatory on 5 January 1992 Zao has been observed by several groups to determine its rotational period.

Its light curve was observed by Pravec, et al. between 1992 and 1995 with the intention of determining its rotational period.

From its nearly constant brightness it was determined that the asteroid is roughly spherical and has a rotational period of ≥ 21.7 hr.

[4] Another group led by Wisniewski conducted an approximately 5 hour observation and were unable to conclude a rotational period.

[5] Zao was again observed in 2001 by Delbó, et al. using the Keck telescope.