Combining all available ground-based data in 2000 decreases the error by an order of magnitude. Next, assuming that the rotation period did not change between 2000 and 2001, constraints from the HST 2001 data (Weaver, H.A., Stern, S.A., Parker, J.Wm. [2003]. Astron. J. 126, 444–451) yield three possible rotation periods: P = 1.088 ± 0.003 days, P = 1.108 ± 0.002 days, and P = 1.135 ± 0.003 days, which are consistent with our initial derivation of P = 1.08 ± 0.04 days (Mueller, B.E.A., Samarasinha, N.H. [2002]. Earth Moon Planets 90, 463–471).
These three periods are further refined and the error bars further improved by another order of magnitude by linking the combined ground-based data from 2000 to the nuclear orientation of Borrelly at the DS1 encounter in 2001 (see Table 2). Due to aliasing, there are seven possible rotation periods around P = 1.088 days, five possible periods around P = 1.108 days, and six possible periods around P = 1.135 days, with precisions of the order of 0.0002 days (≈17 s).