The mass of asteroid (21) Lutetia
Measurements obtained during the 10 July 2010 Rosetta flyby of asteroid (21) Lutetia enabled Rosetta scientists to determine the mass of Lutetia with greater precision than ever before. The mass estimate of 1.7 x 1018 kg turned out to be much lower than expected from ground-based observations. The Rosetta error bar is smaller than the measurement point.
For further details see Pätzold et al., (2011)