The heliocentric orbit of an asteroid can be relatively easily determined, requiring only a handful of astrometric measure-
ments from short imaging exposures, and the orbit provides enough information for the mission to be launched and to arrive at its destination successfully. Not that a high-precision orbit can necessarily be determined from the few-day apparition of a newly discovered small asteroid, but orbits are typically easier to mea-sure than an asteroid's physical and internal properties and this may limit how accurately the density, spin state, taxonomy, etc. of the target is known before the mission proper is launched. Though there is always the option to study the target intensively when it makes a subsequent passage near the Earth, these opportunities may occur only at intervals of years, decades or even longer, and
waiting for them could delay the mission significantly.