REAL AND IMAGINARY KIRKWOOD GAPS

The evidence for resonant structure in the distribution of the semimaj or axes of members of asteroid families and meteoroid streams is revie wed. It is shown that, because families and streams are identified by clustering of orbital elements, great care must be taken to distinguis h between actual removal of objects ('real' gaps) and the failure to i dentify members of families or streams because of resonant perturbatio ns ('imaginary' gaps). If it can be shown that gaps are real then this can lead to important constraints on the time-scale for the removal o f objects. Sample variations of semimajor axis and eccentricity are ca lculated for resonant locations in the Themis asteroid family and the Quadrantid meteoroid stream using high-order expansions of the planeta ry disturbing function. Evidence is presented suggesting that a promin ent gap in the Themis family associated with the 15:7 jovian resonance is real, whereas that associated with the 3:1 resonance in the Quadra ntid meteors is imaginary. Recent claims concerning resonant structure in the Perseid meteor stream are examined in a similar context.