The four monolithic objects have been estimated to have mean diameters in the range 30-120 meters and periods in the range 2-20 minutes. Asteroids of these sizes are expected to have lifetimes against catastrophic disruption of 107 to 108 years (Farinella et al.1998). Therefore, the four objects are likely nonprimordial collisional fragments of larger asteroids. Their sizes give a rough estimate of the size distribution of monolithic subunits in larger, rubble-pile asteroids; they extend up to sizes exceeding 100 meters.
Fragments from catastrophic asteroid disruptions are expected to be in excited, non-principal axis rotation states (Asphaug and Scheeres 1999). The fact that we have found no indication of a tumbling due to a non-uniform rotation in any of the four monolithic asteroids is not surprising, however, since their dumping timescales are estimated to be in a range from 103 to 107 years (Harris 1994), thus likely shorter than their ages.
In Fig. 7, the distribution of spin rate vs. diameter is plotted for 753 asteroids of all sizes (30 m to 1000 km), including the three objects from this paper. (See Pravec and Harris 1999 for a description of the data sample.) The monoliths form a distinct group in the upper left corner. A possible tendency for an increase of the spin rate with decreasing size in this range, though predicted from the theory (Harris 1979), has to be confirmed by further observations.