Discussion

The three objects more than double the sample of known asteroids rotating so fast that they must be monoliths. The other two known cases are 1998 KY₂₆ and 1995 HM, with rotation periods of 10.7 min and (most likely) 97.2 min, respectively (Ostro et al. 1999, Steel et al. 1997). In Table III, physical and orbital characteristics of the five asteroids are summarized. Since the observations of 1995 HM were relatively poor, we discuss just the first four objects in the following two paragraphs.

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.