Asteroids range in size from a few meters to a few hundred kilometres. Observations indicate that asteroids larger than 150 m have rotation periods longer than 2.2 hours. This is because asteroids larger than 150 m tend to be rubble-pile structures bound by gravity. A rotation period shorter than the critical 2.2 hours can cause the asteroid to break apart. In contrast, smaller asteroids are generally coherent monolithic objects, allowing them to have more rapid spin rates.
Nevertheless, two asteroids, 2001 OE84 and 2005 UW163, are both known to be larger than 150 m and have spin periods shorter than 2.2 hours. 2001 OE84 has a diameter of ~700 m and a rotation period of 29 minutes (Pravec et al. 2002), while 2005 UW163 has a diameter of ~600 m and a rotation period of 1.29 hours (Chang et al. 2014). The rotation periods of both asteroids were determined by timing the photometric variations as they spin.
2001 OE84 and 2005 UW163 are rotating too rapidly to be strengthless rubble-pile aggregates. In addition to gravity, other mechanisms such as tensile strength and cohesiveness are required to keep the asteroids from breaking apart. 2001 OE84 and 2005 UW163 could be unusually large coherent monolithic objects. Discovering more of such objects can help reveal their abundance.
Plot of diameters versus rotation period. The green and gray filled circles are objects with well determined rotation periods. The asteroids 2005 UW163 (red filled circle) and 2001 OE84 (blue filled circle) are outliers. Chang et al. (2014).
- Pravec et al. (2002), “Large coherent asteroid 2001 OE84”, Proceedings of Asteroids, Comets, Meteors - ACM 2002.
- Chang et al. (2014), “A New Large Super-Fast Rotator: (335433) 2005 UW163”, arXiv:1407.8264 [astro-ph.EP]