Component size ratio (D_s/D_p) is secondary diameter divided by primary diameter. Symbol sizes indicate component separation (a/R_p) expressed in terms of the radius of the primary. Some values are estimated based on assumed albedoes, densities, etc.

Binary dynamical types are based on taxonomy described by Pravec and Harris (2007) and Lindsay et al. (2015). In summary, type codes are:

• A = small binary (D_p<20 km) with smaller secondary (D_s/D_p<0.5)
• B = small binary (D_p<20 km) with similar sized secondary (D_s/D_p>0.8)
• C = contact binary
• L = large asteroid (D_p>90 km) with small satellite (D_s/D_p<0.25)
• O = outlier, particularly large asteroid (D_p>90 km) with similar sized secondary (D_s/D_p>0.8)
• U = unclassified
• W = wide small binary (D_p<10 km, A₂/D_p>10)

• For main belt and near-Earth asteroids, distinct dynamical types are evident and described in the above references.
• Many of the A type systems are understood to represent components formed from mass shedding during YORP-driven rotational spinup. It appears that 10-20 km is an upper limit on primary sizes for such systems. The lower size limit of ~50 m for secondaries appears to be intrinsic, not observational, given that smaller secondaries should have been detectable by radar for many NEAs. This may represent a lower limit below which rotationally-shed material from the primary cannot efficiently consolidate into a satellite.
• A continuous range of characteristics is seen for the TNO population, including systems with very large separations, stable by virtue of the systems' large distances from the Sun and planets. Note that detection biases introduce cutoffs in known TNO systems, illustrated by the line for constant secondary component size (D₂ = 50 km).

The figure below shows component size ratio vs. primary size with discovery method indicated: SC = spacecraft imaging, GB IMG = ground-based imaging, HST = Hubble Space Telescope (and James Webb Space Telescope), OCC = occultation, RAD = radar, LC = lightcurve, MULT = multiple methods, VIS = visual/telescopic.

References:

• Lindsay, S. S., F. Marchis, J. P. Emery, J. E. Enriquez, and M. Assafin, 2015, Composition, mineralogy, and porosity of multiple asteroid systems from visible and near-infrared spectral data, Icarus, 247:53-70.
• Pravec, P., and A. W. Harris, 2007, Binary asteroid population I. Angular momentum content, Icarus, 190:250-259.

© 2014-2025, 2026 by Wm. Robert Johnston.
Last modified 20 June 2026.
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