Cl

Relation Frequency (per day)

Fig. 37. Rotational range vs. frequency (rotations per day), modified by Scott Shep-pard from [134]. Black dots denote large main-belt asteroids (diameters >200 km) while KBOs are marked as stars. Note that Hektor is a Jovian Trojan while 33128 is a Centaur

Relation Frequency (per day)

Fig. 37. Rotational range vs. frequency (rotations per day), modified by Scott Shep-pard from [134]. Black dots denote large main-belt asteroids (diameters >200 km) while KBOs are marked as stars. Note that Hektor is a Jovian Trojan while 33128 is a Centaur than the trailing. However, the Iapetus albedo asymmetry is a consequence of its synchronous rotation about the planet (which leads to hemispherically asymmetric fluxes of incident charged particles from Saturn's magnetosphere and of Saturn-orbiting dust particles), a circumstance which is not replicated in the KBOs.

Region B shows objects rotating sufficiently rapidly that centripetal distortion of the shape constitutes a likely explanation of the lightcurve. The region is marked for an assumed density p = 1000 kg m-3 and calculated from the figures of equilibrium by Chandrasekhar [17]. Higher (lower) densities would push the left boundary of Region B to the right (left). The implicit assumption is that the tensile strengths are zero and, while this is unlikely to be exactly correct, it is a reasonable approximation for bodies that have been internally fractured by past collisions. Two KBOs fall in Region B; (20000) Varuna (p ā€” 1000 kgm-3 [72] and 2003 EL61 (p ā€” 2600-3340 kgm-3 [127]).

Region C shows locations in the range vs. frequency plot where close and contact binaries would plot. A binary consisting of two spheres viewed equa-torially would have Am = 2.5 log(2) = 0.7 mag. Mutual gravitational deformation would elongate the components, raising Am to 0.9 mag [93]. Objects with Am > 0.9 mag are not explainable as rotationally deformed single bodies and contact binaries are preferred. In the whole Solar system, very few objects have been found with such large photometric range. The main examples are Trojan (624) Hektor, which is believed to be a 150 km scale binary, 200 km main - belt asteroid (216) Kleopatra and ā€”260 km KBO 2001 QG298 [134]. The inferred abundance (admittedly from a single detection) of contact or very close binaries in the Kuiper belt is at least 10-20% [134].

To give a short summary, rotational studies of KBOs have revealed a number of interesting cases for rotational deformation (Varuna and 2003 EL61) and close or contact binaries (the best case remains 2001 QG298 but other KBOs in Region C of Fig. 37, like 2000 GN171, are candidates for contact binaries observed non-equatorially). The appearance of these examples in a still-small (Nā€”40) observational sample is evidence that rotationally deformed and contact-binary structures must be common in the Kuiper belt. Preliminary evidence suggests that the shape distributions of KBOs larger and smaller than 400 km diameter are not the same [86]. If confirmed by future work, this observation might find a natural explanation in terms of collisional effects at small sizes and self-gravity at larger sizes.

4.4 Kuiper Belt Physical Properties: Multiple Objects

About 20 examples of multiple KBOs have been reported as of early 2006 (many are not yet properly published, appearing only in electronic circulars). Multiple KBOs in Table 5 have been collected from [32] and [118] and from a few recent electronic publications. The objects are binaries except for Pluto (three satellites known) and 2003 EL61 (two satellites known), but this is no doubt an effect of observational selection against small, faint companions

Table 5. Multiple KBOs

Object

a [km]a

eb

i[deg]<=

Typed

0[arcsec]'

5 P[days]f

Amag

Pluto

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