the origin and evolution of the binary orbital parame- ters, and therefore may affect the statistics of the orbital parameters in our sample. 

Another important bias is that systems with low in- clinations 4 present nearly edge-on orbits with respect to Earth-based observations, while systems with high rela- tive inclinations are usually seen face-on. Since binaries are discovered when the components are significantly sep- arated on-the-sky, there is a greater likelihood for low inclination secondaries to be unresolvable when observed at a single random epoch. This bias is reduced as the projected semi-major axis grows, but remains significant even at a few times the resolution limit. 

While some binary asteroids have been imaged by HST, ground-based adaptive optics has been employed more often for these brighter systems than their trans- Neptunian counterparts. The much smaller sizes of typ- ical asteroids is partly offset by their increased bright- ness and proximity. Discovery of binaries photometri- cally through double-periodic light curves and/or mutual events is common for near-Earth asteroids (Pravec et al. 2006, note, however that these are not included in our analysis, and are mentioned here for completeness). Other methods are radar observations that often reveal near-Earth binary asteroids (not discussed here), and stellar occultations that can reveal the presence of main belt binary asteroids (e.g. Descamps et al. 2007). The- oretically, the radar and occultations techniques have fewer observational biases than the other more common techniques, but their application is severely limited. 

Binary asteroids are also clearly subject to detection bias: objects with smaller satellites are more difficult to observe. However, since asteroids are searched for bina- ries using ground-based facilities, there is a greater pos- sibility of searching for companions in more than a sin- gle snapshot. Furthermore, the geocentric orientations of these systems change much more rapidly than for es- sentially fixed KBO orbits ( ∼ 60 $^{◦}$yr− 1 for asteroids, vs. ∼ 2 $^{◦}$yr− 1 for KBOs). Therefore, the eccentricity and inclination biases are not as strong as in the Kuiper belt. In binaries discovered through mutual events, there is an obvious bias towards edge-on systems, though "edge-on" can probe a wide range of inclinations. 

Several different processes affect the mutual orbits of BMPs. Some are related to their initial formation and others to their later evolution either as isolated systems, or due to the effects of external perturbations and en- counters with other objects. The studies of these pro- cesses have been focused on a specific type of BMPs such as binary asteroids or binary TNOs. However, all of these suggested processes could in principle be relevant for the formation/evolution of BMPs both close (binary aster- oids) and far (binary TNOs) from the sun. 

The various suggested mechanisms for the forma- tion of BMPs (see refs. Astakhov et al. 2005; 

## 2.2.2. Binary asteroids 

## 3. DISCUSSION 

Richardson & Walsh 2006; Noll et al. 2008, for some overviews) predict different initial orbital configurations. These include the following: 

• Smashed target satellites (SMATS; Weidenschilling 2002; Durda et al. 2004, 2010): Low eccentricity distribution expected from collisionally formed satellites orbiting the main collision remnant body (which typically form as close binaries and are likely to be affected by tides). 

• Escape ejecta binaries (EEBs; Durda et al. 2004, 2010): Intermediate eccentricities from bound ejecta pairs ejected following a collision of two larger bodies. 

• Exchanged binaries (Funato et al. 2004): Typically high eccentricities ( > 0 . 8 ) for high mass ratio bi- naries formed through exchanges. 

• Dynamical friction and chaos assisted capture (CAC) binaries (Goldreich et al. 2002; Lee et al. 2007): Intermediate eccentricities ( 0 . 2 < e < 0 . 8 ) for binaries formed through chaos assisted capture of satellites. 

Unfortunately, only a few studies explored aspects of the inclination distribution of BMPs (Astakhov et al. 2005; Nazzario et al. 2007; Schlichting & Sari 2008; Perets & Naoz 2009). 

After their formation, BMPs can be affected by several processes which can change their orbits. Tidal effect are most important when the BMPs components approach each other at a close distance. These effects couple the orbital evolution of the BMP to the spin of the BMPs components, and the tides raised on the objects serve to dissipate the total angular momentum of the system. Tidal effects can also excite and enlarge the eccentricity and inclination of a given BMP (e.g. Goldreich & Soter 1966). At long enough timescales (depending on spe- cific configuration), however, tidally evolved systems are expected to relax into more circularized configuration, possibly locked configurations and even mergers. Such effects are thought to produce the period-eccentricity dis- tributions of (close) binary systems such as stellar bi- naries and planetary systems, and are likely to play a similar role in BMP systems (Mazeh 2008). 

Another evolutionary process is the Kozai-Lidov (Kozai 1962; Lidov 1962) mechanism, which is the ef- fect of a secular perturbation from a third object (in a triple system, i.e. the Sun serves as the third compan- ion for BMPs) on the (bound) binary system. It could lead to large (order unity) periodic oscillations (Kozai cycles) in the eccentricity and inclination, i.e. it could both raise and lower the inclinations and eccentricities of a system. Note, however, that such a process is ef- fective only for systems with initially high inclinations (( 40 ◦ > i$_{m}$ < 140 $^{◦}$; with a somewhat wider inclination range for initially eccentric systems). 

The combined effects of the Kozai mechanism in ad- dition to tidal friction, (Kozai cycles and tidal friction; KCTF Mazeh & Shaham 1979; Kiseleva et al. 1998), can change the orbital parameters of the BMPs, and re- duce both the eccentricity and separation of the BMPs 

