Asteroid pairs: A complex picture

Abstract

We studied a sample of 93 asteroid pairs, i.e., pairs of genetically related asteroids that are on highly similar heliocentric orbits. We estimated times elapsed since separation of pair members (i.e., pair age) that are between 7 × 10 3 yr and a few 10 6 yr. With photometric observations, we derived the rotation periods P 1 for all the primaries (i.e., the larger members of asteroid pairs) and a sample of secondaries (the smaller pair members). We derived the absolute magnitude differences of the studied asteroid pairs that provide their mass ratios q. For a part of the studied pairs, we refined their WISE geometric albedos and collected or estimated their taxonomic classifications. For 17 asteroid pairs, we also determined their pole positions. In two pairs where we obtained the spin poles for both pair components, we saw the same sense of rotation for both components and constrained the angles between their original spin vectors at the time of their separation. We found that the primaries of 13 asteroid pairs in our sample are actually binary or triple systems, i.e., they have one or two bound, orbiting secondaries (satellites). As a by-product, we found also 3 new young asteroid clusters (each of them consisting of three known asteroids on highly similar heliocentric orbits). We compared the obtained asteroid pair data with theoretical predictions and discussed their implications. We found that 86 of the 93 studied asteroid pairs follow the trend of primary rotation period vs mass ratio that was found by Pravec et al. (2010). Of the 7 outliers, 3 appear insignificant (may be due to our uncertain or incomplete knowledge of the three pairs), but 4 are high mass ratio pairs that were unpredicted by the theory of asteroid pair formation by rotational fission. We discuss a (remotely) possible way that they could be created by rotational fission of flattened parent bodies followed by re-shaping of the formed components. The 13 asteroid pairs with binary primaries are particularly interesting systems that place important constraints on formation and evolution of asteroid pairs. We present two hypotheses for their formation: The asteroid pairs having both bound and unbound secondaries could be "failed asteroid clusters", or they could be formed by a cascade primary spin fission process. Further studies are needed to reveal which of these two hypotheses for formation of the paired binary systems is real.

A common origin for dynamically associated near-Earth asteroid pairs

“…The combination of rotational properties and inferred relative mass ratio of pairs in the Main Belt are largely consistent with formation via rotational fission (Pravec et al, 2010). Similarity in the spin axis orientation of pair components also supports this claim (Polishook, 2014;Vokrouhlický et al, 2017;Pravec et al, 2019). However, the young pair 3749 Balam and 2009 BR60 in the Main Belt suggest that this process is indeed complicated -Balam is also known to host an inner satellite on a nearly circular orbit as well as a more distant, more eccentric outer satellite (Marchis et al, 2008;Vokrouhlický, 2009).…”

“…No attempts were made to correct for phase angle effects such as a change in lightcurve amplitude. Again more data are needed to diagnose the possibility of NPA rotation, which would make EE7 the first known asteroid pair primary in a NPA state (Pravec et al, 2019).…”

Q-type asteroids: Possibility of non-fresh weathered surfaces

“…A few km Q-type asteroids were found also among young asteroids involved in asteroid pairs in the main belt (Polishook et al 2014;Pravec et al 2019). Solar radiation pressure and electrostatic force do not affect small particle exclusion on these km-sized non near-Earth asteroids.…”

“…Solar radiation pressure and electrostatic force do not affect small particle exclusion on these km-sized non near-Earth asteroids. However, since these asteroids were temporarily binary systems (before the secondaries were ejected; see Pravec et al 2019, and references therein), three mechanisms can be considered. One possibility is that radiation pressure and electrostatic force are expected to work effectively to particle release on the binary asteroids because centrifugal force and gravity are expected to be nearly balanced on the binary formed by the rotation fission.…”

“…Delbo et al (2014) proposed that refreshed regoliths were formed by thermal fatigue induced by diurnal temperature variations. However, the time scale required for these formation models is more than several million years except three asteroid pairs in the main belt: 6070 Rheinland -54827 Kurpfalz, 51609 2001HZ32 -322672 1999TE221, and 19289 1996HY12 -278067 2006 to have probable ages of 16 Kyr, ∼540 kyr, and >630 kyr, respectively (Polishook et al 2014;Pravec et al 2019), and if the time scale of space weathering is around thousands years, it is not so easy to explain the surface formation of Q-type asteroids with these models.…”