Formation of asteroid pairs by rotational fission

Abstract

Pairs of asteroids sharing similar heliocentric orbits, but not bound together, were found recently. Backward integrations of their orbits indicated that they separated gently with low relative velocities, but did not provide additional insight into their formation mechanism. A previously hypothesized rotational fission process may explain their formation-critical predictions are that the mass ratios are less than about 0.2 and, as the mass ratio approaches this upper limit, the spin period of the larger body becomes long. Here we report photometric observations of a sample of asteroid pairs, revealing that the primaries of pairs with mass ratios much less than 0.2 rotate rapidly, near their critical fission frequency. As the mass ratio approaches 0.2, the primary period grows long. This occurs as the total energy of the system approaches zero, requiring the asteroid pair to extract an increasing fraction of energy from the primary's spin in order to escape. We do not find asteroid pairs with mass ratios larger than 0.2. Rotationally fissioned systems beyond this limit have insufficient energy to disrupt. We conclude that asteroid pairs are formed by the rotational fission of a parent asteroid into a proto-binary system, which subsequently disrupts under its own internal system dynamics soon after formation.

Effect of main belt perturbers on asteroid-pair age estimation

“…Photometric observations help solidify the estimate of absolute magnitude H, from which a body's dimension can be deduced, assuming albedo is estimated, and the light curves from several apparitions help derive the sense of rotation. The H value and sense of rotation heavily reduced uncertainties in age estimation in the case of the very young pair (6070) Rheinland -(54827) 2001 NQ 8 97 , which seem to have a similar age of ∼17 kyr (Pravec et al 2010). At the present time there are no younger pairs or clusters reported in the literature.…”

“…Information about the integrator used for this task is given in the paragraph below. The basic criteria are listed in Table 1, where a is the semimajor axis, e the eccentricity, i the inclination, Ω the 28 consists of Mars-crossers for which proper elements were unavailable, though its age was estimated to be ≈32 kyr (Pravec et al 2010). The second step in the discovery process is backward integration, where we used the SOLEX package (Vitagliano 1997) 3 .…”

“…It can also indicate when they might have been sufficiently close to have been under each other's gravitational influence (within the Hill sphere). Dozens of such asteroid pairs have been found (Vokrouhlický & Nesvorný 2008;Pravec & Vokrouhlický 2009;Pravec et al 2010), as have a few young clusters (e.g. Nesvorný et al 2002Nesvorný et al , 2003Nesvorný & Bottke 2004;.…”

The young Datura asteroid family

“…Instead, we are content with somewhat weaker condition, notably bringing the asteroids into a distance comparable with the Hill sphere R Hill of the parent body. Using again R Hill 90 a D par in kilometers from Pravec et al (2010), we estimate R Hill 1700 km. Figure 13 shows results of our first method based on convergence of secular angles.…”

“…This is because we know the initial ejection velocities of fragments should be of the order of escape velocity V esc from the parent body of the family. Using V esc 0.6 D par in meters per second from Pravec et al (2010;supplementary materials), and estimating D par 8.3 km (consider (1270) Datura's estimated size from Sect. 1 and results in Sect.…”