Rotational breakup as the origin of small binary asteroids

Abstract

Asteroids with satellites are observed throughout the Solar System, from subkilometre near-Earth asteroid pairs to systems of large and distant bodies in the Kuiper belt. The smallest and closest systems are found among the near-Earth and small inner main-belt asteroids, which typically have rapidly rotating primaries and close secondaries on circular orbits. About 15 per cent of near-Earth and main-belt asteroids with diameters under 10 km have satellites. The mechanism that forms such similar binaries in these two dynamically different populations was hitherto unclear. Here we show that these binaries are created by the slow spinup of a 'rubble pile' asteroid by means of the thermal YORP (Yarkovsky-O'Keefe-Radzievskii-Paddack) effect. We find that mass shed from the equator of a critically spinning body accretes into a satellite if the material is collisionally dissipative and the primary maintains a low equatorial elongation. The satellite forms mostly from material originating near the primary's surface and enters into a close, low-eccentricity orbit. The properties of binaries produced by our model match those currently observed in the small near-Earth and main-belt asteroid populations, including 1999 KW(4) (refs 3, 4).

MarcoPolo-R: Near-Earth Asteroid sample return mission selected for the assessment study phase of the ESA program cosmic vision

“…In particular, rotational disruption of a rubble pile NEA as a result of spin-up above the fission threshold due to the YORP effect (a thermal effect which can slowly increase or decrease the rotation rate of irregular objects) has been shown to be a mechanism that can produce binary asteroids with properties that are consistent with the observed ones (shape of the primary, size ratio of the primary to the secondary and circular equatorial secondary orbit, [16]). Other fission scenarios have been proposed that imply different physical properties of the binary [17].…”

“…Binary formation scenarios therefore place constraints on, and implications for, the internal structure of these objects. One of the most important implications of the model by Walsh et al [16] is that the pole of the primary should be composed of fresh material that was originally buried at some depth in the progenitor. In effect, in this model, when the progenitor is spun up, the material of the pole migrates to the equator and when the centrifugal force exceeds the gravity of the body, this material escapes from the surface to form the secondary (see Figure 5).…”

“…Binary objects represent about 16% of both the NEA and main belt populations [15] and their formation mechanism is still a matter of debate, although several scenarios have been proposed to explain their existence [16,17]. In particular, rotational disruption of a rubble pile NEA as a result of spin-up above the fission threshold due to the YORP effect (a thermal effect which can slowly increase or decrease the rotation rate of irregular objects) has been shown to be a mechanism that can produce binary asteroids with properties that are consistent with the observed ones (shape of the primary, size ratio of the primary to the secondary and circular equatorial secondary orbit, [16]).…”

Earth-viewing satellite perspectives on the Chelyabinsk meteor event

“…If used in synergy with other warning and decision aid systems, including resources designed for this purpose, these satellites could play a complementary role in more rapidly directing our attention and response with regard to possible follow-up threats (e.g., in the case of binary or triple asteroid systems, which are thought to account for ∼15% of the near-Earth asteroid population, ref. 23). …”

MarcoPolo-R near earth asteroid sample return mission

“…Moreover, one can better understand the potential outcome of YORP spin-up (the YORP effect is a thermal effect that can slowly increase or decrease the rotation rate of irregular objects) and test models of formation of a system that represents a non-negligible fraction (15%) of asteroid populations. Relevant to MarcoPolo-R objectives, one of the most important implications of the model of binary formation by YORP spin-up [28] is that the pole of the primary should be composed of fresh material that was originally at some depth in the progenitor. In effect, when spun-up, the material of the pole of the progenitor migrates to the equator and when the centrifugal force exceeds the gravity of the body, this material escapes from the surface to form the secondary.…”