The Active Asteroids

Abstract

Some asteroids eject dust, unexpectedly producing transient, comet-like comae and tails. First ascribed to the sublimation of near-surface water ice, mass-losing asteroids (also called "main-belt comets") can in fact be driven by a surprising diversity of mechanisms. In this paper, we consider 11 dynamical asteroids losing mass, in nine of which the ejected material is spatially resolved. We address mechanisms for producing mass loss including rotational instability, impact ejection, electrostatic repulsion, radiation pressure sweeping, dehydration stresses, and thermal fracture, in addition to the sublimation of ice. In two objects (133P and 238P) the repetitive nature of the observed activity leaves ice sublimation as the only reasonable explanation, while in a third ((596) Scheila), a recent impact is the cause. Another impact may account for activity in P/2010 A2, but this tiny object can also be explained as having shed mass after reaching rotational instability. Mass loss from (3200) Phaethon is probably due to cracking or dehydration at extreme (∼1000 K) perihelion temperatures, perhaps aided by radiation pressure sweeping. For the other bodies, the mass-loss mechanisms remain unidentified, pending the acquisition of more and better data. While the active asteroid sample size remains small, the evidence for an astonishing diversity of mass-loss processes in these bodies is clear.

Asteroid flux towards circumprimary habitable zones in binary star systems

“…The main mechanisms that can induce a relevant mass loss for active (comet-like) or inactive asteroids are ice sublimation and impact ejection (Jewitt 2012). The latter process occurs when smaller asteroids impact larger ones 6 .…”

“…Therefore, we used an averaged albedo A 1 = A 2 =Ā = 0.05; − R is the distance to the star (primary or secondary) expressed in au; − θ is the angle between the incident light and the normal to the asteroid's surface. According to Jewitt (2012),ṁ is weaker for an isothermal surface than at the subsolar point of a nonrotating body. Again, to maximizeṁ, we consider the subsolar case with θ = 0 • ; − is the emissivity of the surface, ∼ 0.9; − σ = 5.67 × 10 −8 W m −2 K −4 is the Boltzmann constant; − T is the equilibrium temperature at the surface expressed in K; − L is the latent heat of sublimation in J kg −1 ; −ṁ is the surface mass loss rate in kg m −2 s −1 .…”

Rotational Properties of the Extraordinary Multi-tailed Asteroid P/2013 P5

“…Below, Figure 4 shows nine of the eleven active asteroids that have been discovered as of March 2012 (Jewitt, 2012). All of these objects, including P/2013 P5, resemble comets in their appearance, making the asteroid-comet categorization more complex.…”

“…On the other hand, the concept of asteroids having a comet-like appearance was puzzling to astronomers who were unsure of how an asteroid, with a limited amount of volatile material, would begin to generate mass loss. Jewitt (2012) describes the eleven current explanations for how asteroids can lose mass. These explanations fall under three main processes related to collisional events, near-surface ice sublimation, and rotational fission.…”

“…Hsieh & Jewitt, 2006) or Active Asteroids (Jewitt, 2012). These are objects that exhibit both asteroid and comet-like properties, typically with a circular, asteroid-like orbit and a comet-like dust tail.…”