Hubble Space Telescope Observations of Active Asteroid 324P/La Sagra
Abstract
Hubble Space Telescope observations of active asteroid 324P/La Sagra near perihelion show continued mass loss consistent with the sublimation of near-surface ice. Isophotes of the coma measured from a vantage point below the orbital plane are best matched by steady emission of particles having a nominal size a 100 μm. The inferred rate of mass loss, dMd/dt 0.2 kg s-1, can be supplied by sublimation of water ice in thermal equilibrium with sunlight from an area as small as 930 m2, corresponding to about 0.2\% of the nucleus surface. Observations taken from a vantage point only 0.6~from the orbital plane of 324P set a limit to the velocity of ejection of dust in the direction perpendicular to the plane, V < 1 m s-1. Short-term photometric variations of the near-nucleus region, if related to rotation of the underlying nucleus, rule out periods 3.8 hr and suggest that rotation probably does not play a central role in driving the observed mass loss. We estimate that, in the previous orbit, 324P lost about 4×107 kg in dust particles, corresponding to 6×10-5 of the mass of a 550 m spherical nucleus of assumed density = 1000 kg m-3. If continued, mass loss at this rate would limit the lifetime of 324P to 1.6×104 orbits (about 105 yr). To survive for the 100 Myr to 400 Myr timescales corresponding, respectively, to dynamical and collisional stability requires a duty cycle 2× 10-4 fd 8× 10-4. Unless its time in orbit is over-estimated by many orders of magnitude, 324P is revealed as a briefly-active member of a vast population of otherwise dormant ice-containing asteroids.