Geminid fireballs and the peculiar asteroid 3200 Phaethon

Abstract

Twelve long-enduring Geminid fireballs from the camera network in western Canada are studied in an attempt to infer properties of their parent object, 3200 Phaethon. The Geminids decelerate from entry velocities of 36 km sec −1 to lower s limit near 15 km sec −1 and disappear at heights above 38 km. No fragments larger than dust particles appear capable of surviving as meteorites. Dynamic masses are derived for the Geminids from observed decelerations using an assumed shape based on data from the recovered fragments of the Innisfree meteorite fall. The luminosity of each fireball is used to obtain a photometric mass, assuming a luminous efficiency, τ, that is independent of velocity. Since Geminids resist crumbling and fragmentation, the dynamic and photometric masses should agree. It is shown that for reasonable values of τ, between 0.02 and 0.06, the masses agree for particle densities between 0.7 and 1.3 g cm −3. A best value of ρ m = 1.0 with τ = 0.034 is suggested. Geminids commonly exhibit flickering in their light, interpreted as an oscillation of the particle in flight. This may indicate the development of a relatively thin meteoroid during ablation and, in this case, the upper limit for the density could be raised to about 1.7. Eleven other fireballs, including four known or suspected meteorites and two fragile members of the Taurid complex, were studied for comparison with the Geminids. The meteorites penetrate the atmosphere considerably deeper than the Geminids, which in turn survive to lower heights than the more fragile objects. The ablation coefficients, σ, are comparable for Geminids and the meteorites, although σ may become meaningless for the low velocities observed in some meteorite events, due to neglect of the energy loss due to deceleration. Classification parameters suggest the Geminids are not as “tough” as meteorites but are more cohesive than many fireballs. It is concluded that Geminid meteoroids have too low a density to associate them with meteorites or normal asteroids. Frequent exposure to intense solar radiation near perihelion (0.14 AU) may lead to a loss of volatiles and may produce relatively tough meteoroids that resemble the crust of a comet rather than its interior.