Jenni Virtanen

(42355) Typhon–Echidna: Scheduling observations for binary orbit determination

Abstract We describe a strategy for scheduling astrometric observations to minimize the number required to determine the mutual orbits of binary transneptunian systems. The method is illustrated by application to Hubble Space Telescope observations of (42355) Typhon–Echidna, revealing that Typhon and Echidna orbit one another with a period of 18.971 ± 0.006 days and a semimajor axis of 1628 ± 29 km , implying a system mass of ( 9.49 ± 0.52 ) × 10 17 kg . The eccentricity of the orbit is 0.526 ± 0.015 . Combined with a radiometric size determined from Spitzer Space Telescope data and the assumption that Typhon and Echidna both have the same albedo, we estimate that their radii are 76 −16 +14 and 42 −9 +8 km , respectively. These numbers give an average bulk density of only 0.44 −0.17 +0.44 g cm −3 , consistent with very low bulk densities recently reported for two other small transneptunian binaries.

Modeling collision probability for Earth-impactor 2008 TC3

Abstract We study the evolution of the Earth collision probability of asteroid 2008 TC 3 using a short observational arc and small numbers of observations. To assess impact probability, we use techniques that rely on the orbital-element probability density function characterized using both Markov-chain Monte-Carlo orbital ranging and Monte-Carlo ranging. First, we evaluate the orbital uncertainties for the object from the night of discovery onwards and examine the collapse of the orbital-element distributions in time. Second, we examine the sensitivity of the results to the assumed astrometric noise. Each of the orbits obtained from the MCMC ranging method is propagated into the future (within chosen time bounds of the expected impact), and the collision probability is calculated as a weighted fraction of the orbits leading to a collision from the Earth. We compare the results obtained with both methods.