(42355) Typhon-Echidna: Scheduling Observations for Binary Orbit Determination

TL;DR

This paper presents a scheduling strategy for astrometric observations to efficiently determine the orbits of binary transneptunian systems, demonstrated on Hubble Space Telescope data of Typhon-Echidna.

Contribution

It introduces a novel observation scheduling method that reduces the number of observations needed for binary orbit determination.

Findings

Typhon and Echidna orbit each other with an 18.971-day period.

The system's mass is approximately 9.49 x 10^17 kg.

The estimated bulk density is very low, around 0.44 g/cm^3.

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) x 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 +14/-16 and 42 +8/-9 km, respectively. These numbers give an average bulk density of only 0.44 +0.44/-0.17 g cm^-3, consistent with very low bulk densities recently reported for two other small transneptunian binaries.