NoteThe upcoming mutual event season for the Patroclus–Menoetius Trojan binary
Introduction
As the first known binary Jupiter Trojan, Patroclus and Menoetius provided the first opportunity to directly measure the mass and thus bulk density of objects from this population. Marchis et al., 2005, Marchis et al., 2006 determined their mutual orbit to have a period of 4.283 ± 0.004 days and a semimajor axis of 680 ± 20 km, leading to a system mass of (1.36 ± 0.11) × 1018 kg. When the plane of their mutual orbit is aligned with the direction to the Sun or to an observer, Patroclus and Menoetius take turns eclipsing or occulting one another. Such an alignment occurs during mutual event seasons, twice every ∼12 year orbit around the Sun. Spitzer Space Telescope thermal observations of Patroclus – Menoetius mutual events enabled Mueller et al. (2010) to determine that the surface thermal inertia is relatively low, consistent with mature regolith. The thermal infrared observations also enabled them to better constrain the sizes of the objects, leading to a mean bulk density of 1.08 ± 0.33 g cm−3, with the density uncertainty being dominated by the uncertainty in the volumes of the two bodies.
The first spacecraft exploration of the Jupiter Trojans is to be done by NASA's Lucy probe, planned for launch in 2021. After flybys of four objects in the L4 (leading) Trojan cloud, Lucy will fly past Patroclus and Menoetius in the L5 (trailing) cloud in 2033. This pending visit motivates additional observational studies of the Patroclus – Menoetius system in support of detailed encounter planning. An opportunity to refine the orbit as well as other properties of the system presents itself with a new mutual event season (see http://fredvachier.free.fr/binaries/phemu.info.php). However, enough time has passed that the uncertainties associated with the Marchis et al. orbit solution are too large to permit precise predictions for the upcoming mutual events. Additionally, it is unknown whether the orbit could be changing over time, due to some as-yet unaccounted for perturbation.
Section snippets
Observations
To obtain a new, independent mutual orbit for Patroclus and Menoetius ahead of the upcoming mutual event season, we observed the system at five epochs during May-June 2017, using the Wide Field Camera 3 (WFC3) ultraviolet and visible (UVIS) camera (e.g., Dressel et al., 2012) of the Hubble Space Telescope (HST). Our observing sequence for each of the five epochs consisted of 16 dithered frames, the majority of them using the broadband F555W filter (the number in the filter name indicates the
Analysis
The positions of the system components in detector coordinates were obtained via familiar PSF-fitting methods using the Tiny Tim PSF model that captures many of the idiosyncrasies of the HST optical system (Krist and Hook, 2004, Krist et al., 2011). We modeled two point sources to simultaneously fit for a pair of x, y positions and fluxes, six free parameters in all. The nearly negligible sky background was removed before fitting by subtracting the mean of the sky.
The first step was to find a
Mutual event predictions
We predicted mutual event times from our circular orbit solution via a simple model that collapses the problem to a two-dimensional search for intersections between circles on the instantaneous sky plane as seen from Earth. A circle of radius 49 km, representing Patroclus, is fixed at the origin. A slightly smaller circle of radius 45 km representing Menoetius is placed at the satellite's position relative to Patroclus as seen from Earth, based on our orbit solution. These sizes are the polar
Summary
We report new, independently-determined Keplerian orbit solutions for the Patroclus – Menoetius binary system that is consistent with the Marchis et al. (2006) orbit solution, but with smaller uncertainties. We use our new orbit to predict timing of mutual events observable from Earth during the coming mutual event season that runs from fall 2017 through summer 2019. Observations of the mutual events can be exploited to learn more about the system ahead of its exploration by the Lucy mission in
Acknowledgments
This work is based in part on NASA/ESA Hubble Space Telescope program 14928. Support for this program was provided by NASA through a grant from the Space Telescope Science Institute (STScI), operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. Additional data were obtained at the W.M. Keck Observatory, a scientific partnership of the California Institute of Technology, the University of California, and NASA made possible by the generous
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