Search

We report on the ephemeris development for Menoetius, the satellite of Patroclus. Our data set consisted of ground–based and Hubble Space Telescope relative astrometry, as well as 42 lightcurves from the mutual events seasons in 2007, 2012, and 2017/2018. Our dynamical model included the effects of oblate, nonspherical shapes of the components, and we assumed that Menoetius contained ∼22% of the system’s mass. We numerically integrated the equations of motion and obtained a set of dynamical parameters that fit the data. We report the fit results in terms of residuals, state vectors, orbital elements and their 1 σ uncertainties. The mean osculating semimajor axis is a = 692.5 ± 4.0 km, the mean eccentricity is e = 0.004 ± 0.004, and the International Celestial Reference Frame pole direction in R.A. and decl. is R. A. = 178.0 ± 0.5 deg, δ = −74.1 ± 0.2 deg. We determined the siderial orbital period of P = 4.282753 ± 0.000023 days. The fit yielded the system GM = 0.0950 ± 0.0012 km ^3 s ^−2 , which, in combination with the system volume determined from the stellar occultation and the assumed volume uncertainty of 20%, suggests a system bulk density of 1.05 ± 0.21 g cm ^−3 (1 σ ). The next season of mutual events starts in February of 2024 and lasts until January of 2025. The Patroclus system is in opposition for the observers on Earth in late September and is suitable for observations of the mutual events with an edge-on geometry in October.

We report on ephemerides for the satellites of Pluto based on the large set of astrometric measurements. Our orbit fit yielded the following masses with 1 σ uncertainties: GM _Pluto = 869.3 ± 0.4 km ^3 s ^−2 , GM _Charon = 106.1 ± 0.3 km ^3 s ^−2 , GM _Nix = 1.50 ± 0.52 × 10 ^−3 km ^3 s ^−2 , GM _Hydra = 2.01 ± 0.27 × 10 ^−3 km ^3 s ^−2 , corresponding to the densities of ρ _Pluto = 1.853 ± 0.004 g cm ^−3 , ρ _Charon = 1.705 ± 0.006 g cm ^−3 , ρ _Nix = 0.88 ± 0.31 g cm ^−3 , and ρ _Hydra = 1.21 ± 0.19 g cm ^−3 . Masses of Kerberos and Styx remain unconstrained, and it is unlikely that we will be able to measure them even if we obtain abundant 1 mas precision astrometry during the next 20 yr. We summarize the results of orbit integration in terms of osculating and precessing ellipse model mean elements. All satellites reside in near-circular orbits, and Kerberos and Styx have 0.4 deg and 0.3 deg inclinations with respect to Charon’s orbit plane. The nodal regression periods for Kerberos and Hydra are ∼9 and ∼14 yr respectively. We found that Charon’s orbit pole can be approximated as: R.A. = 133.0073 + 0.0036 T deg, and decl. = –6.2449 + 2.5 × 10 ^−5 T deg, where T is Julian centuries from the epoch J2000, based on 5000 yr of orbit integration.

We report radar, photometric, and visible-wavelength spectrophotometry observations of NEA 2018 EB obtained in 2018. The radar campaign started at Goldstone (8560 MHz, 3.5 cm) on April 7, and it was followed by more extensive observations from October 5 to 9 by both Arecibo (2380 MHz, 12.6 cm) and Goldstone. 2018 EB was observed optically on April 5, 8, and 9 and again on October 18. Spectrophotometry was obtained on October 19 with the SOAR telescope, and the data suggest that 2018 EB is an Xk-class object. The echo power spectra and delay-Doppler radar images revealed that 2018 EB is a binary system. Radar images constrained the satellite's diameter to ${0.15}_{-0.05}^{+0.02}$ km, but the data were not sufficient for shape modeling. Shape modeling of lightcurves and radar data yielded an oblate primary with an effective diameter D = 0.30 ± 0.04 km and a sidereal rotation period of ${4.3}_{-0.5}^{+0.6}$ hr. Measurements of delay-Doppler separations between the centers of mass of the primary and the satellite, along with the timing of a radar eclipse observed on October 9, resulted in an orbit fit for the satellite with a semimajor axis of ${0.50}_{-0.01}^{+0.04}$ km, an eccentricity of 0.15 ± 0.04, a period of ${16.85}_{-0.26}^{+0.33}$ hr, and an orbit pole constrained to the ecliptic longitudes and latitudes of $\lambda ={93}_{-{43}^{^\circ }}^{+{27}^{^\circ }}$ and $\beta ={48}_{-{18}^{^\circ }}^{+{7}^{^\circ }}$ . The system mass was estimated to be ${2.03}_{-0.08}^{+0.52}\times {10}^{10}$ kg, which yielded a bulk density of ${1.4}_{-0.5}^{+0.6}$ g cm ^−3 . Our analysis suggests that 2018 EB has a low optical albedo of p _V = 0.028 ± 0.016 and a relatively high radar albedo of η _OC = 0.29 ± 0.11 at Arecibo and η = 0.22 ± 0.10 at Goldstone.