  We report observations of the Jupiter Trojan asteroid (3548) Eurybates and
its satellite Queta with the Hubble Space Telescope and use these observations
to perform an orbital fit to the system. Queta orbits Eurybates with a
semimajor axis of $2350\pm11$ km at a period of $82.46\pm0.06$ days and an
eccentricity of $0.125\pm0.009$. From this orbit we derive a mass of Eurybates
of $1.51\pm0.03 \times 10^{17}$ kg, corresponding to an estimated density of
$1.1\pm0.3$ g cm$^{-3}$, broadly consistent with densities measured for other
Trojans, C-type asteroids in the outer main asteroid belt, and small icy
objects from the Kuiper belt. Eurybates is the parent body of the only major
collisional family among the Jupiter Trojans; its low density suggests that it
is a typical member of the Trojan population. Detailed study of this system in
2027 with the Lucy spacecraft flyby should allow significant insight into
collisional processes among what appear to be the icy bodies of the Trojan
belt.
