Creede Dark Skies: Himalia: a small and distant moon of Jupiter

The four Galilean Moons of Jupiter are the most familiar planetary moons in the solar system. They are bright enough to be visible to the unaided eye, if not for being lost in the glare of Jupiter. They are an easy target for binoculars. Ganymede, the largest moon, is larger than the planet Mercury and about 78% the diameter of Mars.

From left to right: Ganymede, Io, Europa, Callisto (11 Jan).

Jupiter currently has 95 moons that are officially recognized, but all of them are much smaller than the big four. The next two largest are Amalthea and Himalia, with diameters on the order of 100–200 km. These two moons are also very faint, with visual magnitudes dimmer than magnitude 14.

Amalthea orbits too close to Jupiter to be picked up with small telescopes. Himalia, on the other hand, is in a very wide orbit and therefore potentially accessible. The problem with finding Himalia is that its orbit drifts rapidly enough over time that software programs with fixed orbital elements will not provide accurate positions. This problem was circumvented by using the Horizons Web Application of the Jet Propulsion Laboratory to generate an ephemeris for Himalia and Ganymede. The location of Ganymede was used to calculate an offset between the JPL coordinates and the coordinate system used by SkySafari. The position for Himalia predicted by SkySafari was determined to be off by about 6.8 arc minutes. (SkySafari is an app for iOS and MacOS that is used to point the telescope and generate star charts.)

The following images were obtained using the Astro-Tech AT72EDII refractor at about 10:30pm on 11 January.

Overexposed Jupiter. The location of Himalia is circled. 2.4-deg wide FOV.

Himalia (circled). 0.5-deg FOV. 30-sec exposure, ISO 1600.

Comparison of the image of Himalia to other stars in the field indicates that it is shining at about magnitude 14.5.

The sky brightness was measured as 19.53 mpsas. This is a very mediocre value for a moonless night in Santa Fe. Some other images obtained from the same session: