Amalthea, or Jupiter V

Amalthea was the fifth satellite of Jupiter to be discovered and so is also named Jupiter V. It was discovered by Edward E. Barnard in 1892; it holds the distinction of being the last satellite to be discovered visually as opposed to by imaging. Amalthea is actually quite bright, at about 14th magnitude, and would be an easy object were it not in such a close orbit around Jupiter. The maximum distance from the satellite to the planet is around 30 arc-seconds and the satellite is usually swamped by the glow from the 4 million times brighter Jupiter

The 21 sub-images, taken over a period of 11 minutes, were stacked on the mean motion of Amalthea. Consequently, the satellite should appear as a circular point and the stars trailed into short lines by their relative motion during the period of observation. At the time, Amalthea moved a little over three arc-seconds in this time.

In the raw images and their stacks, Amalthea is only just barely visible in the background glow and then only if one knows precisely where to look. To remove much of the background and so increase the contrast (though at the cost of introducing artefacts) AstroImageJ was used for its rolling-ball background subtraction feature with a ball diameter of 30 pixels. Amalthea is marked in the resultant image shown here as the circular dot whereas the stars appear as dark lines, generally at the start of one of the artefacts mentioned above. The Galilean satellites are also indicated; from left to right they are Ganymede, Europa, Io and Callisto.