The satellites visible are (from left to right) Europa, Io, Callisto, and Ganymede. Callisto appears fainter than the others because its surface is darker (not because its size is smaller). The satellites are approximately arrayed in a line as they orbit in the plane of Jupiter's equator, which we view edge on.
Processing: This image was made by Calvin student Chris Walker 6 February 2002, pulling out all the stops to get the sharpest image possible. The final image was produced from a series of very short exposures made through clear, blue, green, and red filters (with individual exposure times of 0.1, 0.3, 0.2, and 0.25 seconds, respectively) taken between 7:44 and 8:16 pm EST. The balancing act required is to have enough exposure time to gather lots of light, but not so much as to allow atmospheric variability to blur the image. First, all images were made when Jupiter was fairly high in the sky (above 55 degrees elevation), to minimize the amount of atmosphere through which the light travels. Second, a large number of short exposures were made, the majority of which showed significant atmospheric blurring. However, only those made at moments of steadiness (those that were least blurred) were combined to make the final image. Third, unsharp masking was applied to the final image in each filter to enhance image sharpness. Finally, in combining the colors, the clear filter image (which was naturally of the highest quality, having more light and shorter exposures) was used to determine overall intensity while the remaining images were used only to fix color balance.
Compare the final result here with our first color image from January 2001, which simply combined one exposure each through red, green, and blue filters.
As the satellites were moving over the 32 minutes of the Jupiter observations, only the satellite data from one clear image was added to the final image, with the brightness scale enhanced relative to the planet. For a good view of the satellites in motion, view our Jupiter movie taken 25 January 2002.
Orientation and scale: North is up and East is to the left. The equatorial diameter of the planet is 142,000 km, and subtends 45 arcseconds in this image.