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Another moon of Saturn has been imaged in detail by the Cassini spacecraft.Orbiting Saturn since 2004, therobotic Cassinigot its closest look yet at Saturn’s small moonPrometheus last week.Visible above in an unprocessed image from 36,000 kilometers away,Prometheus’ 100-km long surface was revealed to have aninteresting system of bulges, ridges, and craters. These features, together with the moon’s oblong shape and high reflectivity,are now being studied to help better understand the history ofPrometheus and Saturn’s rings.Prometheus is one of the fewshepherd satellites known, as its gravity, along with its companion moonPandora, confines many smaller ice chucks intoSaturn’s F Ring. Cassini’s next majortargeted flyby is of the moonRhea on March 2.

Charles Messier described the 88th entry in his 18th centurycatalog ofNebulaeand Star Clusters as a spiral nebula without stars.Of course thegorgeous M88is now understood tobe a galaxy full of stars, gas, and dust, not unlikeour own Milky Way.In fact, M88 is one of the brightest galaxies in theVirgoGalaxy Clustersome 50 million light-yearsaway.M88′s beautiful spiral arms are easy to trace in thiscolorfulcosmic portait.The arms are lined with young bluestar clusters, pinkstar-forming regions,and obscuring dust lanesextending from a yellowishcore dominated by an older population of stars.Spiral galaxy M88 spans over 100,000 light-years.

Most galaxies have a single nucleus — does this galaxy have four? The strange answer leads astronomersto conclude that the nucleus of the surrounding galaxy is not even visible in this image. The central cloverleaf is rather light emitted from a background quasar. The gravitational field of the visible foreground galaxy breaks lightfrom this distant quasarinto four distinct images. The quasar must be properly aligned behind the center of a massive galaxy for a mirage like this to be evident. The general effect is known as gravitational lensing, and this specific case is known as the Einstein Cross. Stranger still, the images of the Einstein Cross vary in relative brightness, enhanced occasionally by the additional gravitational microlensingeffect of specific stars in the foreground galaxy.