Venus might not be top of the list for manned missions, but a flock of floating explorers - part drone, part blimp - for its atmosphere are are on the drawing board. Drone-maker Northrop Grumman wants to send its Venus Atmospheric Maneuverable Platform (VAMP) concept, a long-lived, maneuverable, semi-buoyant research platform that would soar through Venus' skies around 30-40 miles …