Birds of a feather

YOU'VE seen it on “Star Wars”—a fleet of spacecraft cruising in perfect formation through icy interstellar realms, precisely controlled by noiseless on-board computers. Now it could become real, in the form of the European Space Agency (ESA) Cluster II mission. When complete (two of its component satellites were launched from Baikonur cosmodrome in Kazakhstan on July 16th, and two more are due to go up on August 9th) its component craft will manoeuvre themselves into an orbit about a third of the way between the earth and the moon. They will then fly in formation at the four corners of a tetrahedron with edges several hundred kilometres long. Barring disasters, this will mean lift-off for an entirely new concept in space science.