News stories make it sound like Rosetta is orbiting the comet. But presumably the comet's gravity is negligible, which means that orbiting it would require continuous acceleration (and therefore continuous use of energy) in order for Rosetta's motion to conform to a circle/ellipse, rather than simply moving in a straight line as it would naturally do without any input of energy. Is Rosetta in fact simply moving alongside the comet in a straight line? Or am I missing something? (Afterthought 5 mins later) Is Rosetta matching the comet's orbit around the sun?