produce any useful results, its the kind of experiment that can only ever really produce a definitive positive and not a definitive negative. Plus that ten years is a very long time in physics and these questions may well all be resolved before it even launches. The way I see it, all this stuff is going to get done - someday. In choosing a priority, we should not just ask, "which is the most worthy mission," but "which do we need to do NOW rather than later?" I feel the IXO falls down on this one. Yes, studying the fringes of black holes would refine our understanding of some very rare and intriguing situations. It might even help develop relativity (or its successor). But it doesn't really seem to matter whether we do it in 2020 or 2080. LISA is much more exciting to me. Its primary mission is similar - study new situations to discover more about the development of the universe, and how space works in general. However unlike IXO, it holds out the hope of seeing something genuinely new and unexpected. It's not just a more sensitive version of last year's gadget: it would be a completely new way of studying space. However EJSM/Laplace is my standout, for the reasons in my opening paragraph. It would still explore a place we'd never seen (just one rather closer to home). It would still stretch the boundaries of science (just different bits). But the thing is, Jupiter is in