The European Space Agency (ESA) decision to go ahead without NASA support in selecting its next large astronomy mission (Nature 471, 421; 2011) has dashed hopes for the International X-ray Observatory (IXO). X-ray astronomy needs a cheaper 'plan B'.

NASA might yet contribute 20% of the funding for a smaller IXO, but that would not be comparable as a flagship for NASA, ESA and the Japan Aerospace Exploration Agency (JAXA).

A radical rethink of IXO might do it. A mission is defined by its science, not its hardware. IXO spanned all three decades of the X-ray band with a single mirror. But X-ray optics need focal lengths proportional to the photon energy reflected, so each sub-band would benefit from having a very different mirror. Turning IXO into a programme of independent small satellites, each optimized for one band, could be the answer.

ESA, NASA and JAXA could each choose one and build it, without financial or technological interdependence, and space the launches over several years.

The X-ray sky is full of complex sources with much interesting detail, but satellites dedicated to different energy bands would aim mostly at point sources, which have no discernable structure. Hence a further satellite with a high angular resolution imager would be needed.

Some inter-agency agreement would help to achieve synergy, coordinating where they should be looking and allowing for multi-telescope proposals.