IF YOU want to send a robot to Mars, two space agencies are better than one. That’s what NASA and the European Space Agency have decided – so long as US law doesn’t block the cooperation.

NASA’s science chief Ed Weiler said last week that the agency had agreed with ESA in principle to join forces in sending robotic spacecraft to Mars. If the plan goes forward, both agencies could fund missions like ESA’s ExoMars rover, to launch in 2016, and a tentative plan to bring a Martian sample back to Earth in the 2020s. Weiler was speaking at a briefing about a …