NASA faces a dilemma in planning to send people to Mars: The scientific desire to search for life there versus the need to prevent any such life from endangering the astronauts or the Earth. The National Research Council (NRC) is recommending that safety take precedence and that missions to the Red Planet try to avoid encountering any possible life forms there. “While the threat to Earth’s ecosystem from the release of Martian biological agents is very low, the risk of harmful effects is not zero and cannot be ignored,” the council said in a report released Wednesday. The NRC urged NASA to establish “zones of minimal biological risk” by sending automated probes to test for organic chemicals or other life forms. Astronauts could then be sent to areas with the lowest possible risk of encountering life that might either pose a threat to them or to Earth if it returned with them.