CAPE CANAVERAL, Fla. — NASA is trying to resuscitate its planet-hunting Kepler spacecraft, in a state of emergency 75 million miles away.The treasured spacecraft — responsible for detecting nearly 5,000 planets outside our solar system — slipped into emergency mode sometime last week. The last regular contact was April 4; everything seemed normal then.Ground controllers discovered the problem Thursday, right before they were going to point Kepler toward the center of the Milky Way as part of a new kind of planetary survey.