Perceived risks associated with a human flight to Mars are just excuses to delay the trip, Mars Society President Robert Zubrin said at Space Congress on Thursday. Zubrin, long an advocate of a direct-to-Mars voyage, compared the nation’s space exploration goals to a rope. A tight rope between two points is the shortest route. A curvy, long rope, like a detour to the moon, means one thing — someone is selling rope, he said.