The shuttle Atlantis was hauled to the launch pad at Cape Canaveral, Fla., Wednesday for blastoff April 28 on a showcase science mission while scientists tried to find out what caused a small leak in one of the shuttle Discovery's main engines. With Discovery just back from the year's first shuttle flight, engineers hauled Atlantis, mounted atop a squat crawler-transporter, from the Vehicle Assembly Building to launch pad 39B. Awaiting Atlantis was the $530 million Magellan Venus radar mapper, the first interplanetary probe built for launch from a space shuttle.