Meanwhile astronomers trace the signal to an asteroid, M-387, and send a radio signal to it. In response the asteroid changes its signal. With the help of Holmes, a yacht builder, Keller, an electronics expert, and several workmen, Joe builds a small spaceship propelled by a reactionless drive based on his modification of the recoilless pistol. Just in time the ship is finished and, with Holmes, Keller, Sandy, and Sandyâs sister Pam aboard, Joe lifts off just as the police arrive to end the project. Pursued by Nike missiles, Joe takes the ship into space and heads for Asteroid M-387.