The Martian Missile

Kermit Langley, a crook hiding out in the desert, rescues the pilot of an extraterrestrial rocket and is turned into an interplanetary unguided missile before the humanoid critter—from Thuban by way of Pluto dies. He's had a message engraved on his armbone, and has four years to get it to Pluto, else he'll begin to vibrate until his bones fall apart. To make the mission not quite impossible, he's given the gift of unnoticeability. People—and things—can look right at him and not realize he's there.