Man who could not be noticed

[Stan Kaplan]

I'm try to locate a novel about a person who is recruited, I believe by aliens, to reach a ship orbiting the solar system. They do not make him invisible, but instead make him not noticeable. He manages to slip onto a Russian probe, starting him on his journey. This may have been a juvenile, but I'm not sure. I would have read it in the late 1950s or early 1960s. Any help would be appreciated.