"I'll set my mertilizer on 'deep fat fry'."
Spaceman Spiff was one of the first of Calvin's (of Calvin and Hobbes) alter-egos to appear in the strip. Spiff's goggles, bad luck, and laser pistol first appeared to the public in the third Calvin and Hobbes Sunday strip*, long before Stupendous Man or the Transmogrifier, but his history is actually a bit longer.
Bill Watterson's take on Snoopy's World War I flying ace fantasy** started in high school, in German class, with the character Raumfahrer Rolf. Rolf met a quick end; he was eaten by a monster on the second page. Rolf next saw the light of day in Watterson's college days, named Spaceman Mort, but school took over and Mort's strip came to an end.
After Watterson graduated, Mort was renamed Spiff, and he was the author's first comic submission to the newspaper syndicates. The original Spiff had a Charlie Chaplin mustache, an everpresent cigar, and flying goggles, which probably made him look not unlike Groucho Marx.
Once Calvin and Hobbes began syndication, Watterson worked his old character into the new strip, mostly because he liked drawing the landscapes. According to The Calvin and Hobbes Tenth Anniversary Book, the landscapes are lifted directly from real-life landscapes in Utah.
Spiff normally appears juxtaposed with Calvin's life during periods of boredom or frustration. The aliens in the story are usually direct re-interpretations of Miss Wormwood (Calvin's teacher), Calvin's parents, or Susie Derkins.
*Thus, it's available in The Essential Calvin and Hobbes, the first Calvin and Hobbes compilation, and The Calvin and Hobbes Lazy Sunday Book.
**Don't believe me? Bill Watterson has claimed Charles Schultz's "Peanuts" as one of the three most influential comic strips in his career.