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a combining form with the meaning “pertaining to stars or celestial bodies, or to activities, as spaceflight, taking place outside the earth’s atmosphere,” used in the formation of compound words:
astronautics; astrophotography.
Historical Examples

Tom was quiet for a moment, and then added, “Same here, astro.”
Stand by for Mars! Carey Rockwell

“The astro One left Earth thirteen years ago,” the man said.
The Quantum Jump Robert Wicks

astro wanted the advantage of the first kick at the ball while the mercury tube inside was still quiet.
Stand by for Mars! Carey Rockwell

astro and Roger followed, dragging their feet and rubbing their eyes.
On the Trail of the Space Pirates Carey Rockwell

They went directly to the power deck where astro was waiting for them, the Geiger counter in his hand.
Stand by for Mars! Carey Rockwell

“Well, it’s not that we’re against speeches,” ventured astro.
On the Trail of the Space Pirates Carey Rockwell

“There are still some things I don’t understand,” said astro.
The Revolt on Venus Carey Rockwell

Corbett, you and astro go back to the shack and give Roger a hand.
On the Trail of the Space Pirates Carey Rockwell

“Maybe you’re trying to make it too small, astro,” commented Connel, looking over the drawing.
Danger in Deep Space Carey Rockwell

“Nothing to worry about,” he said, as astro rushed to his side.
On the Trail of the Space Pirates Carey Rockwell

combining form
indicating a heavenly body, star, or star-shaped structure: astrology, astrocyte
indicating outer space: astronautics

element active in English word formation from mid-18c. and meaning “star or celestial body; outer space,” from Greek astro-, stem and comb. form of astron “star,” related to aster “star” (see star (n.)). In ancient Greek, aster typically was “a star” and astron mostly in plural, “the stars.” In singular it mostly meant “Sirius” (the brightest star).
astro-
A prefix that means “star” (as in astrophysics), “celestial body” (as in astronomy), or “outer space” (as in astronaut).
American Society for Therapeutic Radiology and Oncology