In May 2004 Acaba became the first person of Puerto Rican heritage to be named as a NASA astronaut candidate, when he was selected as a member of NASA Astronaut Training Group 19. He completed his training on February 10, 2006 and was assigned to STS-119, which flew from March 15 to March 28, 2009 to deliver the final set of solar arrays to the International Space Station.

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