Sheila Finch 
Tiger in the Sky [EPUB ebook] 

In this action-packed science fiction adventure, teens from across time must save a scientific base from a destructive—yet adorable—threat. Twenty-fourth-century Earth is a utopia, but when crisis arises aboard a space station on the edge of the solar system, the adults are powerless to help. The teens of this era are amazingly intelligent but they lack the necessary grit for the challenge. Their only option is to turn to Operation Hourglass . . . For five years, Operation Hourglass has yanked resilient teenage heroes from the past and sent them on an array of dangerous missions. This time? A spirited girl and brainy boy from 1999 and a cabin boy from Sir Francis Drake’s ship in 1579 join forces with a twenty-fourth-century girl and an unreliable robot to save a scientific base that’s being threatened by dangerous, hive-mind creatures. The trouble is the little guys are just so cute and fuzzy . . . Winner of the San Diego Book Award for Youth Fiction