A Martian in the Supermarket — Penelope Lively

Judy first sees the Martian when her mom's looking for fish fingers. She isn't scared, but he's a bit nervous. His rocket's engine won't start, so Judy takes him back to her house while they think of a plan. They soon become good friends, and avoid detection in ingenious ways. But the Martian soon becomes homesick, and so Judy must take him back to the supermarket to try to find the rocket and see if it can be prepared. But something has happened to the rocket in the meantime -- will the Martian ever be set free?