Alien Timebomb by David Donohue
Walter Speazlebud, the boy who can spell backwards and travel back in time, returns for a new adventure! Walter Speazlebud - first person on the moon - is back in Nittiburg, going to kung fu lessons with Annie and trying to come to terms with the death of his grandad. He still dreams about his trip to the moon, but he's trying to get back to ordinary life. Until...he receives an urgent message from the American government. There's an asteroid hurtling towards the earth - and they need Walter to stop it! Soon, Walter and his dad are in New York, discovering that an alien nation is holding the world to hostage. They are demanding the return of two of their people, alive. Only problem is, they died in an explosion in 1947. Walter must go back in time to save them. Last time, Walter enabled Man to go to the moon. This time, he can save the earth. But will he be able to do it without Grandad?