Did you just read The wonderful Flight to the Mushroom planet?
"When two boys find a mysterious ad in a newspaper asking for two young boys to build a spaceship, they quickly construct one out of old tin and scrap wood, and bring it to the advertiser. This man is the mysterious Mr. Bass, a scientist living in an observatory who goes unnoticed by most of the townspeople for some reason. He shows the boys a previously undetected satellite of the earth, the eponymous planet, that can only be seen with a special filter he has concocted. He gives them some special fuel he invented to power their spaceship, and tells them to fly to the mushroom planet (after getting their parents' permission). He warns them that their trip will only be successful if they bring a mascot."