A father goes missing in the asteroid belt, his ship is taken back to Earth and his son returns to the asteroid belt to look for him and finds a hollow asteroid that's a ship. I read it over 40 year ago, so somewhere before 1976.UPDATE, I think he had a friend or his brother with him, when they fist landed on the Asteroid they got out to look around, somehow the ship drifted off and they use their O2 tanks to reach the ship. they find a way inside and find few people inside in the middle of the asteroid in a area big enough to have fields and animals. the boys find their father after long search. That all I can remember, but there is a second book where the boys go to Mercury and find little people about a foot tall.
The Forgotten Star by Joseph Greene, 1959. The first in the Dig Allen series. Actually, his father's ship is found drifting in the asteroid belt with no one on board, and Dig is told about it; I don't remember whether the ship was brought back to Earth. And the asteroid is ... Eros!
Summary from Goodreads:
Here is an exciting tale of action and suspense, set in the world of tomorrow. As the story opens, two brothers, Jim and Ken Barry, are traveling by space ship from Earth to a new city on the moon. Aboard the space ship they discover a stowaway, young Dig Allen. Almost at once the three youths are plunged into an adventure that threatens to spell destruction for them all.
Dig has set out to find his father, a space scientist who mysteriously disappeared months before. The Barry brothers agree to help Dig. Their search takes them to the "Graveyard of Space" and to Mars. There they meet Old Dorkas, the one person who is able to decipher the last message received from the missing man. Finally, the boys are forced to set out along in an unauthorized space ship. They soon find themselves marooned on a weird, forgotten world at the outer edges of the solar system where unknown to them, their greatest adventure is about to begin!