On Mars' moon, Phobos, a 150# person would weigh 2 ounces. The movie shows regular earth gravity when on the moon.
The Cyrillic letters CCCP (USSR) can be seen on the side of the "American" rocket ship, betraying the fact that this film used stock footage from a Russian sci-fi film.
Due to the interweaving of Soviet and American footage, Brockman and Grant are abruptly standing still in closeup as they approach the alien ship, when they were both moving in the previous long shot.