Original sentence: In 1981, he proposed a manned mission to the moons
Edited sentence: " The earlier studies were later discovered to have used an overestimated value of 5 cm/yr for the rate of altitude loss, which was later revised to 1.8 cm/yr.. In 1981, he proposed a manned mission to the moons.In 1960 Singer stated in relation to the Russian astrophysicist Iosif Samuilovich Shklovsky's observation of the Martian moon Phobos; "My conclusion there is, and here I back Shklovsky, that if the satellite is indeed spiraling inward as deduced from astronomical observation, then there is little alternative to the hypothesis that it is hollow and therefore martian made.Since they are based on several independent sets of measurements taken decades apart by different observers with different instruments, systematic errors may have influenced them.The big "if" lies in the astronomical observations; they may well be in error