The selection of the Great 7 crew was a real challenge. All the applicants had to be experienced in piloting and under the age of 40. Moreover, there also existed restrictions for candidates' body height and weight. They had to be strictly about 5 feet 11 inches (no taller!) and weigh less than 180 pounds. And for sure, all of them had to be be sound as a bell. That's why as soon as there appeared some troubles with Donald's condition, he was immediately removed from the list . He was the only one of the original 7 astronauts to stay on Earth till 1975.