"NASA is not involved in any sort of cover-up about alien life on this planet or anywhere else in the universe," NASA Headquarters spokesman David Steitz said today. "Dr. Mitchell is a great American, but we do not share his opinions on this issue."
Former astronaut John Herrington, who made three space walks in 2002 on the last space shuttle mission before the Columbia accident, told ABCNews.com that his experience as an astronaut definitely affected his perspective on the universe.
"You see the world from a different perspective -- you have a much grander view," said Herrington. "It does fundamentally change the way you look at the world."
Herrington said that he can also understand the depression that some encounter upon arriving back on earth.
"In anything that you put an incredible amount of effort into, a lot of times you come back from it and things pale in comparison," said Herrington. "There's a little depression, a little let-down."
"You worked hard and now the experience is over," added Herrington.
For Mitchell, the four decades back on Earth have done nothing to sway his belief in extraterrestrial life among the stars.
"There's not much question at all that there's life throughout the universe -- we're not alone in the universe at all," he said last week. "It is a real phenomenon."