See below for the list of sources provided for each of the astronauts included in this post:
"I happen to be privileged enough to be in on the fact that we have been visited on this planet and the UFO phenomenon is real. It has been covered up by governments for quite some time now."
— Captain Edgar Mitchell, Apollo 14 Astronaut
Source: youtube.com/watch?v=RhNdxdveK7c&embeds_referring_euri=https%3A%2F%2Fufopanel.com%2F&source_ve_path=MjM4NTE
"For nearly 50 years, the secrecy apparatus within the United States Government has kept from the public UFO and alien contact information."
"We have contact with alien cultures."
— Astronaut Dr. Brian O'Leary
Source: 8th September 1994, declarations publicly at the International Forum on New Science in Fort Collins, Colorado (Check this into Google Gemini for validation, or Wikipedia).
"...I've been asked [about UFOs] and I've said publicly I thought they [UFOs] were somebody else, some other civilization."
— Astronaut Eugene Cernan, Apollo 17 Commander
Source: LA Times, 1973 (Google Gemini verifies this, but unfortunately no clipping can be found online). Eugene has been very vocal about UFOs in the past.
"I believe that these extraterrestrial vehicles and their crews are visiting this planet from other planets which obviously are a little more technically advanced than we are here on Earth."
— Colonel Gordon Cooper, Mercury & Gemini Astronaut
Source: Letter to the United Nations, November 9. Video - youtube.com/watch?v=dvPR8T1o3Dc&embeds_referring_euri=https%3A%2F%2Fufopanel.com%2F&source_ve_path=MjM4NTE