To paraphrase Winston Churchill, an "iron curtain has fallen" over the flow of news, from the Kennedy Space Center in the east to the Johnson Space Center in the west, about the details surrounding the worst accident in U.S. space history.

While there have been at least three official briefings, even old friends of members of the news media, people who are usually available for personal interviews and chats, have refused to talk about any of the events as they know them during the 74 seconds of the ill-fated flight of the space shuttle Challenger.

Secretaries are now serving as guards, answering telephones and telling reporters not even to expect their phone calls to be returned.

Retired astronauts and space officials are doing the same thing, with a few exceptions. Former chief astronaut Donald K. (Deke) Slayton has told reporters who are friends not to expect him to return their calls. "I've got a stack of more than 200 phone messages that I just can't return," Slayton said.

Former Johnson Space Center Director Gerald Griffin, now executive secretary of the Houston Chamber of Commerce, is saying the same thing through his secretary, who tells reporters in a chilling voice: "No, I don't think he can call you back."

One reporter who got through to a JSC official recounts the following conversation: "Hey, I can't talk to you guys. Go ask the public affairs office to set something up." When the reporter forwarded his request to the JSC Public Affairs Office, he was told, "Hey, we're not doing anything about these things right now."

Part of the reason for what increasingly appears to be a news blackout can be traced to the fact that President and Mrs. Reagan will be in Houston today at a memorial service for the seven dead astronauts. Pandemonium reigns at the Johnson Space Center right now. As many as a thousand reporters are expected from out of town to cover the unprecedented event.

The band from Lackland Air Force Base in San Antonio will play the music. The invocation is to be given by the Rev. Bernard R. Hawley of the First Presbyterian Church of Salina, Kan. He is the father of astronaut Steven Hawley, who is the husband of astronaut Sally Ride, the first female American astronaut to go into space.

Remarks are planned by NASA's acting administrator, William R. Graham, and a lay reading by Marine Lt. Col. Charles F. Bolden Jr., an astronaut.

President Reagan will address the memorial service and the band will play "God Bless America" as four NASA T38 jets fly overhead in the "missing man formation" that signifies the passing of a military aviator.

Another part of the reason for the silence is that members of the space agency appear to be in shock.

The reaction of NASA officials to requests for information is unprecedented in the 27 years of the agency's existence. NASA has almost always been open with the news, depending on that openness to win congressional and public support for its programs.

Said one reporter who has covered the space program longer than any other reporter on the scene: "There's been nothing like it. Even at their worst in the past, I can never remember a clam-up or a stonewalling like this."