But the "secret" is shared by thousands inside the space program and potentially millions on the outside, courtesy of satellite-tracking hobbyists and the Internet.
"Pseudo-security," says James Oberg, an engineer who used to work in Mission Control.
Even the astronauts admit that NASA's new post-Sept. 11 anti-terrorism policy of concealing shuttle launch times until 24 hours in advance is an exercise in futility. They had already given out the launch time to friends and family before the security guidelines were approved March 14.
"It's been fairly public, so I don't know what we're hiding," says astronaut Jerry Ross, a retired Air Force colonel who flew on a truly secret military mission in 1988. "On future missions, it may do a little bit more."
Oberg theorizes that the chief intent is not really to protect Atlantis' flight to the international space station or Endeavour's launch on May 31, but to safeguard Columbia's liftoff in July with the first Israeli astronaut.
This will be NASA's third shuttle flight since the Sept. 11 terrorist attacks. The Dec. 5 and March 1 launches were conducted amid unprecedented security, yet no attempt was made to hide those liftoff times, which had already been made public by NASA.
Fighter jets and attack helicopters will be on patrol Thursday, with no-fly and no-boating zones enforced.
NASA is disclosing only a four-hour launch window, from 2 p.m. to 6 p.m., until the exact time is announced Wednesday afternoon. At that point, the times of all of the mission's highlights, including the shuttle's return, will be released.
Oberg and others say that anyone with modest math skills and computer access can use publicly available information to calculate the launch time for any given day, provided the rocket ship is rendezvousing with something already in orbit.
It takes Oberg, an orbital mechanics expert, about 30 seconds to do it. He estimates a novice could figure it out in a couple of days.
Oberg will not divulge Thursday's launch time. But others have done so.
"It's hard to understand what this buys them, other than it seems to be an attempt to demonstrate that they're doing something," says Ted Molczan, a Canadian amateur astronomer and satellite observer.
NASA will not even confirm the start of the launch countdown until Tuesday, even though the clocks always begin ticking three days ahead of time, which would be Monday.
In other security measures, the seven astronauts' arrival at Kennedy Space Center on Monday was not broadcast live on NASA television, and reporters were barred from the event. The astronauts' breakfast, suit-up and walk to the "astrovan" for the ride to the pad on Thursday will also be tape-delayed.
NASA stopped short of actually classifying the launch time, as it did for seven shuttle missions devoted to Pentagon work from 1985 through 1990. Those launch times were not announced until nine minutes in advance, and virtually the entire flights were blacked out to the public.
For this mission, the launch time is deemed "for official use only," with NASA relying on the patriotism of workers and the cooperation of reporters to keep it confidential.
Kennedy's public affairs office had pushed for business as usual but was overruled by security officials.
"People can figure out what we're doing," concedes Kennedy spokeswoman Lisa Malone. But "the idea is not to give people out there who may want to do us harm any more information than they already have and make it easier."
Atlantis' commander, Air Force Lt. Col. Michael Bloomfield, says he is all for doing whatever it takes to make a terrorist's job more difficult. "In the military, we always called it the fog of war," he says.