CAPE CANAVERAL — The shuttle Atlantis will begin its first flight today on a secret satellite-delivery mission, but NASA's newest spaceplane won't go quietly.
The 350-ton shuttle will thunder skyward between 10:20 a.m. and 1:20 p.m., rattling windows and shaking the ground for miles. The exact launch time is being kept secret until countdown clocks flicker on nine minutes before liftoff.
Scattered clouds and a 40 percent chance of showers are in the forecast for the start of the 21st shuttle mission, the second devoted entirely to the military.
Keeping the launch time secret presumably makes it difficult for the Soviets to track the shuttle. This is only one of a variety of security measures used to prevent adversaries from learning details of Atlantis' flight and future defense space missions.
The Air Force plans to say little during the mission unless something goes wrong in space. A status report on Atlantis will be released four hours after launch and again a day before landing at Edwards Air Force Base in California. The landing time will be announced 24 hours in advance.
Today's liftoff will be controlled from a specially secured firing room at Kennedy Space Center that is designed to prevent unauthorized entry and equipped to prevent someone from electronically eavesdropping on computers or radio transmissions.
The Air Force wouldn't reveal how this is done.
One of two Mission Control rooms at Johnson Space Center in Houston, where shuttle flights are controlled after liftoff, also is secured.
Special cards and pass codes must be used to enter, a television camera monitors those coming in, and the room's walls were built with special metal insulation to prevent electronic snooping from the outside.
In addition, communications between the shuttle and ground will be encrypted, NASA officials said.
Although the top-secret nature of the military mission is much different from NASA's normal policy of openness, the shuttle will be launched and controlled by the same teams of people who run the civilian shuttle missions. Their backgrounds were checked out before they got security clearances, said NASA spokesman Charles Redmond.
''The regulations for us with these clearances are identical to those in the military,'' Redmond said. He noted that in his case, FBI agents talked with his family and 20 other persons.
Although the mission is classified, the two satellites on board are not.
''They are plain old vanilla military communications satellites,'' worth between $75 million and $100 million and used for communications with strategic forces, said John Pike of the Federation of American Scientists in Washington.
Information about the DSCS-3 satellites can be obtained from technical papers, Pike said. But Air Force policy prohibits the military and NASA from commenting about payloads on military shuttle missions.
The satellites are equipped with anti-jamming devices and use special shielding to prevent their electronic systems from being damaged by electromagnetism from nuclear blasts.
The 2,000-pound satellites will enable the president to send emergency messages to U.S. nuclear forces.
Atlantis is the lightest of the four shuttles and, like Discovery, includes insulation blankets rather than tiles on most of its upper white surfaces.
The crew members are Air Force Col. Karol Bobko, commander; Air Force Lt. Col. Ronald Grabe, pilot; and Army Col. Robert Stewart, Marine Maj. David Hilmers and Air Force Maj. William Pailes.