NASA managers meeting at Kennedy Space Center on Tuesday affirmed plans to launch space shuttle Atlantis on April 4 to deliver a massive truss to the International Space Station. But although the agency isn't announcing the target liftoff time, it has been available publicly for months, can be readily calculated on home computers or figured out with simple subtraction.

Atlantis is set to launch April 4 carrying the S0 truss to International Space Station. Photo: NASA-KSC

The 11-day flight of Atlantis marks the first time in the 21-year history of the space shuttle program that NASA will not announce the launch time for a civilian mission. The space agency will only say liftoff will occur sometime between 2 and 6 p.m. EST (1900-2300 GMT), with the precise launch time to be announced officially about 24 hours in advance. The unprecedented step was taken for security reasons, NASA says.

However, Spaceflight Now has been reporting since early January in our worldwide launch schedule that liftoff would occur at 5:14 p.m. EST (2214 GMT).

In addition, anyone with home computer and internet connection can calculate the launch time with relative ease.

NASA officials have frequently explained that shuttles bound for the space station liftoff within 10 minutes of the moment Earth's rotation carries the launch pad into the plane of the station's orbit. NASA's "preferred" launch time is the precise moment the pad is "in plane" with the station. An article in the March issue of the Smithsonian Institution's Air & Space magazine provides an explanation of this principle.

NASA continues to release orbital tracking elements for the space station, precisely defining the complex's orbit. They are known to amateur satellite trackers and ham radio operators as NASA two-line elements, or TLEs.

These elements can be downloaded from numerous sites on the internet and are commonly used by satellite tracking software to predict when the space station is visible in the night sky.

The NASA-provided elements also can be used to predict when Earth's rotation will carry any given site -- including the shuttle launch pads -- directly into the orbital plane of the station.

Using these elements, satellite tracking software indicates Atlantis will take off within a few minutes of 5:16 p.m. EST (2216 GMT) on April 4, close to the time that this site has been reporting for months.

The liftoff time changes slightly over time based on the station's orbit and will be refined by flight controllers in Houston up until 90 minutes before launch when the latest radar tracking of the outpost is computed.

Using this simple math it is also possible to roughly calculate the launch time by taking the last launch time for a shuttle mission to the space station and subtracting 24 minutes for each day that has passed since then. That is the average daily procession rate for the station's orbit. Using this system gives a launch time of 5:19 p.m. EST (2219 GMT).

Spaceflight Now has posted our usual package of mission charts -- including the detailed flight plan and other timelines -- based upon the 5:16 p.m. EST launch time. A comprehensive mission preview story will be online in a few days.

The main goal of Atlantis' mission is to deliver the so-called S0 truss. It will form the central portion of the station's backbone, the eventual nine-piece truss supporting power and cooling systems for the expansion of the orbiting complex.

Atlantis' crew is led by commander Mike Bloomfield, with rookie pilot Stephen Frick, flight engineer Ellen Ochoa and spacewalkers Jerry Ross, Steve Smith, Lee Morin and Rex Walheim. Ross will be making a record-setting seventh space flight, the most of any astronaut in history.

Atlantis' 25th flight, the 109th in shuttle history and second of 2002 is scheduled to land at Kennedy Space Center on April 15.