Houston — For the Challenger astronauts, a change of 51/2 minutes in their launch schedule tells a lot about their unprecedented mission. At this writing, preparations were continuing smoothly at the Kennedy Space Center's Pad 39A for a launch window that was to open at 8:58:08 a.m. Eastern standard time April 6 - slightly earlier than the previously announces 8:58:30 a.m. opening. This gives the launch team nearly 30 seconds of precious extra time in a launch window that is only a few minutes long.
Like virtually everything else having to do with navigation during the early days of the mission, this tightly constrained launch timing is dictated mainly by the need to rendezvous with a satellite already on orbit. This is the Solar Maximum Mission satellite (Solar Max). The astronauts are to retrieve Solar Max during their third mission day, repair it, and later return it to orbit.
But getting to that satellite in the first place is a demanding rendezvous exercise. Previous shuttle crews have tested some of the procedures.
But, as the lead flight director, Jay Greene, has noted, it is ''the first time we've put all that rendezvous business together in one package.''
Challenger cannot approach Solar Max directly. Instead, the space ship must go through a series of complex maneuvers over several days. It is like a stately dance in which Challenger must cover what seems a roundabout path to join an uncooperative partner.
Along the way, during the second mission day, the shuttle crew also must deploy the long duration exposure facility (LDEF) - a satellite with 57 experiments on board.
If Challenger and Solar Max are ever to meet, their orbits must be in the same plane. Also, Challenger must enter its initial orbit so that it is not too far behind Solar Max. Mission planners were only too glad to be able to add another 51/2 minutes to the launch window, which now is 19 minutes, 15 seconds long. By taking a hard look at their requirements, they found they could back up launch time 30 seconds. Earlier in the countdown, by accepting a 85-mile perigee (low point) of the initial orbit - unusually low for a shuttle mission - they bought themselves another five minutes of launch window.
Rendezvous expert Richard Hieb here at the Johnson Space Center explains that two basic facts of orbital mechanics are involved.
* First, it takes a great deal of propellant to change the plane of a spacecraft's orbit.
Controllers strive to minimize such changes. That means Challenger must be launched at a time when the angle between its initial orbital plane and that of Solar Max is smallest.
* Second, the lower the height of a satellite's orbit, the faster the satellite moves around Earth. The greater the orbital height, the more slowly the satellite moves.
Taking advantage of this fact, Challenger's crew can close the distance between it and Solar Max by changing its orbital height.
But for this to be practical, the crew must start out within a certain distance of Solar Max - a range of about 144 degrees of angular separation.
Thus, for a successful launch, Florida must be under the Solar Max orbital plane and Challenger must be within a reasonable angular distance of its target. These requirements tightly constrain the launch timing.
Mission planners also would like to launch so that ground trackers in Hawaii can get a good look at the shuttle's expendable main fuel tank when it reenters the atmosphere. This too constrains launch timing. But it is a secondary consideration.
Also, to save fuel for its orbital maneuvering system (OMS) engines, Challenger was to make the first direct ascent to orbit attempted with a shuttle. Shuttles are launched by two strap-on booster rockets and the shuttle main engines. The main engines continue to burn after the boosters have been jettisoned. Previously, the main engines have taken a shuttle to an intermediate orbit about 80 miles high. Then the OMS thrusters were used to raise the orbit to its final height of, say, 170 miles.
This time, Challenger's main engines were scheduled to carry it directly to a height of some 288 miles.
This eliminated the initial OMS burn, saving fuel for the unusual number of OMS maneuvers needed to catch up with Solar Max.
The flight schedule for first two mission days includes some 10 maneuvers. These should bring Challenger and Solar Max to within about 800 feet of each other.
Then, with what Mr. Hieb calls ''boring slowness,'' Mission Commander Robert Crippen will edge Challenger to within 200 feet of its target by gently firing small thrusters.
The stately dance will be over. The recovery of Solar Max can begin.