Slashdot Mirror
T-43 Hours and Counting
An anonymous reader submits "As seen on NASA TV, for the first time in over two years, the countdown clock has started at 6:00 PM EDT for the Wednesday 3:51 PM EDT launch of Space Shuttle Discovery on the first of the return to flight test flights. The launch is not for certain due to weather issues associated with hurricane Dennis. Currently it is estimated for a 70% chance of launch on Wednesday, with the chances lowering later in the week. If you are confused on how T-43 hours equals almost 3 days, perhaps you should read Countdown 101."
Time. As in liftoff time. Before the liftoff, the clock regesters as "T minus ..." after liftoff it regesters as "T plus..."
T- stands for "Time minus". "Time" is when the main event occurs (the shuttle liftoff). All other times are specified as relative to this event. Thus, T minus 3 minutes is (theoretically) three minutes before the shuttle lifts off, and T plus 6 hours is 6 hours after the shuttle lifts off. The military uses this convention for operations too (e.g. D-Day plus 6 is 6 days after D-Day). This system is good because it allows things to be scheduled without knowing exactly when the main event is going to occur (the launch could be delayed by weather, etc).
The system isn't perfect though, which is why you have "holds" for some things. These "holds" allow time for activities whose duration is really difficult to predict.