SpaceVidcast Daily 27.08.09

NASA has a knack for making even simple concepts confusing. Even something as simple as time. You may notice while watching our live coverage of Space Shuttle launches that there are 2 different clocks. The one NASA has that says T minus or "terminal count" which is the one you're probably used to. But then there is the one we have that says L minus or "launch clock" and of course they don't match. So, let's start with NASA's T minus clock and a bit about how that all works. The T minus clock starts counting at T minus 43 hours and begins counting down to the launch of the Space Shuttle. Once the T minus clock hits T minus 27 hours, it hits the first of what's called a built-in hold. That means that at T minus 27 hours, the clock will actually stop counting down and sit at T minus 27 hours usually for about four hours or so. Now, it's designed to do this. With launching a Space Shuttle or any space vehicle for that matter, it may be necessary to fix some issues on the fly, or delay a bit due to external factors like weather. The built-in holds allow for these kinds of situations. A window where they can fudge the launch time a bit but still take off when the clock hits T minus 0. This means the time you see on the T minus clock is not the time left to the exact launch. In fact, when the clock first starts counting down from T minus 43 hours, there's really about 80 hours left until the shuttle will actually lift off. And that's where the L minus clock comes in. The L minus clock is there to give you a good idea of when the shuttle will actually take off. Until we hit T minus 9 minutes and counting, we don't know for sure when the exact time will be, but we can guess within about 10 minutes, and we're usually only off by a couple of seconds. So when you hear NASA say T minus 20 minutes and holding, but you see on the screen L minus 1 hour, that means the shuttle will actually lift off in about an hour, and that's why we have two clocks. One, the T minus clock, designed to allow the Space Shuttle to launch within its window of opportunity at T minus zero. And the other clock, the L minus clock, designed to give you a rough idea as to when it's really taking off, so you don't sit there expecting the shuttle to launch in 20 minutes when it's really launching in about an hour. Or in other words, don't pay any attention to the T minus clock. It will only mess you up. http://www.spacevidcast.com