Y2K+ 7: Or Why the Shuttle Isn't Supposed to Fly on Dec. 31

HOUSTON, Dec. 5, 2006 — -- Forget the foam. The problem that has many at NASA scratching their heads is YERO -- or year-end rollover. NASA doesn't want Discovery and its seven astronauts in space on New Year's Eve, when 2006 rolls over into 2007.

The Space Shuttle may be the most complex machine ever built (2.5 million parts). But it was designed in the 1970s, with '70s technology and '70s computers.

Engineers at NASA never thought the shuttle would still be flying in 2006. After all, it was supposed to be replaced by something else by now.

But here we are at the end of 2006 with the shuttle Discovery being prepped for a 12-day flight, dubbed STS-116, that is targeted to launch on Dec. 7 and land on Dec. 19. The launch window runs until Dec. 17, and shuttle flights have been known to stay up longer than planned when weather is iffy at the landing site.

Mark Polansky, who commands STS-116, says the problem is simple.

"When somebody designed these general purpose computers that we use to basically run the shuttle, nobody thought that you needed to have the timer such that it needed to reset itself when it went from one year to the next," he says.

To reset the time, the shuttle's main computers would have to be "re-initialized," which would mean there would be no navigation updates or vehicle control. That is a situation NASA obviously wants to avoid.

The space shuttle runs a Julian clock, which counts up from zero. This clock will basically be running at the end of the year on day 364 plus 23 hours, 59 minutes. When it gets to January 1, instead of resetting itself to zero, it just keeps counting itself up and adds a day. So now you are at day 366.

In the meantime, there is a second clock -- called a master timing unit -- which does reset itself to zero on Jan. 1.

"The two talk to each other and say, 'Wait a minute, you are at zero and I am at day 366. Something is wrong so we are going to have to shut ourselves down,'" Polansky says.

"So we now have this problem that requires us to come up with a software, kind of, a patch, a procedure that we would work around to artificially reset all of the clocks, take all of the computers down and bring 'em back up -- something that we don't normally like to do," he says. "I don't know any instance where you would on purpose take every single computer that you use for control, basically put it to sleep, and then bring it back up, and see that it wakes up."