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

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.

Computers That Talk to Each Other

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.

How Do You Reset All the Clocks?

"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."

NASA has confronted this problem before -- in 1999, the year of the big Y2K scare.

Mission controllers brought a shuttle flight back early. The shuttle had been on a mission to service the Hubble telescope, which had several technical problems, but flight directors were willing to drop a planned spacewalk to get the shuttle back on the ground by Dec. 27.

What if the shuttle mission is delayed?

Polansky says engineers are looking at options.

"That's the part that we are hoping we won't come to, but if we have some slips in the mission we could conceivably be in that position, which is why the shuttle program is taking a good look at this with some testing to decide if this is something they would really want to launch into," he says.

Astronaut Joan Higginbotham is the other crew member on STS-116 working on the problem. She says the problem is they just aren't sure what would happen.

"The issue here is the unknown," she says. "We don't know exactly how they are going to act. Probably nothing is going to happen, they are probably just going to roll over and we will start all over again, but it has not been certified and that is the issue. Everything on the shuttle has been tested and tested and tested, and this particular item has not been tested to that extent or certified."

The only way, then, that STS-116 could spend New Year's Eve in space is if it's in trouble -- if there's been a problem that would prevent a safe landing, and another shuttle is being readied for a rescue mission.

So when Mission Control plays "I'll be Home for Christmas" as wake-up music during this next mission, it will be a promise, not just a wish.

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