NASA hopes to get its next space shuttle off the launch pad and back on the ground by the end of 2006 in order to avoid computer problems similar to those once ascribed to 鈥榊2K鈥�. It is now considering moving the shuttle Discovery鈥檚 planned lift-off ahead by one day, to 6 December.
The space shuttle鈥檚 computer software is about 30 years old and does not recognise when the calendar year switches. On 1 January 2007, for example, it will think it is day 366 of 2006 鈥� a problem NASA calls 鈥榶ear-end rollover鈥�.
To reset the time, the shuttle鈥檚 main computers would have to be 鈥榬einitialised鈥�, which would mean a period without navigation updates or vehicle control, a situation NASA obviously wants to avoid.
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NASA had already moved the shuttle鈥檚 target launch date from 14 to 7 December 鈥� in part to avert the year-end issue and in part to allow shuttle workers to rest over the holidays. Now, it is considering moving lift-off one day earlier, to 6 December, to give launch teams another chance to get the shuttle off the ground before the new year.
鈥淚t looks like we will not try to execute the flight over the year end,鈥� says NASA shuttle programme manager Wayne Hale. He points out that avoiding the calendar change is simply a long-standing recommendation rather than a requirement 鈥� NASA may change the policy in the future.
Cutting it close
If the shuttle were to launch on the last day of its launch window, on 17 December, it could land on 28 December. But if there were unforeseen problems or bad weather during the planned landing, NASA could tack on a few extra days to the mission, pushing the landing close to New Year鈥檚 Eve.
Hale says the shuttle will probably be able to take off before the end of the window, however: 鈥淲ith the more benign weather we have in Florida in December, we think that will probably be adequate to get us off [the ground].鈥�
This is not the first time that the shuttle programme has been faced with the year-end rollover problem. On a Hubble servicing mission in 1999, the year of the overblown Y2K computer scare, the shuttle landed on 27 December (see Fuel fault delays space repair). To make sure the shuttle got back on the ground before 31 December, mission managers decided to drop one of the four planned spacewalks.
Despite its concerns, NASA approved a plan to fly a rescue shuttle over the critical period if the upcoming mission runs into trouble. And Discovery crew members Joan Higginbotham and Mark Polansky recently did flight simulations of a mission operating on 1 January.
鈥淚t went flawlessly,鈥� Higginbotham told New 杏吧原创. But she says flying an actual mission on that day is another story: 鈥淲e鈥檝e never done it in the history of the shuttle; it鈥檚 just an unknown.鈥�
Night launches
Fortunately, the International Space Station (ISS) is a much newer vehicle 鈥� its first US components were launched in 1998 鈥� and its computers do not have the same problem. They were designed to stay in space year after year.
On this flight, Discovery鈥檚 seven crewmembers will help reconfigure the space station鈥檚 power and cooling systems. This will also be the first night launch since the Columbia accident in 2003.
Night launches were put on hold after the accident so NASA could train cameras on the shuttle during its climb to space to look for pieces of foam or ice coming off the external tank. If NASA did not allow night flights to resume, it would not finish construction of the ISS by 2010 鈥� the shuttles鈥� retirement date.
But NASA says cameras and radar will still be able to identify potentially dangerous pieces of debris that fall from the shuttle during lift-off. 鈥淚 don鈥檛 think there is any significant difference between processing for a launch in the dark than in the day,鈥� Hale says. 鈥淚 feel confident that we would see a large piece [of debris].鈥�
NASA has five shuttle flights to the ISS scheduled for next year. The first was recently delayed 鈥� from February to 16 March, as were the next two launches.
Modified ramps
The delays are due to fixing so-called 鈥榠ce/frost ramps鈥� on the shuttle鈥檚 external fuel tanks. The ramps are designed to cover metallic brackets outside the tank to prevent atmospheric water vapour from condensing and forming ice on the brackets.
The March shuttle will be the first to fly with modified ice/frost ramps, which are the areas on the tank currently most prone to shedding foam.
The new ramp is a more blunt design than the ramps currently flown on the tank. It also uses 50% less foam than the old ramps, although wind tunnel tests reveal it still sheds small pieces of foam insulation during a simulated launch.
Engineers will hold a design review for the modified ramps at the Michoud Assembly Facility in Louisiana, US, on 13 November.
In about a year, NASA plans to eliminate the foam ramps altogether, using titanium 鈥� which conducts heat poorly and therefore does not accumulate ice 鈥� for the underlying brackets.
