Right now, the Space Shuttle Atlantis is sitting on launch pad 39A, waiting for its May 12th liftoff to service the Hubble Space Telescope.
Interestingly, on Friday the Shuttle Endeavour will be rolled out to pad 39B, which will provide us with one of those very rare times two Shuttles will be out on the pads simultaneously. Why is NASA doing this now?
|Atlantis and Endeavour wait for liftoff before the mission to Hubble was scrubbed back in September 2008. Credit: NASA|
After Columbia, a new rule was made that a Shuttle must always have a safe haven, a place it can go in space if it's found it has sustained so much damage that a safe landing is in doubt. When there are missions to the International Space Station this is no issue, because it can hang out there (hopefully) for long enough to get another Shuttle launched for a rescue mission.
But the Shuttle cannot both get to Hubble and have enough reserve fuel on board to get to the ISS. The orbits are just too different, and would take too much fuel. So NASA is getting Endeavour set up on 39B just in case it's needed to rescue the good folks going to help out Hubble. If everything is OK, and once Atlantis is cleared to land, Endeavour will be moved to pad 39A, the primary launch facility.
This was done before, last September, again for the Hubble servicing mission. But that launch was scrubbed due to a failure of a component on Hubble. The launch was delayed until May of this year so that a spare part could be moved into the Shuttle manifest to be installed on the observatory.
If Atlantis launches, this will be the last time ever we'll have this sight of two Shuttles ready to go.
Endeavour will start its move to the pad at 12:01 p.m. (18:01 UTC) on Friday, and it'll be covered on NASA TV.