Seven astronauts are stranded in orbit after their shuttle is damaged during launch. Unable to repair the ship, they hunker down with dwindling supplies while four more astronauts board a second spacecraft and blast off on a daring rescue mission.
NASA executives would like to keep this scenario in the realm of science fiction. But they're preparing for it just the same on the slim chance the shuttle Atlantis is crippled during the May 11 repair mission to the Hubble Space Telescope.
A second shuttle - Endeavour - is poised for liftoff from Cape Canaveral if there's a call for help from Atlantis. It's the first time NASA has ever readied a shuttle expressly to serve as a rescue vehicle.
"The likelihood of it ever flying ... [is] extremely low," said NASA spokesman Kyle Herring. Nevertheless, "the seriousness with which the agency takes having that capability is obvious."
Scientists and engineers in Maryland and around the world have waited anxiously for the trip to repair Hubble. The telescope is operated by engineers at the Goddard Space Flight Center in Greenbelt, and Hubble's scientific work is managed by scientists at the Space Telescope Science Institute in Baltimore. During five years of cancellations and delays, scientists who observe the cosmos with Hubble have watched with concern as some of its scientific instruments and other hardware have broken down.
The special safety preparations for the repair shuttle stem from the Columbia disaster in 2003, in which undetected launch damage doomed seven astronauts to a fiery death during re-entry.
Before Congress and NASA would clear the way for a resumption of shuttle flights to the International Space Station, the space agency's engineers were directed to develop tools and procedures so astronauts could repair damaged heat shields in orbit. If repairs failed, they could take refuge aboard the space station for up to 90 days, until another shuttle could bring them home.
But astronauts on a servicing mission to Hubble would find no oxygen or stores of food and water. There would be no "safe haven," Herring pointed out. Their own supplies would run out in 23 to 28 days.
So after the Columbia tragedy, then-NASA Administrator Sean O'Keefe canceled the fifth and final Hubble repair mission, to the dismay of astronomers around the world.
That policy finally changed in 2006 under a new NASA chief, Mike Griffin, provided the agency readied a rescue shuttle to "launch on need."
NASA managers hope and expect it won't be needed. "Flying a rescue mission is risky in itself," Herring said.
No one has calculated the chances that the Atlantis crew will need rescuing. But in general, the danger of a catastrophic shuttle failure is thought to be 1 in 80. The risk of a crippling strike by a micro-meteor or space junk during this mission is put at 1 in 221.
Endeavour's countdown could begin even as in-orbit repairs were attempted on Atlantis, said flight director Tony Ceccacci. If the repairs proved futile, Endeavour and its crew of four would be launched on a race to rendezvous with the crippled shuttle.
Mission planners rejected as too risky a proposal to have the two spacecraft dock directly, or transfer the crew with the shuttles' robot arms. They decided the rescuers would fly to within 24 feet of Atlantis, close enough for Endeavour's robot arm to grasp the crippled ship, with the twin craft maintaining their positions without burning any fuel.
One puzzle remained: How to transfer all seven Atlantis astronauts to Endeavour. Only four - those designated for the Hubble repair work - are trained spacewalkers with their own spacesuits on board. NASA had to choreograph the movement of seven astronauts across the deadly vacuum between the two spaceships.
If they get the order, here's how it would happen:
All four members of the rescue crew would stay put inside Endeavour. All of the movement between shuttles would be done by the seven stranded Atlantis astronauts.
First, three Atlantis crew members would suit up and string a safety tether between the two shuttles to make sure no one drifts away during the hand-over-hand transfer between ships. The three would stay aboard the rescue ship for the night.
Left behind on Atlantis would be their four crew mates - with only one spacesuit. So before quitting for the night, one of the spacewalkers would carry over two spacesuits from the rescue shuttle to Atlantis and go back to Endeavour.Thus, at the end of the first day, four astronauts would remain aboard Atlantis with three spacesuits.
The next day, one of the rescued astronauts already aboard Endeavour would suit up again and return to Atlantis to help two of his crew mates don their suits. He'd carry with him one more empty spacesuit.
On their way back to Endeavour, the trio would have different baggage - four of the seven orange re-entry suits and helmets the rescued crew members would need for the flight home.
Finally, on a third and final spacewalk that same day, another of the rescued Atlantis crew members would put his suit on and float back over to Atlantis to help the two remaining castaways don their suits and cross to the rescue craft. They'd bring with them the remaining three re-entry suits.
If they haven't forgotten anything (or anyone), all 11 astronauts would be safely aboard Endeavour, with 11 re-entry suits.
Endeavour would then pull away and head for home. The vacant Atlantis would be set on course for a fiery re-entry, with debris falling somewhere in the Pacific Ocean north of Hawaii.
Although shuttles can be landed without a crew, Herring said, "if this vehicle is damaged at a level that's unsafe for the crew to fly home ... we would be risking whatever populated area we fly over during re-entry. More than likely, you wouldn't do that."
If Atlantis is condemned and scuttled before its crew completes the planned repairs and upgrades to the Hubble telescope, it would be an intense disappointment to astronomers.
It could also slow or end the 28-year-old shuttle program, now in a race to complete construction of the International Space Station before congressional funding expires in 2010. Nine missions to the space station remain on the shuttles' flight manifest.
The shuttles' successor craft - the Constellation system of Ares boosters and re-usable Orion crew capsules - is not expected to begin manned flights before 2015. Until then, the U.S. will be entirely dependent on Russian Soyuz craft to carry astronauts to the space station.
On the other hand, if all goes well, Endeavour will be moved to another launch pad for a planned June 13 construction mission to the space station. Its former pad will be rebuilt for the launch of manned Orion capsules to the moon.