In January 2004, responding to a recommendation from the Columbia Accident Investigation Board, President Bush ordered NASA to complete the International Space Station and retire the shuttle fleet by the end of fiscal 2010. The president said the money saved would help finance development of new rockets and spacecraft designed to carry astronauts back to the moon by around 2020. The goal was the establishment of Antarctica-style lunar bases.

Credit: NASA

NASA managers then prioritized the remaining shuttle payloads to ensure completion of the space station with a reduced number of flights. One payload -- the Alpha Magnetic Spectrometer -- was dropped from the manifest to make room for more critical space station components.

Ting and the project's supporters never gave up hope and even while developing plans for the possible use of an unmanned rocket, they continued lobbying for an additional shuttle flight to ferry the particle detector to the space station. Their arguments swayed several key lawmakers, including Sen. Bill Nelson, a Florida Democrat who flew aboard the shuttle in 1986.

During a swing through Florida in the midst of the 2008 presidential campaign, Obama said he would support an additional shuttle flight as well as the Constellation moon program NASA was developing to carry out the Bush administration's 2004 directives.

After the election, Obama came through on his promise to add an additional shuttle flight to ferry AMS into space. But the administration chartered a blue-ribbon panel in 2009 to re-assess NASA's manned space program in light of lower long-range budget projections.

When all was said and done, Obama ordered NASA to stop work on Constellation and to focus instead on development of private-sector rockets and a new heavy-lift system for deep space exploration. He also approved a five-year extension to keep the space station operational through 2020.

When AMS was designed, the instrument featured a powerful cryogenically cooled superconducting magnet. Designed to operate for three years, NASA originally planned to bring the detector back to Earth when it ran out of coolant, making space for a follow-on experiment.

When AMS was put back on the post-Columbia manifest, it was penciled in as the last mission. But planners revised the schedule to maximize space station resupply and AMS ended up scheduled for launch in July 2010. A final flight by the shuttle Discovery was planned for the following September.

In late 2009 and early 2010, when it was becoming clear the administration was going to support a five-year extension to the life of the space station, Ting began re-thinking the AMS mission. The cryogenically cooled magnet, while more powerful than an uncooled magnet used during an AMS test flight, would run out of coolant in three years.

By switching back to the uncooled magnet and adding additional detectors and computer processors, the team could achieve the same resolution as the super-cooled magnet. And the uncooled magnet could operate through the life of the space station.

"We had built a superconducting magnet based on the assumption ... that we would be on space station for three years and space station would be deorbited in 2015," Ting said. "So we test the magnet. The magnet would last 28 plus or minus six months, close to three years. But now at the end of last year (2009) we learned space station would go to 2020 and maybe even go to 2028. So after three years, AMS would become a museum piece. And so we quickly decided to change to a permanent magnet."

The permanent magnet, which flew a test mission aboard the shuttle in 1998, is five times less powerful than the superconducting magnet originally envisioned. But it requires no maintenance and testing shows its magnetic field has not changed in 12 years.

Because of the weaker field, "we put in more detectors ... to increase the measurement accuracy," Ting said. "And so the detector resolution with the permanent magnet is not compromised."

Replacing the magnet and adding the additional detectors forced NASA to delay Endeavour's launch until after Discovery's flight, first to November, then December and eventually February. Then, problems with suspect ribs, or stringers, in Discovery's external tank ultimately delayed that flight to February 2011 and Endeavour's launch to mid April.

While all of that was going on, NASA managers were lobbying for permission to convert a stand-by launch-on-need rescue flight for Endeavour's crew into a full-blown space station mission to deliver a final load of supplies. That flight, by the shuttle Atlantis, is scheduled for launch June 28. While a dedicated rescue flight will not be available for Atlantis' crew, NASA developed plans for the three-man one-woman crew to fly back aboard Russian Soyuz spacecraft if anything goes wrong with Atlantis.

Funding for the Atlantis mission was included in the compromise continuing resolution that is keeping the government in operation through the end of fiscal 2011.

Atlantis will use an external tank that is similar in design and history to the one used by Discovery. The aluminum alloy used in that tank's stringers was from a batch that did not meet strength requirements. Built-in stress, coupled with exposure to cryogenic temperatures during fueling, caused cracks to develop, requiring extensive repairs.

Similar repairs have been carried out on the tanks used by Endeavour and Atlantis. But Endeavour's tank is of a different vintage.

"ET-122 was in one of our production cells down at Michoud (Louisiana) when Hurricane Katrina hit and several small chunks of concrete were dislodged from the room and fell on the tank, damaging the foam," Shannon said. "We kind of put that tank to the side while we were doing normal processing.

"I asked the team several years ago to go back and look at ET-122 and see if it was a viable flight tank. All the foam in that area was dissected. The LOX tank, they did eddy current (testing), they did all kinds of non-destructive analysis on it. It was a very good tank, so they replaced that foam, they went to the intertank area, there was one stringer that had been nicked, they took that stringer off, put a new one on, re-foamed that area."

The foam insulation on the tank is nearly 10 years old. To make sure it was still up to the rigors of launch, "they did pull tests all over the tank, they did assessments to make sure it's a good tank and safe to fly," Shannon said. "Then they did all of the return-to-flight modifications that we had done on tanks after Columbia."

"We have a lot of confidence in ET-122," he said. "It doesn't look real pretty because we did some foam patches, it looks a little more like the hail-damaged tank that we flew, which I think was ET-120. But from all out testing and experience, we have high confidence in that tank."

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