When the most intensely examined shuttle mission in history ended July 17, NASA officials were cautiously optimistic that the three orbiters were ready to return to a more regular flight rotation and begin a tight schedule that calls for about four missions a year for the next four years.

NASA Administrator Michael Griffin said July 17 that the external tank modifications that were done to improve shuttle flight safety after the Columbia accident, as well as improved heat shield inspection techniques, seemed to be a stunning success. He cautioned, however, that NASA engineers will have to complete a comprehensive analysis before a final evaluation can be made.

“This is the cleanest orbiter anybody remembers seeing,” Griffin said. “What’s behind that, we’ve got to dig in and look. Honestly, when we know, we’ll tell you.”

Aside from some thick clouds, a finicky air data probe, and a nicked heat-resistant tile near Discovery’s nose landing gear door, the spacecraft’s return to Earth surpassed all expectations, NASA officials said.

For NASA launch director Michael Leinbach, Discovery’s smooth landing July 17 provided a closure of sorts. The touchdown marked the first successful Kennedy Space Center shuttle landing since the loss of Columbia, which was heading towards the Shuttle Landing Facility here when it broke apart over Texas.

“Columbia’s landing day was a horrible day,” Leinbach said. “Today was a great day.”

During the 13-day mission, the international space station (ISS) received a new pantry chock full of fresh food, equipment and other vital supplies after shuttle astronauts installed a fresh cargo pod at the orbital laboratory.

STS-121 mission specialists Lisa Nowak and Stephanie Wilson maneuvered the Italian-built Leonardo cargo module into a slot outside the space station’s Unity node where it stayed throughout Discovery’s eight-day stay at the ISS.

Known as a Multi-Purpose Logistics Module, Leonardo carried more than 2,267 kilograms of the total 3,175 kilograms Discovery delivered.

Tucked among the thousands of kilograms of cargo aboard Leonardo were a couple of vital items for space station crews.

The module carried a new Oxygen Generation System, a U.S.-built piece of equipment that will separate water into breathable oxygen and waste hydrogen once installed inside the space destination’s Destiny laboratory. The 664-kilogram U.S. oxygen generator functions much like the Russian-built Elektron device currently used aboard the ISS. Together, the two systems will help support larger, six-person crews once the U.S. generator is brought online.

“It will need some other software capabilities and some additional hardware before it can be operational,” said Debbie Hahn, Discovery’s STS-121 payload manager, in an interview prior to Discovery’s July 4 launch.

Leonardo delivered a new exercise stationary cycle, known as the Cycle Ergonometer with Vibration Isolation and Stabilization, that astronauts use for exercise during their long missions so they can maintain muscle strength during the long months of weightlessness.

Leonardo also delivered a pair of new science tools for the U.S.-built Destiny laboratory. A Minus Eighty Laboratory Freezer for ISS will allow station astronauts to store biological specimens for later transport back to Earth, while the European Modular Cultivation System includes a set of four centrifuges that can be spun up to different speeds to check how plants grow under different stresses.

Comments: tmalik@space.com