While NASA’s Discovery astronauts prepare for the agency’s first shuttle flight in more than two years, another orbiter crew also is training hard for its own launch — now less than three months away.

“We’re working really hard, and it’s getting busier,” Mark Kelly, pilot for NASA’s STS-121 mission, said in a telephone interview. “You would think that a two-month launch slip would give you a lot of extra time, but it doesn’t.”

Kelly and his crewmates are set to ride the space shuttle Atlantis into orbit between Sept. 9 and Sept. 24 in what is scheduled to be the second orbiter launch since the 2003 Columbia disaster. The spaceflight’s earlier launch window, which stretched from July 13-31, now is reserved for NASA’s first return-to-flight mission, STS-114 aboard Discovery.

Both shuttle missions are test flights to demonstrate new safety tools and procedures put in place after the loss of seven astronauts aboard the Columbia orbiter, which broke apart during re-entry Feb. 1, 2003. Wing damage sustained from external tank foam debris at launch later was identified by investigators as the accident’s cause.

“To me, professionally, it’s just about getting the space shuttle program back to where we were before the accident,” Kelly said of his mission . “The fact is we learned from Columbia and will hopefully operate [the shuttle] safer than before.”

Kelly, an experienced pilot and U.S. Navy commander, said that while he is aware of the risks posed by human spaceflight, he believes space exploration is a worthy endeavor. “I, personally, would not take a lot of risk with something that has little benefit to me or the country as a whole,” he said. “And spaceflight has given our country and the rest of the world a lot of technology we might not have had otherwise.”

Many of the return-to-flight tasks during STS-121, such as external tank photography, will be simple repeats of STS-114 experiments. But Atlantis’ mission is expected to take some tasks, especially orbital repair tests, to the next level.

“Our flights work together,” STS-121 commander Steven Lindsey said of the two upcoming shuttle missions in an earlier interview. “The [return-to-flight] objectives the STS-114 crew don’t get to, we’ll do … that’s where we fit in.”

Like Discovery’s STS-114 spaceflight, the STS-121 crew will test a 50-foot (15-meter) orbital boom designed to scan sensitive shuttle areas for damage using a laser and camera sensor package. But while the STS-114 mission will test new instrument’s utility, the STS-121 crew will put one its spacewalking astronauts at the tip of boom for additional tests.

STS-121 astronauts said the manned orbital boom test is slated for the mission’s first extravehicular activity (EVA) with the boom attached to the end of Atlantis’ robotic arm.

“We want to test the feasibility of the structural loads,” Lindsey said. “We need to determine if the arm and boom together is a stable enough platform for orbital repair … with a crewmember attached.”

All three of the STS-121 mission’s 6.5-hour spacewalks, to be executed by mission specialists Michael Fossum and Piers Sellers, will test some facet of return-to-flight hardware, ranging from the manned orbital boom test to repair methods for Atlantis’ protective thermal tiles and reinforced carbon-carbon (RCC) panels.

The second spacewalk includes space station support activities, in which Fossum and Sellers will attach spare parts to an exterior platform.

“In the middle of that we’ll test the CIPAA tile repair goo gun,” Kelly said. “And EVA 3 will be dedicated to RCC repair tests.”

Short for Cure in Place Ablator Applicator, the backpack-mounted CIPAA device is designed to mix and deliver a pink, ablative goop known as STA-54 into damaged shuttle tiles.

During STS-121’s third spacewalk, RCC plug experiments and additional tests of a black, heat-resistant substance called non-oxide adhesive experimental are scheduled, Kelly added.

Unlike Discovery’s upcoming flight, which will rely on its entire seven-astronaut crew to accomplish NASA’s return-to-flight goals, the STS-121 crew must make do with six astronauts.

The final seat aboard Atlantis is currently reserved for European Space Agency astronaut Thomas Reiter, who will join the two astronauts aboard the international space station for a long-duration mission instead of returning home with the STS-121 crew.

“We’ve got the same amount of work [as STS-114] with one less person,” Kelly said, adding that Reiter will help with some standard shuttle tasks in the three days before space station docking. “So we’re going to be pretty busy, but we’re ready.”

In addition to Kelly and Lindsey, STS-121’s crew complement includes mission specialists Lisa Nowak, Stephanie Wilson, Fossum and Sellers.

As shuttle engineers ready Atlantis for launch — the orbiter and its crew must be prepared to launch to the space station within about 35 days after Discovery as part of an emergency rescue plan — the STS-121 crew is preparing for a July 19th check on their Italian-built Leonardo cargo pod, Kelly said. A launch countdown dress rehearsal is slated for mid-August, he added.

The inevitable target of the STS-121 spaceflight is the orbiting space station, where Expedition 11 commander Sergei Krikalev and flight engineer John Phillips will be awaiting the arrival of both Reiter and the cargo aboard the Leonardo module.

NASA grounded its three remaining shuttles after the Columbia accident, leaving only Russian Soyuz spacecraft available to launch fresh crews to the space station , and Russian unmanned Progress vehicle to deliver vital food and other supplies. Additional space station construction has been delayed because many of the new trusses and modules can only be launched via NASA’s shuttles.

Comments: tmalik@space.com