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NASA

will continue the onorbit testing of new equipment and repair procedures for space shuttles when mission STS-121 visits the International Space Station later this year. STS-121, the second Return to Flight test mission, will carry on demonstrations of safety improvements that debuted on the first Return to Flight mission, STS-114,

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by Brad Thomas

deliver critical hardware and a new crewmember to the space station, bringing the orbital outpost's crew complement back to three. "The main goals of STS-121, first and foremost, are that we're the second and final Return to Flight test mission," Lindsey said. "The second one... is purely space station objectives. We will be

The remote manipulator system robot arm and its 50-foot extension, called the orbiter boom sensor system, flex above the Earth during STS-114. The boom will return to action during STS121 when it is used during inspections of Space Shuttle Discovery's heat shield.

and build upon those tests. STS-114, which launched in July 2005, was the first shuttle mission to fly since the loss of Columbia and the STS-107 crew on Feb. 1, 2003. Discovery and its crew, led by Commander Steven Lindsey, will also

transferring back and forth and resupplying the station with transfer items, and also taking off the stuff that's used up, old food trays and things like that." Lindsey, who served as the pilot on two previous shuttle missions, commanded his first shuttle flight when

STS-104 flew to the station in 2001. His new crew is a mixture of veterans and first-time fliers. The pilot is Mark Kelly, who will make his second trip to the station. Kelly, a commander in the U.S. Navy, served as the pilot of STS-108 in 2001. Mission Specialist Piers Sellers, who holds a doctorate in biometeorology, will make his second flight into space. Sellers served as a mission specialist on STS-112 in 2002. Mission Specialists Mike Fossum, Stephanie Wilson and Lisa Nowak will make their first spaceflight on STS-121. Wilson and Fossum worked for NASA before joining the astronaut corps. Nowak came to NASA from the Navy, where she holds the rank of commander. Also scheduled to fly to the station on Discovery is European Space Agency (ESA) astronaut Thomas Reiter. When Discovery leaves the station, Reiter will stay behind to work under an agreement between ESA and the Russian Federal Space Agency. Reiter's presence will give the station a three-member crew for the first time since Expedition 6 returned to Earth in May 2003. He will also be the first ESA astronaut to live aboard the space station for a long-term mission. Reiter said that STS-121 and the arrival of a third station crewmember will be a good sign for the future. "I think this moment signifies that we are getting back on track, if I may say so," Reiter said. "There has been an interruption of three years where only two

people were working onboard the station, and now we are actually back in a state that we can continue with the assembly." STS-121 will begin its journey to the station when Discovery lifts off from Kennedy Space Center in Florida. After entering orbit, the crew will spend about 48 hours preparing for Discovery's arrival at the station and conducting inspections of the orbiter's heat shield. After docking with the space station on Flight Day 3, the STS-121 crewmembers will conduct joint operations with the station's expedition crew. Activities will include cargo transfers and three spacewalks. Discovery is slated to undock from the station on Flight Day 11 and land at Kennedy's Shuttle Landing Facility on Flight Day 13. STS-121 will be the 115th shuttle mission and Discovery's 32nd flight. Also, it will be the 18th space shuttle mission to visit the space station. The STS-121 crewmembers are scheduled to perform three spacewalks to conduct tests for on-orbit inspection and repair techniques for the orbiter's heat shield, perform station maintenance and install spare parts for future use on the station. Sellers and Fossum will perform all three excursions. Sellers has accumulated 19 hours and 41 minutes of spacewalking time during three spacewalks during STS-112. Fossum will conduct his first spacewalks on this mission. STS-121's three spacewalks will be performed from the station's airlock while the orbiter is docked to the complex. During one spacewalk, Sellers and Fossum will test a 50-foot robotic arm boom extension as a heat shield repair and inspection platform. "The idea is that we're going to test the suitability of this system for something to stand on and work from while doing a repair on a shuttle," Sellers said. "We're going to put this whole system through a series of tests to see how well it works as a stable platform." During another spacewalk, Sellers and Fossum will contribute to the construction of the space station by installing a spare part on the outside of the station for future use. The spare part is a pump for the station's thermal control system. They

will also take on tasks to restore operating capability of the station robotic arm's mobile railcar. One of two power, data and video cables for the system was inadvertently cut in December 2005. A third spacewalk will include tasks to test techniques for inspecting and repairing the reinforced carbon-carbon (RCC) segments that protect the orbiter's

devoted to later shuttle flights is going to come in our MPLM." On Flight Day 4, the station's robotic arm will lift Leonardo from the payload bay and attach it to the station's Unity Connecting Module. Then the station and shuttle crews will spend the next several days unloading the cargo and refilling Leonardo with trash, equipment and

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The Leonardo Multi-Purpose Logistics Module rests in Space Shuttle Endeavour's payload bay during a previous trip to the space station. Leonardo will make its fourth trip to the orbital outpost during STS-121.

nose cone and wing leading edges. The inspections will be done with an infrared camera, and the crew will use special sealants and plugs to repair simulated RCC segments in Discovery's payload bay. In addition to a new crewmember, Discovery will deliver supplies and equipment to the station. More than two tons of cargo will make the trip to the station inside NASA's Italian MultiPurpose Logistics Module (MPLM), known as Leonardo. This pressurized cargo compartment will ride in the space shuttle's payload bay along with the spare parts for the station. Kelly said that Leonardo will deliver a wide range of cargo to the station. "We've got a lot of supplies for the crew of the space station that will be inside," Kelly said. "We have some racks that will go in the U.S. Laboratory and then a lot of gear. Things tend to wear out in space, so we're bringing a lot of supplies that will replace things within the space station and on the outside of the space station, stuff that's

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The space station soars above the Earth after Space Shuttle Discovery undocked during STS-114 in August 2005. Discovery is scheduled to make its second consecutive visit to the station during STS-121.

experiments that will return to Earth. The robotic arm will return Leonardo to the payload bay before Discovery undocks. STS-121 will be Leonardo's fourth trip to the station. The STS-121 mission has a number of objectives. "It's going to be a full plate," Fossum said, "and there's no one thing that I could say would be my definition of mission success. We're going for all of it."

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High-flying junk

by Brad Thomas

In

addition to the International Space Station and other satellites, millions of objects are orbiting the Earth. The majority of these objects are the remnants of robotic and human spacecraft and are known as orbital debris, or space junk. The Department of Defense (DOD) uses radar to track debris, but not every piece can be tracked. It is important for NASA to know what the entire orbital debris environment is like and the associated risks. Since tracking only gives part of the picture, NASA's Orbital Debris Program attempts to paint the rest of it. "Our principal job is to characterize the orbital debris environment and analyze the risk to orbital spacecraft," said Nicholas Johnson, chief scientist and program manager for orbital debris. Orbital debris is defined as human-made materials orbiting the Earth that no longer fulfill a useful function. Space debris includes both orbital debris and meteoroids, which are natural in origin. "Everything now in orbit around the Earth is (human-made), except the moon," Johnson said. Johnson said that debris larger than 10 centimeters in diameter are tracked by the DOD's Space Surveillance Network, headquartered in

Cheyenne Mountain, Colorado Springs, Colo. The Orbital Debris Program office is responsible for assessing the environment for debris smaller than 10 centimeters. Currently, the DOD is tracking more than 13,000 objects that are larger than 10 centimeters. Johnson said that more than 100,000 pieces of orbital debris that are between one and 10 centimeters and millions of pieces smaller than one centimeters are circling the Earth. The threat that debris poses to spacecraft is one that NASA and the DOD take seriously. Johnson said Cheyenne Mountain personnel search every eight hours for objects that could come close to the station. If an object's orbit places it close enough to the orbital track of the station, with a greater than one in 10,000 chance that it will hit the station, Mission Control will initiate a collision avoidance maneuver. Johnson said an avoidance maneuver is performed about once a year. To date, if a collision avoidance maneuver had not been performed, the closest that a tracked piece of debris would have come to hitting the station was one kilometer in 2003. The Orbital Debris Program's work is not limited to human spaceflight endeavors. "Every NASA

John Opiela, orbital debris scientist, displays the distribution of space debris orbits around the Earth.

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project has to do an orbital debris assessment prior to launch," Johnson said. "Every report comes through our office and we evaluate if the reports are compliant--if not, we help them." Johnson said there are four basic principles that the orbital debris group checks. The first principle is the risk of inadvertently creating debris that is not necessary; the second is to limit the probability that an explosion can occur in space; the third involves studying what happens to a spacecraft or a rocket stage once its mission is completed; and the fourth is to look at the danger that any debris can pose to humans on the ground. Johnson said that all U.S. government agencies and the international community have adopted these four basic principles, which were formulated by NASA in the 1990s. The Orbital Debris Office also creates models to describe the environment. But before models can be created, data must be collected. Lead Measurements Scientist Gene Stansbery is one of the people who collects the data. "My job is to figure out how much junk is up there in space and to figure out where it is and where it is going," Stansbery said. Stansbery said the group uses information gleaned from statistical sampling based on data gathered from a worldwide network of radars, telescopes and other tools. The orbital debris group assesses the debris environment up to 40,000 kilometers. Stansbery said the greatest concentration of orbital debris is found at an altitude of 850 to 1,000 kilometers. Fortunately, the shuttle and station, which orbit the Earth at an altitude below 600 kilometers, are in an area that is less dense. "The shuttle and station are flying in an area where atmospheric drag cleans out debris," Stansbery said. "The issues we have with debris would be worse if they were at a higher altitude." After the data are collected and analyzed, the orbital debris team creates a series of models. Dr. Mark Matney, a space scientist, is the modeling lead.

Heather Rodriguez, orbital debris scientist, places a simulated orbital debris sample on a robotic arm to study its optical properties.

"The objects we see are the tip of the iceberg," Matney said. "There are a lot that are too small to track but just as serious." Since they are too small to track, one must fall back on statistical modeling. "We take the data from the instruments and turn it into useful information," Matney said. In addition to the environmental models, there are numerous other models. One of them is used to predict how much debris would be created by an explosion. Another model predicts what will happen in the debris environment if no further action is taken. The group also models what can happen during a shuttle mission. The models fill in the gaps not covered by known data. "Modeling is behind everything," Matney said. "We have to make sense of the data to hand off to the engineers." Matney said orbital debris should be a concern for all. "The orbital debris environment has no international boundaries," he said. Like pollution on Earth, orbital debris is a worldwide problem. Johnson, who is also the U.S. expert to the United Nations for

orbital debris, said that if orbital debris was left unchecked the problem would get worse. Johnson said that NASA and other groups are making progress on space operations with the guidelines and other steps. He credits the four guidelines that NASA adopted in the 1990s with the reduction of newly created orbital debris. "It is easier not to pollute than to clean it up," Johnson said. The Orbital Debris Program is also involved in identifying objects that fall back to Earth as well as predicting what objects will reenter the atmosphere. Four days before an object is forecast to fall back to Earth the DOD will begin providing information on the reentry point. Johnson said the group gets a few calls a year from people who claim to have pieces from a spacecraft, and in some instances they do. "We can confirm satellite debris within minutes," Johnson said. So far, orbital debris has not caused harm to humans or property on Earth's surface. "In almost 50 years, there have been no reports of injuries or significant damage," Johnson said.

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