COST- BENEFIT ANALYSIS OF TWO DISSIMILAR WARM STANDBY SPACE SHUTTLE SYSTEM SUBJECT FAILURE DUE TO DAMAGE OF THE THERMAL PROTECTION SYSTEM , ATMOSPHERIC HOT GAS AND DEPLETION OF OZONE LAYER

Ozone depletion describes two distinct but related phenomena observed since the late 1970s: a steady decline of about 4% per decade in the total volume of ozone in Earth's stratosphere (the ozone layer), and a much larger springtime decrease in stratospheric ozone over Earth's polar regions. The latter phenomenon is referred to as the ozone hole. In addition to these well(known stratospheric phenomena, there are also springtime polar troposphere ozone depletion events. The details of polar ozone hole formation differ from that of mid(latitude thinning, but the most important process in both is catalytic destruction of ozone by atomic halogens. The main source of these halogen atoms in the stratosphere is photo dissociation of man(made halocarbon refrigerants, solvents, propellants, and foam(blowing agents (CFCs, HCFCs, freons, halons). These compounds are transported into the stratosphere after being emitted at the surface. Both types of ozone depletion have been observed to increase as emissions of halo(carbons increased. CFCs and other contributory substances are referred to as ozone(depleting substances (ODS). Since the ozone layer prevents most harmful UVB wavelengths (280–315 nm) of ultraviolet light (UV light) from passing through the Earth's atmosphere, observed and projected decreases in ozone have generated worldwide concern leading to adoption of the Montreal Protocol that bans the production of CFCs, halons, and other ozone(depleting chemicals such as carbon tetrachloride and trichloroethane. It is suspected that a variety of biological consequences such as increases in skin cancer, cataracts, damage to plants, and reduction of plankton populations in the ocean's photic zone may result from the increased UV exposure due to ozone depletion. The Space Shuttle Columbia disaster occurred on February 1, 2003, when Columbia disintegrated over Texas and Louisiana as it reentered Earth's atmosphere, killing all seven crew members. During the launch of STS(107, Columbia' s 28th mission, a piece of foam insulation broke off from the Space Shuttle external tank and struck the left wing. Most previous shuttle launches had seen minor damage from foam shedding, but some engineers suspected that the damage to Columbia was more serious. NASA managers limited the investigation, reasoning that the crew could not have fixed the problem if it had been confirmed. When the Shuttle reentered the atmosphere of Earth, the damage allowed hot atmospheric gases to penetrate and destroy the internal wing structure, which caused the spacecraft to become unstable and slowly break apart. After the disaster, Space Shuttle flight operations were suspended for more than two years, similar to the aftermath of the Challenger disaster. Construction of the International Space Station (ISS) was put on hold; the station relied entirely on the Russian Federal Space Agency for resupply for 29 months until Shuttle flights resumed with STS(114 and 41 months for crew rotation until STS(121. Several technical and organizational changes were made, including adding a thorough on(orbit inspection to determine how well the shuttle's thermal protection system had endured the ascent, and keeping a designated rescue mission ready in case irreparable damage was found. Except for one final mission to repair the Hubble Space Telescope, subsequent missions were flown only to the ISS so that the crew could use it as a safe haven.

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