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The National Aeronautics and Space Administration (NASA) Kennedy Space Center (KSC) requires accurate gas mixtures containing argon (Ar), helium (He), hydrogen ([H.sub.2]), and oxygen ([O.sub.2]) in a balance of nitrogen ([N.sub.2]) to calibrate mass spectrometer-based sensors used around their manned and unmanned space vehicles. This also includes space shuttle monitoring around the launch area and inside the shuttle cabin. NASA was in need of these gas mixtures to ensure the safety of the shuttle cabin and the launch system. In 1993, the National Institute of Standards and Technology (NIST) was contracted by NASA to develop a suite of primary standard mixtures (PSMs) containing helium, hydrogen, argon, and oxygen in a balance gas of nitrogen. NIST proceeded to develop a suite of 20 new gravimetric primary PSMs. At the same time NIST contracted Scott Specialty Gases (Plumsteadville, PA) to prepare 18 cylinder gas mixtures which were then sent to NIST. NIST used their newly prepared PSMs to assign concentration values ranging from 100 to 10 000 [micro]mol/mol with relative expanded uncertainties (95% confidence interval) of 0.8-10% to the 18 Scott Specialty Gases prepared mixtures. A total of 12 of the mixtures were sent to NASA as NIST traceable standards for calibration of their mass spectrometers. The remaining 6 AIRGAS mixtures were retained at NIST. In 2000, these original 12 gas standards at NASA had become low in pressure and additionally NASA needed a lower concentration level; therefore, NIST was contracted to certify three new sets of gas standards. NIST prepared a new suite of 22 PSMs with weighing uncertainties of

During the space shuttle era, policy makers have repeatedly wrestled with the issue of fleet size. The number of shuttles had both practical and symbolic significance, reflecting the robustness of the space transportation system and US preeminence in space. In debating how many shuttles were needed, NASA and other government entities weighed various arguments to determine the optimum number of vehicles for human spaceflight. Deliberations and decisions about shuttle fleet size reflected changing policy priorities and attitudes about the role of the shuttle. That history frames issues that may arise again in planning for new space transportation vehicles beyond the shuttle.