Hydrazine is similar to chlorine or ammonia in that it affects the lungs and breathing tissue.

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The hazardous compounds are: two forms of hydrazine, nitrogen tetroxide and a pure form of ammonia.

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Raw hydrazine fueled the shuttle's auxiliary power units, which generated hydraulic pressure for various functions in the shuttle.

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The satellite, fueled with hydrazine, could have posed health risks had the satellite landed in a populated area.

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The Air Force also will have a plane in the air that can detect the release of hydrazine gas.

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The microbes didn't use the hydrazine to launch themselves into orbit, but as an intermediate step in a larger process.

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"The intent here was to preserve human life... it was the hydrazine we were after, " Gen Cartwright said on Thursday.

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Here were bugs that created hydrazine, an extremely unstable compound, used as rocket fuel, that is poison to most living things.

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Its engines are fueled partly by methane, which the agency says is easier to handle and store than other propellants such as liquid hydrogen or hydrazine.

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Nitrogen tetroxide and monomethyl hydrazine are "hypergolic" propellants, meaning they ignite spontaneously when they come in contact with each other, a useful property for maneuvering systems.

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The satellite is likely to burn up during reentry, but there's concern that it could contain hydrazine fuel, which may be hazardous to people on the ground.

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Another form of the chemical, monomethyl hydrazine, fueled the shuttle's maneuvering systems while it was in space, said Randy Azera, a former NASA engineer and consultant to CNN.

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These tests apparently showed that the system used to ignite the craft's hydrazine fuel could not operate effectively at the cold temperatures expected during the flight to Mars.

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Both forms of hydrazine are clear, flammable liquids.

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It depends on hypersonic gliding maneuvers, the largest supersonic parachute ever deployed, eight hydrazine rocket engines firing in sequence and an elaborate system of tethers called a "sky crane" that are meant to lower the landing craft gently to the ground.

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