Re: Why the Soyuzes can't stay more than 6 months in space
He said the fuel meant to slow the spacecraft back to an earthbound path is made of H2O2 (heavy water) which has a tendency in space to decompose into H2O and O2 ; the water evaporates into space, risking not having enough fuel to return back.
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I think your excellent mastery of the English language failed you here. Heavy water is deuterium. H2O2 is hydrogen peroxide which does decompose to water and O2 as you stated.