<p>A mysterious leak of coolant on the international space station compelled two astronauts to make an extraordinary, unplanned spacewalk Saturday in which they tried in vain to find the source of the streaming ammonia....</p>
<p>“So far no smoking gun, an hour and a half into the spacewalk,” an announcer said on NASA TV, which streamed images of the astronauts patrolling a truss of the station and aiming their cameras at the innards of machinery packed with tubes and wires.</p>