What, exactly, makes a rocket fuel environmentally friendly?

Automobiles aren’t the only vehicles turning to more environmentally friendly fuel sources. As we reported recently, NASA are testing a new type of rocket propellant made of a mixture of water and “nanoscale aluminum” powder they claim could provide a cleaner way to launch rockets, power long-distance space missions and generate hydrogen for fuel cells. A number of readers wondered, not unreasonably, what qualifies a rocket fuel as eco-friendly. We now have a few more answers.

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