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Both are highly toxic as well as "hypergolic," which means they react violently when in contact with each other.

Mission controllers detected higher-than-appropriate amounts of “hypergolic fumes,” or fumes that could explode when coming in contact with one another.

Mission controllers detected higher-than-appropriate amounts of “hypergolic fumes,” or fumes that could explode when coming in contact with one another.

But hypergolic fuels must be handled carefully, Westmoreland said.

The very purpose of the SuperDraco engines — almost instantly spiriting away astronauts from the rocket during an emergency — explains the decision to use those so-called hypergolic propellants, said David Barnhart, director of USC’s space engineering research center.