A notebook owned by Leonardo da Vinci more than 500 years ago contained his concept for a parachute. His design used cloth stretched over a rigid, pyramid-shaped frame to create drag. He never tested it, but others did, much later, and it basically worked. It wasn’t exactly like the parachutes that would be perfected in the four centuries after his death, but the principle on which it operated—harnessing atmospheric friction to create drag and resist freefall—was the same.

That principle may work on da Vinci’s planet, with its thick, protective atmosphere. But what if you’re trying to drop something to the ground elsewhere in the solar system, like Mars? Creating enough drag to decelerate a spacecraft to a safe touchdown speed in that planet’s thin, shallow atmosphere is about as easy as trying to sew yourself a parachute after you’ve already jumped.

This is the problem that plagues Rob Manning, a flight systems engineer who’s been around NASA’s Jet Propulsion Laboratory long enough to see two decades of Mars missions firsthand (“The Mars Dilemma,” Oct./Nov. 2014). Now he’s on the team trying to figure out how to get the next generation of landers—the precursors to human missions—safely to the surface.

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