The biggest hurdle to mounting an interstellar mission may be humanity's short attention span. 

It'll take several decades of sustained, focused, coordinated and costly work to pull off a project such as Breakthrough Starshot, which plans to blast fleets of sail-equipped robotic nanocraft toward potentially life-supporting exoplanets at tremendous speeds using powerful, Earth-based lasers.

And our species doesn't exactly excel at taking the long view. [Breakthrough Starshot in Pictures: Laser-Sailing Nanocraft to Study Alien Planets]

"It took us a decade to go to the moon, and you could argue that that [relatively rapid pace] was largely because we had this — perceived at least — existential threat," Zac Manchester, a member of the Breakthrough Starshot advisory committee, said during the Breakthrough Discuss conference at Stanford University this past April. (That perceived threat, of course, was the Soviet Union, the United States' space-race rival.) 

"That's the part that worries me: What's going to keep us motivated, and keep us kind of organized, and keep us pushing forward?" added Manchester, who's an assistant professor of aeronautics and astronautics at Stanford.

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