Don’t peek at the answer until you’ve given this a good try! Then, scroll down a bit and the truth will be revealed to you.

(Image Credit: NASA/MSFC Illustration)

The schematics to a graduation cap? A wind surfer in space? Well, the second guess is the closest. It’s an illustration of an interstellar probe being pushed along by sunlight reflected from an ultrathin sail. It’s a concept currently being explored by NASA centers, though the idea itself dates back nearly 400 years ago when astronomer Johannes Kepler observed comet tails being blown by a solar breeze; Kepler suggested that vessels might likewise navigate through space using appropriately fashioned sails. In fact, it’s all about "pressure." Astronomers now widely recognize that sunlight does indeed produce a force which moves comet tails and a large, reflective sail could be a practical means of propelling a spacecraft.

Such a solar sail would measure half a kilometer wide. but the sail itself would not be unfurled until the spacecraft was out in space. According to Robert Nemiriff (Michigan Technological University) and Jerry Bonnell (Universities Space Research Association) continuous pressure from sunlight would ultimately accelerate the craft to speeds about five times higher than possible with conventional rockets — without requiring any fuel! If launched in 2010 such a probe could overtake Voyager 1, the most distant spacecraft bound for interstellar space, in 2018 going as far in eight years as the Voyager will have journeyed in 41 years.