Perhaps the most impressive thing about LightSail is that it was funded almost entirely by citizen donations to The Planetary Society, a non-profit space advocacy organisation co-founded by the late astronomer Carl Sagan and currently headed by Bill Nye, the Science Guy.  

"After six years of development, we're ready at last to see how LightSail flies," said Nye in a press release.  

"LightSail is technically wonderful, but it's also wonderfully romantic," he said. "We'll sail on sunbeams."

The planned launch is an important first step toward demonstrating solar sailing technology, which some people say is humanity's best bet to achieve interstellar space travel (even if it might take a sail the size of Texas or the help of some fancy, super accurate lasers to get beyond our Solar System). 

The LightSail spacecraft won't be up for any Matthew McConaughey-style interstellar missions just yet. In fact, this first flight won't even take it beyond Earth's orbit, or the limitations posed by atmospheric drag. Nevertheless, it's still pretty impressive.  

The LightSail consists of three 10cm by 10cm cubesats stacked together, and is about the size of a standard loaf of bread. Folded inside its external casing are four triangular sails. These reflective sails will unfurl along 4-metre-long retractable booms, creating a large 32-square metre reflective surface that will catch the Sun's energy and momentum-carrying photons - much like fabric sails on a boat catch wind.  

The sails have been made of Mylar, which is a type of aluminium-coated plastic film developed by chemical engineering company DuPont in the 1950s. It traps and reflects heat, and has been widely used by NASA - and other space agencies - to protect and insulate spacecraft. It's also the stuff used to make emergency blankets. The Mylar sheets employed on LightSail are about one-quarter the thickness of a plastic garbage bag. 

The Planetary Society's new LightSail website explains the theory behind solar sailing: 

"Light is made of packets of energy called photons. While photons have no mass, a photon traveling as a packet of light has energy and momentum. Solar sail spacecraft capture light momentum with large, lightweight mirrored surfaces—sails. As light reflects off a sail, most of its momentum is transferred, pushing on the sail. The resulting acceleration is small, but continuous. Unlike chemical rockets that provide short bursts of thrust, solar sails thrust continuously and can reach higher speeds over time." 

When the sails are deployed (likely sometime in June, depending on the launch date), the craft will become visible from Earth. 

And while this test flight won't demonstrate true solar sailing, it will pave the way for a second test in 2016

This second flight will be on a SpaceX Falcon Heavy rocket, and will reach an altitude where solar radiation can be used to maneuver the craft. 

In 2005, the Planetary Society attempted to launch its first solar sail spacecraft, known as the Cosmos 1. However, 83 seconds into the launch, a rocket failure ended the mission.   

As IEEE's Spectrum reports, since that failed attempt, "other spacecraft have used radiation pressure to adjust their orbits," and there have been successful solar sail missions.

In 2010, Japan's space agency, JAXA, launched a solar sail-powered craft called IKAROS on a mission past Venus, which was eventually meant to travel around the Sun. This bold demonstration was followed by NASA successfully deploying NanoSail-D in low-Earth orbit in 2011. 

Source: The Planetary Society IEEE's Spectrum