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Members of the LightSail 2 team declared their mission a success in a teleconference Wednesday. The citizen-funded spacecraft is the highest-performing solar sail to date and the first to demonstrate the ability to orbit Earth in a controlled way. 
 
"This is a very exciting day for us, and for me personally," said Bill Nye, chief executive officer  of the Planetary Society, the organization behind the mission. "This idea that you could fly a spacecraft with nothing but photons is surprising, and for me, it's very romantic that you could be sailing on sunbeams." 
 
LightSail 2 is the latest demonstration of solar sail technology, which uses the gentle pressure of photons — the particles of light — on a lightweight, reflective surface to propel a craft through space, similar to the way the wind pushes a sailing ship across the ocean. However, instead of canvas, solar sails are made of thin sheets of Mylar, the same crinkly silver material often used for helium-filled balloons. 

Faster speeds
 
Although the pressure of the sun's rays is no greater than the weight of a paperclip dropping on the sail, sunlight is a constant source of energy. Scientists expect that as long as sunlight reaches them, solar sails will keep accelerating to much higher speeds than what is provided by traditional propulsion methods using chemical or nuclear fuel. 
 
By tracking the location of the spacecraft, the team found that it had traveled 1.7 kilometers (1.1 miles) farther from Earth in just four days thanks to the gentle influence of sunlight. This is the first time solar propulsion has been successfully demonstrated in Earth's orbit. 
 
The technology has been tested before. In 2010, the Japan Aerospace Exploration Agency launched a spacecraft called IKAROS, which used a solar sail to propel it past Venus and into orbit around the Sun. 
 
The Planetary Society, which aims to advance space exploration, deployed its first solar sail in 2015. The LightSail 1 mission successfully unfurled a solar sail before re-entering Earth's atmosphere a week later. 
 
LightSail 2 follows the same trajectory. The $7 million project was funded by Planetary Society members as well as individuals who contributed to a Kickstarter campaign in 2015. 
 
The goal of this project is to demonstrate that solar sails can be used to propel small satellites called CubeSats. These tiny satellites weigh as little as 1 kilogram and can carry scientific instruments like cameras. Specifically, LightSail 2 is carrying a 5-kilogram (11-pound) CubeSat into a controlled orbit around Earth. 

Small package
 
LightSail 2 was launched on June 25, 2019, carefully folded into a spacecraft the size of a loaf of bread. Last week, it successfully unfolded to its full 32-square-meter (344-square-foot) extent — about the size of a boxing ring. 
 
The spacecraft orbits Earth along an elliptical path. Propelled by sunlight, the spacecraft will rise to a higher orbit through Aug. 23, 2019. As the maximum distance between Earth and LightSail 2 increases, part of its orbit will inch closer to Earth. Eventually, the spacecraft will dip low enough into the atmosphere that it will begin to slow down, re-enter Earth's atmosphere and burn up, concluding its mission. 
 
The team members acknowledged the mission's 50,000 financial backers, who hail from 109 countries. Jennifer Vaughn, chief operating officer of the Planetary Society, thanked the people who "have the dream and who are willing to put down their own financial support to make it happen." 
 
LightSail 2's success is encouraging for the future of solar sailing. Nye said solar sails may enable us to travel to distant destinations in the solar system, including his personal goal to "ferry cargo to Mars, look for signs of life and change the course of human history." 
 
Nye added that LightSail 2 is part of the bigger idea that humanity seeks to explore the universe and understand our place in it. 
 
"Space exploration brings out the best in us," he said.