By Shannon Stirone

New York Times News Service

The ability to sail across the cosmos, powered by the energy of the sun, is finally becoming a reality.

Engineers in California pressed a button Tuesday that unfurled the sails on a satellite that can be steered around Earth, advancing long-held hopes for an inexhaustible form of spaceflight and expanding the possibilities for navigating the voids between worlds.

For centuries, it was only a dream: traveling through space propelled by the sun’s photons. It was first imagined in the 1600s by Johannes Kepler, the German astronomer who described the laws of the planets’ orbits. In 1964, ­Arthur C. Clarke moved it into the realm of science fiction in “Sunjammer,” a short story. Carl Sagan, the cosmologist, believed it could be more than a speculative fantasy and in the 1970s began promoting the building of solar sails for space exploration.

After 10 years of planning and more than 40,000 private donations worth $7 million, that idea took flight Tuesday as LightSail 2, a spacecraft built for the Planetary Society, co-founded by Sagan, began what its creators hope will be a year of sailing around Earth.

“This is still one of the most feasible pathways to have real interstellar space travel in the future,” said Sasha Sagan, a writer as well as the daughter of the astronomer.

If it succeeds in its mission, it will contribute to overcoming one of the greatest limitations on the outer bounds of space travel — that the power that steers spacecraft, usually hydrazine fuel, eventually runs out.

In contrast, the sun is a source of constant energy. It is always releasing photons into space. While these particles don’t have mass, they have momentum. Solar sailing relies on the ever so gentle nudge of photons to push a sail forward, moving whatever is attached to the sail in the desired direction.

Other fuel sources, such as solar power and ion propulsion, can power spacecraft for decades, but solar sailing could eliminate their need for fuel altogether.

“There is a limitless supply of solar pressure,” said Dave Spencer, an aeronautics professor at Purdue University in West Lafayette, Indiana, and LightSail’s mission manager.

Sailing could be one of the most fuel efficient options for space travel. While the force exerted on a solar sail is about the same as what you might feel from the weight of a piece of paper in the palm of your hand, the momentum is able to build, increasing the speed of the sail over time.

The spacecraft was launched last month by a SpaceX Falcon Heavy rocket and has since been orbiting Earth while its managers on the ground prepared to unfurl its sails like a space-lotus.

“I’m really excited, things went just perfectly,” Spencer said.

During the deployment, two wide-angle cameras on the cubesat were to capture 32 images. “It will effectively give us a kind of movie of the sail deployment,” Spencer said.

LightSail 2 is the first steerable solar sail ever launched into orbit around Earth. Its solar sail is about the size of a boxing ring and made from a thin Mylar material. It blossomed Tuesday and began to collect the sun’s energy.

As LightSail orbits Earth, engineers on the ground will attempt to extend the farthest point in its orbit, called apogee. To do this, the sail must get enough of a push from the sun and also rely on steering from the ground.

“If everything goes perfectly, we ought to be able to raise the apogee by about 1,640 feet per day,” said Spencer in an interview before the deployment.

The primary mission is to last around a month, and after that LightSail 2 could orbit Earth for up to a year. Sometimes it will be visible from Earth with the naked eye, and the Planetary Society will provide updates on where it can be seen.

Eventually, Earth’s gravitational pull will drag the cubesat back toward the atmosphere, where it will burn up.

As the spacecraft’s name implies, LightSail 2 follows LightSail 1, launched in 2015 as a test. While accomplishing some of its goals, the test was hindered by a number of engineering snags.