After 10 years of planning and over 40,000 private donations worth $7 million, that idea took flight on Tuesday, as LightSail 2, a spacecraft built for the Planetary Society, co-founded by Mr. 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.

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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.