For reasons not understood, LightSail ignored the command.

“I was despondent,” Mr. Nye said. “I was a little down. This mission has had me down many times.”

On the next orbit, about two hours later — the last chance for the day — the team sent the command again. The electric motor that was to extend four 13-foot booms to pull out almost 345 square feet of Mylar started turning.

When the spacecraft passed out of radio range, the tiny motor had turned 67,000 times, halfway to the 134,200 needed to fully deploy the sail. “There was no reason to expect it wouldn’t keep going,” Mr. Nye said.

On Monday, the spacecraft is to send down photographs to confirm that the sail is spread out.

The technology, using sunlight to traverse the solar system in the same way mariners once crossed oceans in sailing ships, is not a new idea, but it has not been widely used. While particles of light impart only a smidgen of momentum, the force is continuous and provides propulsion without fuel.

LightSail, packed into a box about the size of a loaf of bread, was one of 10 payloads that last month hitchhiked on a rocket that took an unmanned United States Air Force space plane into orbit. LightSail was successfully deployed and worked for two days before its computer crashed because of a software flaw.