A software flaw had caused a file of telemetry data to grow without bound. When that part of its memory filled up, the computer crashed, with no one around to press the restart button.

LightSail’s systems had been tested on the ground, but not quite long enough to turn up the computer bug, which occurred after about 40 hours.

Fortunately, the problem arose in space, where high-energy particles like protons from the sun and cosmic rays from distant supernovas whiz around all the time. Usually they are a nuisance to space missions, garbling computer data and calculations.

But in this case, engineers expected that a high-energy particle would jolt LightSail’s memory, causing it to restart.

After eight days, that is what happened, and a radio chirp was detected at the ground station at California State Polytechnic University.

“Jubilation was my response,” said Barbara Plante, the lead software engineer and the president of Boreal Space, a contractor involved with the project. “We were all monitoring the pass when the data came in. It was quite a relief. That just felt like the mission was going to go forward.”

To prevent the problem from recurring, the team is restarting LightSail’s computer about once a day. The team also decided to accelerate deployment of the solar sail — four large triangular pieces of Mylar stretched out between four 13-foot booms.