They include Michael Coplan, a physicist at the University of Maryland who, along with his students, helped build the ion composition instrument, which counted different types of charged atoms. Dr. Coplan had largely forgotten about the experiment after they received the last data in the late 1980s. This year, to clear space in a laboratory he would be sharing with another scientist, he threw out his ISEE-3 data notebooks.

Then, in June, one of his former students heard about the reboot project and told Dr. Coplan, who went to the waste bin and found the notebooks. “That sat around for a while, fortunately,” he said.

Mr. Wingo said scientists were already comparing ISEE-3’s data with observations from newer spacecraft. “We’re already in touch with the larger scientific community,” he said.

He said his team should be able to send commands to the craft for four more months before it is too far away, and moderate-size radio telescopes will be able to hear the spacecraft’s signals for at least the next year or two.

“There is still much we need to learn about the sun, and ISEE-3 will continue to contribute to gathering science data,” Mr. Wingo said.

Still a mystery is what happened to the nitrogen, especially as the first firing of the thrusters, on July 2, worked perfectly. Firings on July 8 also initially seemed to work, but then stopped. Two weeks of troubleshooting indicated that everything was working properly and the spacecraft had plenty of fuel.

But the nitrogen tanks were empty. “All of the options are equally unlikely, but one of them has to be the case,” Mr. Wingo said.