An astronaut apologized on Thursday for a data tampering incident that occurred a few years back, with the country's space agency issuing a warning to the individual who is slated for a second stint on the International Space Station later this year.

Satoshi Furukawa, 58, the third Japanese person to have completed a long-term mission in space, was responsible for an experiment conducted between 2016 and 2017 that simulated life on the ISS. He prioritized conducting research over immediately reporting a mistake, a source linked to the experiment said earlier.

"I sincerely apologize for undermining the trust of the people," Furukawa said during a news conference in Tokyo. On his planned second stint on the ISS, he said, "I would like to carry out my assigned duties faithfully."

The experiment assessed the stress levels and mental well-being of people confined for about two weeks in a closed environment, meant to recreate the conditions inside a space station, in Tsukuba, Ibaraki Prefecture.

Two researchers who conducted interviews to ascertain the mental state of the participants fabricated data, including making it seem as if other researchers had also participated. Furukawa had supervisory responsibility for the experiment.

The Japan Aerospace Exploration Agency publicly revealed in November that data tampering had occurred.

The source has said the mistake, discovered in January 2018, was reported to Furukawa, but he did not convey the information to JAXA's ethics committee immediately.

Some JAXA executives, including President Hiroshi Yamakawa, were also reprimanded over the case.

"We lost trustworthiness in our research because of lackluster data management," Yamakawa said in a separate statement. "We apologize for not having arranged an appropriate research environment."