Japan’s space agency, JAXA, has confirmed the failure of a mission intended to test technology for clearing up debris in space. The Kounotori 6 cargo transporter returned to Earth and burned up in the atmosphere on Monday, officials said. Though the experimental segment of the mission was a failure, Kounotori did successfully deliver supplies to the International Space Station after launching in December.

Kounotori 6 carried a 700-meter (2,296-foot) metal tether that was designed to slow down space junk and bring it back to Earth with electromagnetic force. JAXA says there was an issue with the mechanism to release the tether, however, and technicians were unable to fix it. It’s the second notable setback to hit JAXA in recent weeks after the agency failed to put its SS-520-4 rocket into orbit last month.

NASA estimates that there are about 500,000 pieces of debris larger than half an inch across in low orbit, posing a potential danger to the 780-odd satellites operating in the area. The ISS, for example, is shielded for objects over half an inch across, but can only track pieces of debris larger than two inches.