Japanese space agency JAXA has confirmed a major disappointment in a mission to test technology for cleaning up space junk: the Kounotori 6 cargo transported burned up in the atmosphere while returning to Earth on Monday. The mission was really a failure, as the attempt to return the transporter was experimental and it completed its main mission of delivering supplies to the International Space Station successfully.

Kounotori 6 had a 2,300-foot metal tether to slow down space junk and bring it back into the atmosphere with electromagnetic force, but there was a problem with the tether’s release. It’s the first major failure for JAXA since the SS-520-4 rocket failed to get into orbit last month.

There are an estimated 500,000 pieces of space debris larger than half an inch orbiting our Earth, which creates a risk to the 800 or so satellites operating in low-earth Orbit. The ISS must be shielded from objects but has difficulty picking up pieces that are smaller than two inches, and as more stuff gets put into space, the space junk problem worsens, leading authorities to try to figure out ways to clean it up.