Basically the satellite-tracking version of IPv4 exhaustion — five-digit NORAD catalog IDs cap at 99999, and mega-constellations are burning through the remaining space fast.

I think the number actually runs out at 69999:

https://celestrak.org/NORAD/documentation/gp-data-formats.ph...

In the linked bluesky thread, they have been estimating when they will need 6 digit numbers, and seems like it was a few days earlier than their previous prediction.

https://bsky.app/profile/tskelso.bsky.social/post/3mlg2dpuqd...