It's a type of sidechain that convinced two idea, one very good and one very bad

The very good idea is called blind merge mining, the inventor of spacechains (Ruben Somsen) figured out a way to do it without a soft fork

The very bad idea is that in order to get bitcoins "onto" the spacechain, you destroy them, like literally you provably delete your bitcoins... And once they are on the spacechain you can never get them back again

I thought they were interesting enough to implement and someday I hope to fix the "destroy your bitcoins" part