Spacechains
Links
Introduction
Spacechains are a model for creating blockchains that attach themselves to Bitcoin through a blind merged mining mechanism that doesn't require Bitcoin miners or nodes to be aware of the existence of these blockchains, they just see normal Bitcoin transactions that may (or not) overpay in fees (due to the economics of spacechain mining) and include these in their blocks.
Spacechains can be used to emulate all kinds of protocols that require a blockchain, like protocols for transferring fungible and non-fungible tokens, open decentralized DNS replacements, single-use seals and other ideas that involve ownership and require double-spend protection.
Many of these ideas have been tested already in altcoins, but invariably these include a native currency to be mined or staked in order for the blockchain to be operated, which inevitably perverts the incentives and plague the usefulness of these ideas. Spacechains' main benefit is that they don't require any native currency, although these can also be used with safeguards against pump-and-dump scams.
Technical Requirements
It is possible to build an inefficient, fragile version of a Spacechain without covenants, but it only becomes a serious idea once it uses some kind of covenant that allows the chain of BMM transactions to be immutable, which requires either APO or TXHASH. CTV could also work, although with a tradeoff that makes BMM riskier and costlier, so it is it less appealing.