At a time when the proof-of-work technique for the Bitcoin blockchain consumes as much electricity as the entire country of Denmark, and other techniques require a 2/3 majority of "stake," or fraction of ownership, yet can still be subverted, there is a simple energy-parsimonious solution to ensure the integrity of blockchains that, incidentally, also gives rise to some cool puzzles.
The idea is to implement a broadcast network through a satellite (or one or more cubesats), here we call the "Primary." Users sign their transactions and send them to satellites that bounce them to the Primary, which collects all the transactions received during some time period, say, one second, into a block, signs the block with its private key, and includes the hash of that block and the hash of the previous block. The Primary then sends the block, possibly partitioned into packets, to many sites on Earth.
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