# Satellite-Aided Consensus Protocol for Scalable Blockchains
## 1 Introduction  
Blockchain is currently blossoming in sectors such as internet of things (IoT) [1, 2], 6G wireless networking [3, 4], edge computing [5, 6], sensor networks [7], and smart city [8]. For such a secured distributed ledger technology, consensus protocols serve as the very foundation. Based on specific consensus mechanisms, blockchain spurs miners to participate in maintaining an ordered public ledger via proper financial incentives. PoW, the most popular and recognized consensus protocol, was first introduced in 1992 [9] and developing into a vital part of Bitcoin since its launch in 2008 [10]. The success of PoW-based cryptocurrencies in the last decade indicates its robustness and resilience against misbehaviors and adversarial attacks [10, 11]. However, PoW has an obvious weakness, as it consumes immense amounts of resources and energy. The estimated energy consumed by Bitcoin was around 70.05 TWh in 2020 [12], equivalent to the annual electricity consumption of the entire country of Czech Republic. Alternatives to PoW leverage other capabilities instead of intensive computations but often cause new security concerns [13]. Constrained by the consensus protocol (as well as the chain structure), the existing public blockchains exhibit scalability problems. Transactions per second (TPS), as the measure of information throughput in blockchain, is rather low in traditional blockchains, e.g., at most 7 TPS in Bitcoin and 15 TPS in Ethereum [14]. As a benchmark, centralized entities such as PayPal and VISA can achieve hundreds or thousands of TPS [15]. In short, high consumption and low scalability present two main barriers that severely constrain the advances of blockchain technologies. Currently, there are several works concerned with how to utilize blockchains for the space sector [16; 17; 18], such as mobile satellite communication networks (MSNET) [16] and satellite monitoring [17]. However, only a few works and projects were investigating the potential applicability of satellites on blockchain. In both of [19; 20], the authors proposed to use satellites as relays to help forward and propagate blocks in a space-terrestrial network. The project of BitSats [21] planned to turn satellites into crucial full Bitcoin nodes in space as early as 2014. Another project named SpaceChain [22] aims to bring an open-source decentralized software model to space and has leased two satellites in February and October of 2018, respectively. More recently, BlockStream [23] a startup in Canada, has launched six satellites to broadcast full Bitcoin blockchain for air-gapped areas across the globe. Another project ChainSat [24] has a similar plan to facilitate an off-grid network for enterprise blockchain ledgers via direct satellite communications. Space Impulse [25] and SupremeSat [26] also are reported to introduce blockchains into the space industry. Notably, satellites can bring substantial benefits to next-generation blockchain protocols because of the extensive coverage and ubiquitous connectivity. As standardized by, e.g., Digital Video Broadcasting Second Generation Satellite (DVB-S2) and Digital Video Broadcasting Second Generation Satellite Extensions (DVB-S2X) [27; 28], the satellites multicast transmission is now mature enough to deliver information to massive terrestrial receivers simultaneously, which would be helpful to solve the scalability issue of blockchain. (We will show how multicast transmissions affect the blockchain scalability in Section 5). Moreover, the latency of the terrestrial peer to peer (P2P) networks (e.g., the Internet) is often long-tailed due to the number of hops [29], whereas the propagation delay of satellite communications is nearly fixed and more controllable [19; 20]. Furthermore, satellite systems can significantly reduce the vulnerability against Internet attacks and network interrupts [30; 31]. Satellites are becoming the next game changer for blockchains.