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Space industry upstarts are deploying thousands of satellites to offer global Internet service. These plans promise large improvements in coverage and latency, and could fundamentally transform the Internet. But what if this transformation extends beyond network transit into a new type of computing service? What if each satellite, in addition to serving as a network router, also offers cloud-like compute, making the new constellations not just global Internet service providers, but at the same time, a new breed of cloud providers offering "compute where you need it"? We examine, qualitatively and quantitatively, the opportunities and challenges of such in-orbit computing. Several applications could benefit from it, including content distribution and edge computing; multiuser gaming, co-immersion, and collaborative music; and processing space-native data. Adding computing hardware to a satellite does not seem prohibitive in terms of weight, volume, and space hardening, but the required power draw could be substantial. Another challenge stems from the dynamics of low Earth orbit: a specific satellite is only visible to a ground station for minutes at a time, thus requiring care in managing stateful applications. Our exploration of these trade-offs suggests that this "outlandish" proposition should not be casually dismissed, and may merit deeper engagement from the research community. CCS CONCEPTS • Networks → Cloud computing.

Social bookmarking communities are now major content production platforms. There, millions of users interact every day on a great variety of knowledge domains, creating new contents, linking to existing ones, and engaging in constructive discussions. Relevant domain-specific content is often mixed with less useful contributions, and domain experts often have to find their way through lurkers and Web trolls. Such a diversity in topics and quality is a distinctive property of this class of Web sites. This diversity interferes with the ability to locate relevant content and users, and this hinders the usage of social bookmarking communities for tasks such as structured knowledge creation, or crowdsourcing. In this paper we investigate how relevant domainspecific content, in the form of submissions shared by (expert) users, can be effectively located in the social bookmarking platform reddit. We contribute with a framework process for the identification and characterisation of domainspecific content and knowledgeable users, and apply it to the reddit platform. Our work provides novel insights into the properties and dynamics of reddit, and represents an important step towards a better use of social bookmarking communities as a source of knowledge and expertise.

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