## 1 Introduction  
A satellite network is a type of communication network that can provide beyond-line-of-sight communication services [1]. It consists of a space segment, a ground segment and a user segment. The space segment is mainly composed of a satellite (s) that has communication functions, such as signal processing and routing. The ground segment is mainly composed of gateway stations that can interconnect with terrestrial networks, such as the 4G/5G mobile network. The user segment mainly contains satellite terminals used by end users. It is widely accepted that the 6G network is a space-ground integrated network. As such, the satellite network is an important part of 6G [2]. Compared with traditional geosynchronous (GEO) satellite networks, the low-Earth-orbit (LEO) network has some intrinsic advantages, such as relatively low propagation latency between the terminal and the LEO satellite, seamless global coverage and so on [3]. Consequently, in recent years, the LEO satellite network has achieved considerable development. Many companies, such as SpaceX, OneWeb, Amazon and Telesat, have deployed their own LEO satellite networks [4]. These networks are expected to meet the growing demand for reliable and accessible Internet connection worldwide. Despite the fact that the LEO network has many advantages and has attracted much attention in recent years, it cannot provide high-quality services in some scenarios:  
* Computation offloading: If a UAV or a user terminal wants to offload computation-intensive tasks to the network, it has to offload the task to the cloud on the ground. However, the end-to-end delay between the user and the cloud is usually very large because the distances between user links (between satellite terminal and the access satellite) and inter-satellite links (ISLs) are very long, which means that the signal propagation delay is very large;
* Remote-sensing data processing: Traditionally, after the remote-sensing payload on the UAV or after the satellite obtains the original data, the UAV or the satellite has to transmit these original/preprocessed data to the cloud on the ground where the data are processed and useful information is obtained [5]. However, these original/preprocessed data are usually very large. For example, the size of a single image taken by a Gaofen-1 satellite is up to several gigabits, and the size of satellite-based synthetic-aperture radar (SAR) echo data for a single image is up to 50 Gbits [6]. As remote-sensing technology evolves, this size becomes larger and larger. This type of remote-sensing data transmission presses hard upon the ISLs and feeder links. Therefore, we believe that it is necessary to introduce edge computing into LEO networks, which conforms to the trends in the integration of computation and the networking of the 6G system [7]. By deploying computation and storage resource on the LEO satellite, it is possible to build a collaborative space-based edge-computing system that is composed of many satellite-based computing platforms. Introducing edge computing into the LEO network can yield obvious benefits:  
* Low-delay computation offloading: By offloading computation tasks to the LEO satellite (instead of the cloud on the ground), which is the edge of the LEO network, it is possible to support users' latency-sensitive, computationally intensive applications;
* Data analysis nearby the source: By processing the remote-sensing data nearby the data sources (the UAV-based or satellite-based remote-sensing payload), the bandwidth occupation for transmitting huge amounts of original/preprocessed data is saved.