  With the rapid evolution of space-borne capabilities, space edge computing
(SEC) is becoming a new computation paradigm for future integrated space and
terrestrial networks. Satellite edges adopt advanced on-board hardware, which
not only enables new opportunities to perform complex intelligent tasks in
orbit, but also involves new challenges due to the additional energy
consumption in power-constrained space environment. In this paper, we present
PHOENIX, an energy-efficient task scheduling framework for emerging SEC
networks. PHOENIX exploits a key insight that in the SEC network, there always
exist a number of sunlit edges which are illuminated during the entire orbital
period and have sufficient energy supplement from the sun. PHOENIX accomplishes
energy-efficient in-orbit computing by judiciously offloading space tasks to
"sunlight-sufficient" edges or to the ground. Specifically, PHOENIX first
formulates the SEC battery energy optimizing (SBEO) problem which aims at
minimizing the average battery energy consumption while satisfying various task
completion constraints. Then PHOENIX incorporates a sunlight-aware scheduling
mechanism to solve the SBEO problem and schedule SEC tasks efficiently.
Finally, we implement a PHOENIX prototype and build an SEC testbed. Extensive
data-driven evaluations demonstrate that as compared to other state-of-the-art
solutions, PHOENIX can effectively reduce up to 54.8% SEC battery energy
consumption and prolong battery lifetime to 2.9$\times$ while still completing
tasks on time.
