  Computing tasks are ubiquitous in space missions. Conventionally, these tasks
are offloaded to ground servers for computation, where the transmission of raw
data on satellite-to-ground links severely constrains the performance. To
overcome this limitation, recent works offload tasks to visible low-earth-orbit
(LEO) satellites. However, this offloading scheme is difficult to achieve good
performance in actual networks with uneven loads because visible satellites
over hotspots tend to be overloaded. Therefore, it is urgent to extend the
offloading targets to the entire network.
  To address the network-wide offloading problem, we propose a metagraph-based
computation and transmission fusion offloading scheme for multi-tier networks.
Specifically, virtual edges, between the original network and its duplicate,
are generated to fuse computation and transmission in single-tier networks.
Then, a metagraph is employed to integrate the fusion in multi-tier networks.
After assigning appropriate edge weights to the metagraph, the network-wide
offloading problem can be solved by searching the shortest path.
  In addition, we apply the proposed scheme to solve the spatial computation
offloading problem in a real multi-tier network. The superiority of the
proposed scheme over two benchmark schemes are proved theoretically and
empirically.
  Simulation results show that the proposed scheme decreases the weighted
average delay by up to 87.51% and 18.70% compared with the ground offloading
scheme and the visible offloading scheme, respectively.
