  Low Earth orbit (LEO) satellites have been envisioned as a significant
component of the sixth generation (6G) network architecture for achieving
ubiquitous coverage and seamless access. However, the implementation of LEO
satellites is largely restricted by the deployment of ground stations.
Inter-satellite links (ISLs) have been regarded as a promising technique to
fully exploit the potentials of LEO mega constellations by concatenating
multiple satellites to constitute an autonomous space network. In this article,
we present the merits of implementing ISLs in LEO mega constellations and the
representative applications empowered/inspired by ISLs. Moreover, we outline
several key technical challenges as well as potential solutions related to LEO
satellite networks with ISLs, including performance analysis for system design,
routing and load balancing, and resource allocation. Particularly, the
potential of using ISLs in enhancing in-flight connectivity is showcased with a
preliminary performance evaluation. Finally, some open issues are discussed to
inspire future research.
