  Low Earth Orbit (LEO) satellites undergo a period of rapid development driven
by ever-increasing user demands, reduced costs, and technological progress.
Since there is a paucity of literature on the security and reliability issues
of LEO Satellite Communication Systems (SCSs), we aim to fill this knowledge
gap. Specifically, we critically appraise the inherent characteristics of LEO
SCSs and elaborate on their security and reliability requirements. In light of
this, we further discuss their vulnerabilities, including potential security
attacks launched against them and reliability risks, followed by outlining the
associated lessons learned. Subsequently, we discuss the corresponding security
and reliability enhancement solutions, unveil a range of trade-offs, and
summarize the lessons gleaned. Furthermore, we shed light on several promising
future research directions for enhancing the security and reliability of LEO
SCSs, such as integrated sensing and communication, computer vision aided
communications, as well as challenges brought about by mega-constellation and
commercialization. Finally, we summarize the lessons inferred and crystallize
the take-away messages in our design guidelines.
