  The next sixth generation (6G) networks are envisioned to integrate sensing
and communications in a single system, thus greatly improving spectrum
utilization and reducing hardware costs. Low earth orbit (LEO) satellite
communications combined with massive multiple-input multiple-output (MIMO)
technology holds significant promise in offering ubiquitous and seamless
connectivity with high data rates. Existing integrated sensing and
communications (ISAC) studies mainly focus on terrestrial systems, while
operating ISAC in massive MIMO LEO satellite systems is promising to provide
high-capacity communication and flexible sensing ubiquitously. In this paper,
we first give an overview of LEO satellite systems and ISAC and consider
adopting ISAC in the massive MIMO LEO satellite systems. Then, the recent
research advances are presented. A discussion on related challenges and key
enabling technologies follows. Finally, we point out some open issues and
promising research directions.
