The emerging demand for Earth observation (EO) to address environmental challenges has driven unprecedented growth in its primary carrier, Low Earth Orbit satellites, in recent years. Ground stations (GSs), the egress points of these networks, are congested due to the massive volume of EO traffic, and their deployment is constrained by geographic, political, and budgetary factors. Although inter-satellite links (ISLs) can partially relieve this congestion by forwarding traffic to alternative GSs, existing ISL-based approaches can hardly address traffic contention caused by biased GS distribution and may also raise sustainability concerns due to prolonged ISL paths. In this paper, we propose OrbitTransit, a pickup-carry-offload (PCO) approach that leverages satellite mobility for data \textit{delivery} and integrates ISLs for traffic \textit{diffusion} to alleviate the resource contention inherent in PCO delivery. The proposed orbit-as-node framework and contention-avoidant delivery jointly determine the optimal hybrid PCO-ISL path, minimizing energy consumption and balancing GS traffic. Extensive experiments show that OrbitTransit reduces battery consumption by $47.16\%$, decreases task failures by $1.09\times$, and improves GS load balancing compared with state-of-the-art GS selection and routing algorithms.

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