Constellations of sensor equipped pico-satellites hold the promise to be the replacement of traditional and cost ineffective monolithic space missions. The similarity of pico-satellite constellations with Wireless Sensor Networks (WSNs) facilitates the reuse of existing networking techniques already employed in terrestrial applications. However, the protocol mechanisms can cause communications to fail due to the large propagation delay experienced in space. This paper focuses on the Medium Access Control (MAC) and shows how the non negligible propagation delay degrades the performance of a MAC protocol used in terrestrial sensor networks. The paper also demonstrates how a slight modification improves the protocol's performance