This paper proposes a proactive fragmentation mechanism for an optimal DTN message transfer. DTN message transfer is presented to give coverage to a terrestrial sensor network by using a nanosatellite communications link. Satellite communication is modeled with a multi-access mechanism with priority in downlink traffic (ALOHAGP). This new multiple access protocol is based on unslotted ALOHA extended with an adaptive contention mechanism. It uses satellite feedback to implement the congestion control, and to dynamically adapt the channel effective throughput in an optimal way. The effective throughput has been optimized by adapting the protocol parameters as a function of the current number of active sensors received from satellite. Also, regarding the optimal size of a bundle, there is a lack of standard negotiation methods of bundle sizes that can be accepted by a bundle agent in satellite communications. Thus, too large bundles are dropped and too small messages are inefficient. We have characterized this kind of scenario obtaining a probability distribution for frame arrivals to nanosatellite and visibility time distribution providing an optimal proactive fragmentation of DTN bundles. We have found that the proactive effective throughput (goodput) reaches in some cases almost 97% of the reactive fragmentation approach.