Delay-/disruption-tolerant networking (DTN) offers a solution to highly stressed communications in space environments. Very little work has been done in investigating how to accommodate highly asymmetric space communication channels for high performance of DTN transmission in space. We study the effect of aggregation of data bundles in space communications characterized by asymmetric and low channel rates and intend to find the number of bundles which should be aggregated within a block for the best performance over highly asymmetric cislunar channel rates. A channel ratio (CR) threshold is derived considering the lengths of an aggregated data block and an acknowledgment (ACK) packet and channel rates, and it must be maintained to avoid delay of ACK transmission of DTN Licklider transmission protocol (LTP) due to space channel asymmetry.