The design of random media access control (MAC) protocol renews great attention for emerging challenged wireless environments, where the propagation delay is long, heterogeneous, and/or varying, such as satellite or underwater acoustic sensor networks. In these environments, the existing MAC solutions based on slotted transmissions, carrier sensing, or channel reservation by control packets are no longer favorable or even feasible. In this paper, we propose the asynchronous flipped diversity ALOHA (AFDA) to tackle the challenges based on a new diversity transmission scheme. Different from the existing diversity transmission schemes, each data packet and its flipped replica are transmitted back to back, and the zigzag decoding technique is adopted to resolve collisions. The performance of AFDA has been evaluated by analysis and simulations. The results show that, without time synchronization or handshaking requirements, the performance of AFDA is unaffected by the duration or variation of the propagation delay, and it substantially improves system performance in terms of throughput, packet loss ratio, and network admission region.