In space, the impact of radiative particles, such as neutrons and heavy ions, can change the node states of a flip-flop, thus resulting in loss of data. In this paper, a Highly reliable and Low power Radiation-hardened-by-design (RHBD) Flip-Flop cell, namely HLRFF, completely hardened against double-node-upsets (DNUs), is proposed for aerospace applications. The HLRFF is a master-slave structure. The master latch is mainly constructed from two 2-input C-elements (CEs) and one 2-input clock-gating based CE, while the slave latch has an additional keeper at the output stage. The verification results demonstrate that the proposed HLRFF is completely DNU-tolerant. Furthermore, compared to the state-of-the-art radiation-hardened FF cells, the proposed HLRFF can reduce power consumption by approximately 69%. However, only the proposed HLRFF is not only completely DNU-tolerant but also insensitive to high-impedance-state.