In history, the Earth has encountered frequent and massive impacts. Until today, it is still a collided target for Potentially Hazardous Objects (PHOs), gradually making planetary defense a concerned and important research topic in the field of aerospace science. Against this background, this work clarifies and discusses some important scientific issues and various critical technologies of planetary defense. First, the current status of planetary defense research is summarized. Subsequently, the classification and characteristics of PHOs are introduced, focusing on their potential impact on planetary defense and related scientific research issues. For the purpose of protecting the Earth and maintaining space safety, the monitoring and characterization, deflection, and mitigation of PHOs have basically occupied all the attention of planetary defense researchers. Therefore, the critical technologies in planetary defense are discussed from these three aspects, and are also compared and analyzed in depth. Based on the current research progress in planetary defense, the challenges for planetary defense are discussed, and future development directions are also analyzed. Finally, a summary of the entire study is provided.

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