This paper summarizes three definitions of the solar system boundary, i. e., planetary orbit boundary, heliospheric boundary and solar gravitational boundary, presents the difference between interplanetary space and interstellar space, and discusses the scientific objectives and key scientific problems of the solar system boundary exploration, which includes four missions,namely, exploration of no man's land, panoramic view of the heliosphere, sweeping of major planets, and archaeology of the solar system.
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