Simplify the following text for a Grade 9 native English-speaking person.
Original: The Space Race was a mid-to-late twentieth century competition between the Soviet Union ( USSR ) and the United States ( US ) for supremacy in outer space exploration . Between 1957 and 1975 , Cold War rivalry between the two nations focused on attaining firsts in space exploration , which were seen as necessary for national security and symbolic of technological and ideological superiority .
Simplified: The Space Race was a competition of space exploration between the Soviet Union and the United States , which lasted from 1957 to 1975 .