Planets, stars, galaxies, meteorites, asteroids and the void between them.
According to Cambridge Dictionary, space is the empty area outside Earth's atmosphere, where the planets and the stars are.
More scientifically, space is an almost perfect vacuum, nearly void of matter and with extremely low pressure, where sound doesn't carry because there aren't molecules close enough together to transmit sound between them. Not quite empty, bits of gas, dust and other matter floats around "emptier" areas of the universe, while more crowded regions can host planets, stars and galaxies.
The astronauts prepared for their mission to explore the mysteries of space beyond our solar system.