alien – adj. from somewhere other than the planet Earth

comet – n. an object in outer space that develops a long, bright tail when it passes near the sun

asteroid – n. any one of thousands of small planets that circle around the sun

millimeter – n. a length equal to 1/1000 meter

probe – n. a device that is used to obtain information from outer space

galaxy – n. any one of the very large groups of stars that make up the universe