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