Explanation:

- Geography: This is the study of Earth's landscapes, environments, and the relationships between people and their environments. It does not focus on celestial objects or space.

- Astrology: Astrology involves studying the positions and movements of celestial bodies with the belief that they influence human affairs and natural events. It's not a scientific study of celestial objects.

- Geology: This science deals with the Earth's physical structure and substances, its history, and the processes that act on it. It doesn't cover celestial objects and space.

- Astronomy: This is the scientific study of celestial objects like stars, planets, comets, and galaxies, as well as the universe as a whole. It explores space beyond Earth's atmosphere.

- Astronomy is the correct one-word substitution for science which deals with celestial objects and space.