The force that pulls two objects toward each other; the greater the mass of an object, the greater the attraction; the further apart the objects, the weaker the attraction.
The investigation of physical conditions in space and on stars, planets, and other celestial bodies through the use of satellites, space probes, or spacecraft with human crews.
A celestial body that revolves around a planet.
A building or place given over to or equipped for observation of natural phenomena.
The four largest and brightest moons of Jupiter, which were discovered by Galileo Galilei: Ganymede, lo, Callisto, and Europa.
Any of the rocky, terrestrial planets of Mercury, Venus, Earth, and Mars, whose orbits are inside the asteroid belt.
The Sun together with the group of planets and other celestial bodies that are held by its gravitational attractions and revolve around it.
An engine that provides thrust directly through the burning of its fuel without requiring the intake of air.
The gravitationally-curved path of an object around a point in space.
A person trained to be part of a spacecraft crew.
Large and small rocks or metallic masses orbiting the sun; made up of materials similar to those that formed the planets.
An instrument for viewing or detecting distant objects.
A ball of gas in space that produces its own light and heat.
A small object that enters Earth's atmosphere from space and burns due to friction, emitting light.
A vehicle for exploring the surface of an extraterrestrial body (Such as the Moon and Mars).