Albert Einstein concluded that the mass of objects can bend space-time.

A gravitational force, in simpler terms, is the attractive force between two separate bodies. Gravity is related to mass: the greater the mass of an object, the greater the gravitational force it exerts on other objects. It is one of the essential forces responsible for the structure of the universe. Although it is one of the most easily observable cosmic forces, the principles of gravity are still not fully understood by modern science.

The Space Shuttles used a combination of liquid-fuel main rockets and solid-fuel rockets to reach orbit.

Gravity is popularly associated with Sir Isaac Newton, the 17th century English scientist, but scientists had been studying gravity centuries before Newton. Galileo, a few decades earlier, had shown that objects fall at the same rate, regardless of their weight. The fall of an apple inspired Newton to study gravitational force, although the apple didn’t really hit his head and bring instant understanding; he has spent more than 20 years on the subject. He realized that the gravitational force also kept the moon in orbit around the Earth and was able to prove this by using the moon’s orbit to verify his calculations.

The Sun’s gravity acts as a centripetal force that keeps the planets orbiting in the Solar System.

Newton realized the same principles applied to all objects everywhere in space; His theory became known as the law of universal gravitation. He freely admitted, however, that he did not understand how the gravitational force is transmitted from one object to another, seemingly instantaneously, over vast distances. Despite this flaw, Newton’s law became an established scientific principle for the next three centuries. It has repeatedly proven itself correct in predicting, for example, the location of planets that had not yet been discovered.

Isaac Newton codified the law of universal gravitation.

Newton’s law, while correct in most general applications, contained minor inconsistencies that became obvious as the advance of science became more precise. Einstein, working in the early 20th century, finally explained these inaccuracies with his theory of general relativity. According to relativity, the movement of bodies through space is governed by the effect of these bodies on the structure of spacetime itself. Newton’s law is still used by scientists because it’s easier to calculate and is correct for practical questions like how much force a rocket needs to leave Earth’s atmosphere.

The gravitational force keeps the moon in orbit around the Earth.

Even Einstein did not fully explain the gravitational force. In modern times, gravity is understood to be one of the four fundamental forces that bind the universe together, along with electromagnetism and the weak and strong nuclear forces. The last three were substantiated by the discovery of the subatomic particles that transmit these forces at a distance. A similar particle to gravity, the so-called graviton, remained undefined in 2010. Other principles of gravitational force also remain unexplained by modern physics.