Before we go into orbit, let's discuss an important difference in physics. The difference between weight and mass. Mass is a property of an object that can describe as the ability for that object to resist acceleration. Weight on the other hand, depends on the local gravitational field. Mass always stays the same. If my mass is 75 kilograms, I'll be 75 kilograms whether I'm here on Earth, on the Moon, or somewhere in deep space. But weight is actually a measurement of the force felt by an object within a gravitational field. Which means that weight can change in different gravities. It's a product of the mass and the local gravitational field. So, weight is equal to m times g, in multiples of one Earth gravity. On the moon where gravity is roughly one-sixth of Earth's gravity, my mass is still 75 kilograms, but my weight is reduced by a factor of six. That means that on the moon, my weight would be 12.5 kilograms even though my mass is still 75 kilograms. What does it mean to be weightless if weight depends on the local gravity? Well imagine a region of space so far from stars and planets that the local gravitational field is very close to zero. What would someone with a mass of 75 kilograms feel when there is zero gravitational force on them. They would feel a weight of zero kilograms. So, when an astronaut is floating freely in space, are they weightless? No, it's a common misconception that astronauts experience weightlessness when they are above Earth's atmosphere where gravity is weak. In fact, there's still enough gravity in the environment around Earth that they have a measurable weight. However, this is different from experiencing free fall acceleration in a gravitational field that is not restricted by any other forces. Astronauts feel weightless because both the spacecraft that they're in and the astronauts themselves, are in a state of free fall above the earth. A body is in free fall whenever gravity is the only force acting upon it. If I release a ball either here or from up high in space, the only force acting on the ball while it's moving is gravity. When it's moving it's in a state of free fall and experiences weightlessness. Since the force of gravity acts on it, the ball accelerates and moves towards the earth until it hits the Earth's surface. What do you think would happen when the ball is thrown horizontally? Newton was the first to imagine what would happen if you climbed a tall mountain in order to fire a cannonball horizontally. Newton reason that the cannonball would curve towards the Earth due to gravity. If the cannonball was fired at a faster speed, it would go a longer distance. Eventually, if the cannonball could be fired fast enough, it would fall towards the ground on a curved trajectory that matches the curvature of Earth's surface. This was the first time someone had reckoned about orbital motion. This is very similar to how flying is described in Douglas Adams' A Hitchhiker's Guide to the Galaxy. Where it is stated, "There's an art to flying or rather a knack. The knack lies on learning how to throw yourself at the ground and miss." When an astronaut orbits the Earth in the International Space Station, the only force acting on the astronaut is gravity. The astronaut is travelling in a stable orbit around the Earth. So although gravity is pulling on the astronaut towards the earth, the circular motion makes it possible for the astronaut to miss the Earth. One way to experience weightlessness without being in orbit or at a vast distance from the earth, is to fly in an airplane on a parabolic trajectory. Special aircraft that can withstand many times the force of gravity navigate to a high altitude before climbing into an inverted parabolic flight path. During the arc of the parabola the airplane and the occupants within it only experienced the force of gravity and therefore they feel weightless. These moments feel like zero gravity but they only last about 20 seconds. The airplane can't stay in free fall for very long, for obvious reasons. "Guidance is internal. 12, 11, 10, 9, ignition sequence starts. 6, 5, 4, 3, 2, 1, 0. All went in line. We have a lift-off Apollo 11." Rockets like the Saturn V, that carried the crew of the Apollo 11 mission to the Moon must expend energy to climb through Earth's gravitational field. The speed of a spacecraft dictates how high it will go in a given scenario. So just how much energy is required for a rocket to escape from a planet entirely. Let's consider an example of a rocket escaping from Earth. Kinetic Energy is the energy associated with the speed of an object, which is supplied to a rocket by burning fuel and expelling it from the rocket's nozzles. The energy required to break the gravitational grasp of a planet like Earth, depends on the mass of the planet as well as its size. When a speed is associated with kinetic energy of a departing rocket, we call it the Escape Velocity. Earth has an escape velocity which is roughly 11.2 kilometers per second, which is more than 40,000 kilometers per hour. But let's not get too carried away, getting to space is much more complicated than merely getting a vehicle to the right speeds. This calculation considers the pure physics involved in climbing out of the gravitational potential well. So, we ignore otherwise important factors like air resistance. 11.2 kilometers per second is the instantaneous velocity you'd need traveling directly upwards from earth's surface in order to escape earth's gravitational well. At sea level,11.2 kilometers per second is equivalent to Mach 33, which is fast enough to make the air around the spaceship into a boiling plasma, so instead, rockets accelerate out of our atmosphere starting from a standstill. Although we used Apollo 11 to introduce you to the concept of escape velocity, it's worth pointing out that in order to reach the moon, the astronauts never exceeded Earth's escape velocity at all. The moon is gravitationally bound to Earth and a voyage there hasn't escaped from Earth's gravitational sphere of influence. The moon itself is also trapped within Earth's gravitational well. Out of all the spacecraft launched by humanity, only a few have achieved Earth's escape velocity. Those spacecraft which traveled to other planets in our solar system. But a small subset of spacecraft have voyaged well beyond the earth's grasp and escaped from the gravitational pull of the entire solar system. One such spacecraft Voyager 2, launched in 1977 and is now considered to be an interstellar traveler. The red line in this graph represents the changes in speed experienced by Voyager 2, from 1977 to 1989 on its journey past the outer planets. In order for Voyager 2 to achieve escape velocity from our solar system, it needed a gravity assist from the planet Jupiter. A gravity assist is a way for a space probe to boost its kinetic energy by stealing the orbital energy from a heavy body like Jupiter. Over the course of Voyager 2's transit through the solar system, it was repeatedly boosted by encounters with planets, Saturn, Uranus and Neptune. At present, Voyager 2 is traveling at 15.4 kilometers per second on its way to the outermost edge of our solar system. By contrast, the fastest humans have ever traveled was accomplished by the crew of Apollo 10 in 1969, achieving a top speed of nearly 11.08 kilometers per second. However, their speed record was on their way back through Earth's atmosphere and not on the way out. But even faster than the Voyager spacecraft, the current speed record held by a human object is the New Horizons probe, which took pictures of Pluto in a Flyby in 2015. New Horizons accelerated away from Earth achieving a whopping 16.26 kilometers per second, making it the fastest spacecraft ever launched. Deriving the formula for escape velocity is relatively straightforward. It involves setting the gravitational potential energy equation of an object on the surface of a body which is GMm over r, equal to its kinetic energy which is equal to one-half m v squared. Since the little m mass, which represents the mass of the object you want to move, appears on both sides of the equation, we can eliminate those from the equation altogether. This means that the escape velocity of an object does not depend on its own mass. We can finally rearrange the terms of this equation solving for ve the escape velocity. So, escape velocity ve, can be calculated by multiplying two times the universal gravitational constant G, times the mass of the body capital M and dividing by the radius of the body's surface r, all taken underneath a square root sign. So, increasing the mass of a body will increase its escape velocity and decreasing the radius of a body, will also increase its escape velocity. In order for spacecraft to escape from Earth, escape velocity is required to ensure the gravitational potential well can be climbed. In fact, if you'd like to explore these ideas further, we've created an escape velocity calculator that you can use to plan a mission across the Solar System. What could escape velocity possibly have to do with black holes? Well, chew on this for a minute. What if we calculated the density of an object for which the speed of light is equal to the object's escape velocity? The answer is something sinister, something dark.
