What is escape velocity? Escape velocity is the minimum speed an object needs for it to be able to escape the gravitational pull of a celestial body.
Everything in the universe has gravity. Even dark matter has gravity, which is how astronomers know it is there even though they can’t see it. The gravity of the dark matter pulls on light that is passing close to it. The more mass something has, the greater its gravity is. The celestial body doesn’t have to be large to have high gravity because its mass can be compressed down so that it has a very high density. For example, a neutron star is only about 20 km across, on average, but they have more mass than a sun. A black hole, as well, is not very big, but has more mass than anything else in the universe.
If you want an object to leave the planet, you have to make it go fast enough so that it can break free of the gravitational pull. If you throw a ball, for example, it will go up for a short while, but gravity will pull it back down pretty quickly. If you throw it harder, by putting more kinetic energy into it, the ball will stay up for longer, but it will ultimately come down. The best baseball players can throw a ball over 130 m, but nobody would argue that a ball fired from a canon would go further than any person could throw. The canon-fired ball would have more energy. Long jumpers are a good example of this. The best long jumpers are also incredibly fast sprinters because they can put so much speed into their run-up that they have a huge amount of energy when they take off. There are obviously a lot more factors at play than just speed, but it is enormously important. Again, though, the fastest sprinter couldn’t jump as far as a motorbike, which has a lot more speed at the point of take-off.
The easiest way of thinking about escape velocity is to keep thinking about that baseball. The more energy the baseball player can give to the baseball, the further it will go. With 10% more energy, it might cross the stadium. With 50% more energy, it might get halfway across the town. With 2,000% more energy the ball might cross into the next county. As you keep increasing energy, the ball will go further. At some point, the ball would have enough energy to circle the Earth and come back to the point where you threw it from. Of course, baseballs are not perfectly aerodynamical and as you increase the speed, the fiction will build up as well. The baseball would probably burn up long before you could get to even a fraction of the energy necessary for it to orbit the Earth.
This is the theory behind satellites. If they have enough energy, they will keep circling the Earth. They will be pulled by the Earth’s gravity, but they will have enough energy that they keep circling. However, the pull of the Earth’s gravity is strong, and unless you keep giving the satellites more energy, through occasional boosts from their rockets, they will be pulled back down to Earth. No satellites can stay in orbit permanently without additional energy. The International Space Station, for example, travels at roughly 7.7 km per second. It needs to use its rocket boosters about once a month to keep its orbit stable. Once it is retired, in late 2030, it won’t be boosted anymore and its orbit will be allowed to decay. It will burn up in Earth’s atmosphere. The ISS and these satellites don’t leave Earth’s orbit though. They don’t have enough energy to do that. To leave Earth’s gravity, you need even more energy. And that is the escape velocity.
For something to leave the orbit of a celestial body, to escape its gravity, it has to have a speed that is greater than the pull of gravity. For Earth, that is 11.2 km per second. If we kept accelerating the ISS to 11.2 km per second, it would leave Earth’s orbit and fly off into space. As we said earlier, the more mass a celestial body has, the greater its escape velocity. To calculate the escape velocity, you multiply the gravitational constant by the mass of the celestial body. The gravitational constant is the strength of gravity and doesn’t change. You then have to divide that by the radius of the celestial body because the escape velocity decreases the further away from the center you are. 11.2 km per second is the escape velocity from the surface of Earth. From closer to the center, it would be higher, and from above the surface, it would be lower. (So, in fact, the escape velocity for the ISS would actually be 10.9 km per second). Then you have to take the square root of all that to get the escape velocity. The escape velocity of Earth is 11.2 km/s. From Jupiter, it would be 59.5 km/s. From the sun, it would be 618 km/s. From a neutron star, it would be 150,000 km/s (about half the speed of light). From a black hole, the escape velocity would be higher than the speed of light, which is why light (or anything else) cannot escape a black hole. And this is what I learned today.
Sources
https://www.qrg.northwestern.edu/projects/vss/docs/space-environment/2-whats-escape-velocity.html
https://www.eeweb.com/the-physics-of-the-long-jump
https://en.wikipedia.org/wiki/Escape_velocity
https://www.energy.gov/science/doe-explainsneutron-stars
https://school.careers360.com/articles/physics-of-jumping-premium
https://en.wikipedia.org/wiki/International_Space_Station
https://www.space.com/what-is-the-gravitational-constant
Photo by Edvin Richardson: https://www.pexels.com/photo/space-shuttle-launch-during-nighttime-796206/