What are the three major cosmic velocities? Why is the fourth cosmic velocity rarely mentioned?

We all know that nearly 400 years ago, physicist Newton discovered gravity inspired by an apple, but many people may not know that Newton also used a cannon to describe to us how to overcome the earth's gravity.

Of course, Newton's cannon does not really exist, but is imaginary. The general idea is this: if a cannon is placed on a high mountain, as long as the cannon is powerful enough and the speed of the cannonball is fast enough, then the cannonball can always revolve around the earth without falling to the ground. This thought experiment is called "Newton's cannon". Newton not only proposed the concept of "Newton's cannon", but also deduced the speed of this cannonball that never falls, which is 7.9 kilometers per second, which we now call the "first cosmic speed".

In Newton's time, 7.9 kilometers per second was an unattainable speed. Even in modern times, the muzzle velocity of most main battle tanks cannot exceed 2 kilometers per second.

But it doesn't matter. Now we have rockets. Through launch vehicles with thousands of tons of thrust, we can send the spacecraft they carry into the sky and keep them in orbit around the earth. These spacecraft are called artificial satellites. Among the three commonly mentioned cosmic speeds, the first cosmic speed is the most used by humans at this stage, because most of our space activities are within the gravitational range of the earth, including the moon landing. The moon is the natural satellite of the earth and is still within the gravitational range of the earth, so in a broad sense, the moon landing does not really leave the earth.

So how do we truly leave the Earth? This is where the second cosmic velocity comes into play.

The second cosmic velocity is also called the "Earth escape velocity". As long as it has this initial velocity, the spacecraft can completely get rid of the Earth's gravity and explore other planets in the solar system. This speed is 11.2 kilometers per second. The contemporary human space industry is no longer satisfied with just wandering around the Earth. Humans have begun to challenge more distant space, and Mars is the planet that humans are most interested in. If you want to launch a spacecraft to Mars, you must reach an initial velocity of 11.2 kilometers per second.

Is it enough to just wander around the solar system? Of course not, our goal is the sea of ​​stars.

Because humans have such far-sighted rationality, they launched spacecrafts, Voyager 1 and Voyager 2, with the goal of going outside the solar system as early as the 1970s. Whether it is the Earth or Mars, all celestial bodies in the solar system are bound by the gravity of the sun. The sun is much larger than the Earth. It is not easy to get rid of the sun's gravity and leave the solar system. It is necessary to achieve the third cosmic speed, which is 16.7 kilometers per second. In order to enable Voyager to have enough speed to leave the solar system, it also used the gravitational slingshot effect of Jupiter to accelerate. The first, second, and third cosmic speeds are the three most familiar to us, but in addition to the three cosmic speeds, there is actually a fourth cosmic speed. So what is the fourth cosmic speed? Why is it rarely mentioned? Because we really don't want to mention it.

The so-called fourth cosmic velocity is also known as the "Galactic escape velocity". As the name suggests, it is the initial velocity that an object must have to escape the gravitational pull of the Milky Way. The estimated value of this speed is 120 kilometers per second.

My goodness, this number is obviously not on the same order of magnitude as the first three. Why is this number an estimate? To calculate the escape velocity, the mass of the celestial body and the celestial system is a condition that must be known. We can calculate the mass of the earth and the mass of the sun, but we can't do anything about the entire Milky Way. The Milky Way is too big, and we don't know how many stars there are in it. Maybe it's 100 billion, or maybe it's 400 billion. Since we don't even know how big the Milky Way is and how many stars there are in it, of course we can't accurately calculate its escape velocity, so the fourth cosmic velocity is just an estimate.

We don't know the exact value of the fourth cosmic velocity, which is one reason why we rarely mention it. Another reason is that the fourth cosmic velocity makes us deeply realize our own insignificance.

The so-called cosmic speed is essentially the confrontation between the kinetic energy of an object and its gravitational potential energy. For humans at this stage, it is already a great effort to make the kinetic energy of an object overcome the gravitational potential energy of the earth. It is impossible to fight against the Milky Way. In addition, although the fourth cosmic speed is the escape velocity of the Milky Way, even if humans really achieve the fourth cosmic speed, it does not mean that they can leave the Milky Way, because the radius of the Milky Way is as high as hundreds of thousands of light years. Not to mention 120 kilometers per second, even if it is 300,000 kilometers per second, it will take hundreds of thousands of years to leave the Milky Way, so the fourth cosmic speed may never be used.

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