Nearly 400 years ago, the physicist Newton discovered gravity because of the apple, which is something we all know, but many people may not know that Newton also used a cannon to describe to us how to overcome the gravitational pull of the earth.
Of course, Newton's cannon is not real, but imaginary, roughly meaning this: a cannon is mounted on a mountain, as long as the cannon is powerful enough to make the shell fast enough, then the shell can always rotate around the earth without falling to the ground, this thought experiment is called "Newton's cannon". Newton not only came up with the idea of "Newton's cannon", but also deduced from this that the speed of this never-falling shell was, which is 7.9 kilometers per second, which we now call the "first cosmic speed".
In Newton's day, 7.9 kilometers per second was an unattainable speed, and even in modern times, the shell exit rate of most main battle tanks could not exceed 2 kilometers per second.
But it doesn't matter, now that we have rockets, through thousands of tons of thrust launch vehicles, we can send the spacecraft we carry into the sky and keep them in orbit around the Earth, which we call artificial satellites. Among the three major cosmic velocities commonly spoken of, the first cosmic velocity 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 Earth's natural satellite, and it is still within Earth's gravitational range, so broadly speaking, the moon landing is not really leaving Earth.
So what does it take to really leave Earth? This requires the second cosmic speed.
The second cosmic velocity is also known as the "Earth escape velocity", as long as it has such an initial speed, then the spacecraft can completely get rid of the gravitational constraints of the Earth to explore other planets in the solar system, and this speed is 11.2 kilometers per second. Contemporary human space is no longer satisfied with just hanging around the earth, human beings began to challenge more distant space, and Mars is the planet that human beings are most interested in, in order to launch spacecraft to Mars, then it is necessary to achieve an initial speed of 11.2 kilometers per second.
Is it okay to just walk around the solar system? Of course not, our goal is a sea of stars.
It is precisely because human beings have such a lofty rationality that spacecraft targeting the outside of the solar system, Voyager 1 and Voyager 2, were launched as early as the 1970s. Earth or Mars, all celestial bodies in the solar system are bound by the gravitational pull of the sun, the sun can be more than the earth, to get rid of the gravitational pull of the sun away from the solar system is not easy, you must achieve the third cosmic speed, that is, 16.7 kilometers per second. In order to allow Voyager to have enough speed to leave the solar system, it also used Jupiter's gravitational slingshot effect to accelerate. First, second, third, these three cosmic speeds are more familiar to us, but in addition to the three major 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, also known as the "Milky Way Escape Velocity", as the name suggests, is the initial velocity that an object needs to have to get rid of the gravitational bondage of the Milky Way, and the estimated speed is 120 kilometers per second.
Well guys, this number is obviously not on the same order of magnitude as the first three, so why is this number an estimate? To calculate the escape velocity, the mass of celestial bodies and celestial systems is a necessary condition, we can calculate the mass of the Earth, we can also calculate the mass of the Sun, but for the entire Milky Way, we can do nothing, the Milky Way is too big, we have no idea how many stars there are, maybe 100 billion, maybe 400 billion. Since we don't even know how big the Milky Way is, how many stars there are, and of course we can't accurately calculate its escape velocity, the fourth cosmic velocity is only an estimate.
We don't know exactly what the exact value of the fourth cosmic velocity is, which is one reason why we rarely mention the velocity of the fourth universe, and another reason is that the fourth cosmic velocity gives us a deep appreciation of our own smallness.
The so-called cosmic speed, in essence, is the object's kinetic energy and gravitational potential energy confrontation, as far as the current stage of human beings, let the kinetic energy of the object overcome the earth's gravitational potential energy has been exhausted, want to fight with the milky way is simply impossible. In addition, although the fourth cosmic speed is the escape velocity of the Milky Way, even if human beings 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.