# How the stars float

Everyone must have played Drifting, right? It is very exciting to be able to make a beautiful multiple floats. But did you know that apart from rocks, missiles, manned spacecraft return capsules and even stars in outer space can also float in the water! Let us find out today.
Drakes is a very serious scientific question
many people have played ducks and drakes of the game, and the trick is to choose a moderate weight flat stones in the water, then add some water lateral rotation throw out stones will be in the water Jumping into the distance hop by hop. If the technique is good enough, the cigar-shaped stone can also be used to make water drifts. It is said that the world record for drifting is 91 jumps.
So why does the sinking stone jump up again? This is actually a more complicated mechanics problem. The speed of the stone is very fast when the water floats, so the layer of water in contact with the stone will get a high speed. According to Bernoulli’s law in fluid mechanics, the greater the velocity of the fluid, the lower the pressure. Therefore, the water in the layer next to the stone is fast and the pressure is low, but the water in the lower layer is almost static, so the pressure is higher. This pressure difference will push this layer of water upward, causing the rock to jump up.
Of course, the shape, weight, posture, angle of entry, initial speed, rotation and other factors of the stone will affect the effect of the water drift. The French physicists Lidari Boque and Christopher Kranney have been on the water drift. They did a very in-depth study. They used an aluminum disc with a radius of R and a thickness of h to simulate a stone (such as the analysis diagram of the mechanical model of a rock in the water). The attitude angle of the stone itself (angle of attack) is α , And the angle between the instantaneous velocity direction of the stone and the water surface and the horizontal direction is β, the angular velocity of the stone rotation is Ω, and the velocity vector of the stone is U. By changing parameters such as speed and attitude, and using multiple experiments, we have obtained the conclusion of whether it can be successful. When the speed of the stone is 3.5 m/s and the angle of entry is greater than 55 degrees, the water can not be floated, and when the angle of entry is about 20 degrees, the best bounce effect will be obtained.
During the Second World War, the famous British inventor Barnes Wallis invented the famous ricochet after conducting in-depth research on dipping. At that time, the British army wanted to blow up three very important German dams. In order to prevent the British army from dropping torpedoes by air, the German army had already arranged a large number of torpedo nets in the water. Wallis used the principle of kick-off and drifted through precise calculations to study the speed and altitude of the aircraft, referring to the weight of the bomb and other parameters. The made ricochet could jump on the water like a rock, and finally broke through the German water. Under the torpedo defensive net, the target was successfully blown up.
Also liked to skip asteroid in space
under certain conditions, the Earth’s atmosphere can also be used as “water”, so that aircraft and a number of small bodies achieve “space hydroplaning.” The return capsule of China’s lunar exploration project is to enter the atmosphere at a slower speed by drifting. The atmosphere changes from thin to thick from space to the ground, and it can also be seen as a fluid interface similar to the surface of water. Since the speed of the return capsule is very fast, if it enters the atmosphere perpendicular to the interface, the aircraft may burn out due to frictional heating. And if the angle between the interface and the atmosphere is too small when returning, it is equivalent to drifting in the atmosphere, and it may not be able to land and eventually jump out of the earth. In order for the return capsule to decelerate through the water drift without being bounced away, scientists need to perform precise calculations and designs.
The space is full of rocks and ice. Some are asteroids and comets left over from the formation of the early solar system, and some are rocks produced by asteroids impacting planets and sputtering. These stones will fly to the earth from time to time, pass by in many cases, and sometimes fall into the earth’s atmosphere to form meteors; the stones that are not completely burnt will fall to the ground and become meteorites. In addition to these two situations, these space rocks may also enter the upper atmosphere of the earth and be bounced back into space, just like a “water drift” in the earth’s atmosphere. Analogous to the rocks passing the water surface and the aircraft reentering the atmosphere, to realize the “space water drift” of such asteroids, certain conditions must also be met.
The shape of asteroids is usually irregular, with dumbbells and cigars. Due to the long-term influence of physical processes such as the YORP effect (an effect in which asteroids rotate due to the irregular shape of asteroids and the radiation of stars), asteroids are usually rotating, and their angular velocity is usually along the axis with the largest moment of inertia. The dumbbell-shaped and cigar-shaped asteroids spin like bamboo dragonflies. Like the boulders that float in the water, if the angle is right, the asteroid can be bounced away by the earth’s atmosphere, completing a “space float”. However, because asteroids are usually very fast, they can only complete a “water drift” in the upper atmosphere of the earth. Like a water drifter, using rocks to bounce dozens of times on the water surface is almost impossible for asteroids. a task that can not be done.
In reality, humans have observed the “space water drift” of asteroids. In 1972, an asteroid hit a perfect “water drift” on the interface of the atmosphere without falling down. In the future, with the improvement of observation capabilities, we may see more and more such “space water drift” events. This will provide us with more information about the interaction between the upper layers of the Earth’s atmosphere and the asteroids.
Extrasolar asteroids and interstellar hydroplaning
in addition to the solar system’s asteroids, in recent years scientists have discovered asteroids from the outer solar system, which is called an Austrian street street (Oumuamua), which is a cigar shape (there are also studies Indicates a disk-shaped asteroid. Compared with the asteroids in the solar system, Oumamo moves much faster. This is an important evidence that we can confirm that it comes from outside the solar system. Like the asteroid in the solar system, this asteroid from interstellar space also rotates.
There are many asteroids in interstellar space, which may come from regions similar to the Oort Cloud. Except for the foreign asteroids that enter the sun like Oumomo, at the boundary between the solar system and the interstellar medium, will the foreign asteroids rotate and create beautiful “interstellar water drifts”? This question is left to you in the future to explore!