In 1894, Konstantin Tsiolkovsky depicted a "space lodge" orbiting the Earth, which was actually a space station for astronomical observations and plant cultivation. Nowadays, the space station is not a strange concept to the public, but in the space station, what advantages do the ground do not have, and what experiments can be done that are unattainable on the ground? What are some peculiar physical phenomena in space?
Under the conditions of space microgravity, the law of material movement has undergone many changes, and some strange phenomena that cannot be observed on the ground have occurred, involving fluids, combustion, materials, basic physics and other aspects.
NASA astronaut Tim Copra performs the operation in a microgravity science glove box at the International Space Station laboratory. Copyright/NASA
/ Large bubbles appear in space boiling
Under normal gravity (left), buoyancy causes bubbles to rise upward from the surface of the heater over surface tension. In microgravity (right), buoyancy is weak, bubbles usually attach to the heater due to the presence of surface tension, and due to the continuous input of energy from the heater, more steam is generated, and the bubbles become larger.
Buoyancy convection in space has basically disappeared, and for the case of water heating, the hot water cannot rise, but is close to the heat source, getting hotter and hotter. At the same time, water away from heat sources remains relatively cold. When the heated fluid reaches its boiling point, the bubbles do not rise to the surface. Instead, the formed bubbles polymerize into a large bubble located on the surface of the heater. There is precious thermal energy in the foam, but these thermal energy is trapped in the bubble. By implementing boiling experiments with variable gravity on the space station, scientists can study the various factors that affect boiling, making it possible to drive improved cooling system design and increase the efficiency of refrigeration technology, which will have a positive impact on the global economy and the environment.
/ A spherical flame burning in space
Comparison of flames on the ground (left) and in a microgravity environment (right).
On Earth, when a flame burns, it heats the air around it, and under the action of gravity, the colder, denser air is drawn to the bottom of the flame, thus discharging the hot air that rises. This convection process transports fresh oxygen into the fire, and the upward flow of air causes the flame to form a droplet shape and makes it flicker. But strange things happen in space, without gravity, hot air expands but doesn't move upwards. Due to the diffusion of oxygen, the flame persists, and oxygen molecules randomly drift into the fire to maintain the flame, and its flame will stretch out in all directions, forming a spherical flame.
However, because the amount of oxygen molecules floating into the flame is much less than in the Earth's environment, and it is hindered by the surrounding exhaust gases, the combustion is not sufficient. So space will burn more slowly than on Earth. The difference between a flame produced under normal gravity conditions and a flame in a microgravity environment is clear at a glance. Scientists study the combustion process in a microgravity environment, further revealing the laws of combustion and heat transfer under the cover of gravity, which in turn helps to improve combustion efficiency and clean energy research and development.
/ Magical "Cold Flame"
Cold flame phenomenon found on the International Space Station.
There is an interesting flame extinguishing experiment on the International Space Station, which is to burn droplets of heptane and methanol in different pressure and gaseous environments. In a large number of droplet experiments, scientists observed the unexpected phenomenon of "cold flame". After the combustion of the heptane droplets, the flame is obviously no longer visible, which is the so-called "extinguished" state, but the droplets evaporate continuously, rapidly, and almost stably, showing the same state as when there is a visible flame, and scientists define this process as "cold flame".
The combustion temperature of the ordinary visible flame is generally 1500 Kelvin (1226.85 degrees Celsius) to 2000 Kelvin (1726.85 degrees Celsius), and the "cold flame" is burned at a relatively low temperature of 500 Kelvin (226.85 degrees Celsius) to 800 Kelvin (526.85 degrees Celsius). Moreover, their chemical reactions are completely different, ordinary flames produce soot, carbon dioxide and water, while "cold flames" produce carbon monoxide and formaldehyde. "Cold flames" also exist on Earth, but they are only fleeting. In the space station, the "cold flame" can last for a long time. The discovery of the "cold flame" process can be applied to the development of fuel engines, helping to improve the efficiency of fuel engines and reduce pollution emissions, with great application potential.
/ Robots can also have muscles
Artificial muscles in space.
When we think of the image of a robot, we may think of complex structural mechanisms, motors and controllers, a bunch of metal. In April 2015, the Falcon 9 rocket carrying artificial muscle material was launched from Florida into the International Space Station in order to test the radiation resistance of this material on the International Space Station, and in the future it will be installed on robots to enable it to perform tasks in special environments. This artificial muscle is a material made of electroactive polymers, a new type of intelligent polymer material. Under the action of an applied electric field, when the charge is reversed, it contracts, expands, bends, tightens or expands with the current, simulating the movement of our own muscles.
Artificial muscles have very good radiation resistance in addition to being able to simulate our muscle movements, so this material is installed on robots to help humans better explore space, such as performing Mars missions, and can also perform rescue and repair maintenance tasks after nuclear power plant failure. This artificial muscle can withstand 20 times the radiation limit that humans can withstand, which is comparable to radiation on Mars, and still maintains good conductivity, strength and durability after 45 hours of radiation testing. Similarly, artificial muscles do not change under conditions of minus 271 degrees Celsius and work well in environments of 135 degrees Celsius, which is much higher than the boiling point of water.
Based on the superiority of artificial muscles in all aspects, the researchers sent artificial muscles to space for testing, testing their ability to adapt to the harsh environment of space and the surface of alien planets. In the future, these artificial muscles can be used to make human prosthetic limbs, which can do subtle movements like our own bodies, restoring mobility and freedom for injured people. At the same time, robots fitted with artificial muscles can perform tasks that require fine motor skills in potential nuclear disasters and other dangerous areas.
/ Precise space clock
Astronaut Tim Pique is experimenting with hardware configuration. Copyright/ESA
In the Tiangong-2 space laboratory, scientists have achieved the highest precision space cold atomic clock in the world, with a daily stability of 7.2× 10^-16 seconds, which can be roughly described as an error of less than 1 second in 30 million years. On the ground, due to the action of gravity, the ultra-cold atomic pellet after laser cooling and captivation is always in a variable speed state, and the macroscopic movement can only do fountain-like movement or parabolic motion, which makes the atomic clock based on the precision measurement of atomic quantum states be limited in time and space in two dimensions. In the space microgravity environment, the atomic mass can do ultra-slow uniform linear motion, based on the fine measurement of this motion can obtain more precise atomic spectral line information than on the ground, so that a higher precision atomic clock signal can be obtained. Therefore, space cold atomic clocks have become an important high-precision time frequency system.
Since the space cold atomic clock can transmit and calibrate the time signal of the spaceborne atomic clock on other satellites without interference in space, so as to avoid the influence of the changing state of the atmosphere and the ionosphere, it can provide timing services for the global satellite navigation system, with more accurate and stable operation capabilities; at the same time, it can support the verification of general relativity, the measurement of the basic physical constant, the measurement of the earth's gravitational position, the space cold atom interferometer, the cold atom gyroscope and other major scientific research and applied research.
——This article is excerpted from the February 2022 issue of China National Astronomy
About the Author /
Wei Zhang is a researcher at the Center for Space Application Engineering and Technology, Chinese Academy of Sciences.
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Source: National Astronomical State of China
EDIT: just_iu