This article is based on responses to similar questions from netizens. It is merely a thought experiment; while not entirely impossible in practice, it presents significant challenges. Addressing these questions serves to disseminate basic knowledge, offering a way to relax, learn, and find enjoyment in discussion over the weekend.
As enthusiasm for lunar exploration and development grows, with many countries, including China, planning manned moon missions, and the US implementing plans to return to the Moon, a plethora of unconventional ideas about the Moon have been circulating online, such as the notion of running an electrical wire from Earth to the Moon for power supply.
However, the Earth rotates on its axis, and the Moon orbits around Earth, making it highly improbable to run a wire to the Moon. Even if it were possible, the wire would quickly become twisted like a rope. Moreover, how thick would this wire need to be? What would be the mass requirement for wiring to reach the Moon?
The average distance between the Earth and the Moon is 384,000 kilometers. Using international standards, if we were to use a 2.5 square millimeter stranded copper wire, which weighs approximately 22.25 kilograms per kilometer, the total weight for 384,000 kilometers would be 8,554 tons. This would require 86 heavy-duty trucks, each with a 100-ton capacity, just to transport the wire. And this calculation is only for the bare wire, not including insulation, which would significantly increase the weight.
Furthermore, the Moon's orbit is elliptical, with the furthest distance from Earth being about 406,000 kilometers, which would increase the total mass of the wire even more.
Additionally, a 2.5 square millimeter wire has a radius of less than 0.9 millimeters, which is too thin to withstand significant pulling forces; this aspect we can practically ignore. The temperature difference in space between the Earth and the Moon is substantial, with temperatures dropping to -200°C and rising to 130°C when exposed to sunlight. How would one manage such thermal expansion and contraction?
Therefore, running an electrical wire to the Moon is practically unfeasible.
Let's entertain this hypothetical scenario: if we actually ran a wire to the Moon, how long would it take for the charge to travel from Earth to the Moon once the switch is flipped? This question might seem straightforward, so we won't delve into the topic of electron drift velocity, which is less than 1 millimeter per second. Instead, we'll discuss the propagation of electric current (or electric field), which travels at the speed of light, approximately 300,000 kilometers per second.
The average distance between Earth and the Moon is about 384,000 kilometers. With simple calculation, if there were a 384,000-kilometer wire connecting the two, the bulb on the Moon would light up 1.28 seconds after the switch on Earth is turned on. If the person on Earth could observe this bulb, it would take an additional 1.28 seconds for the light to travel back, meaning they would see the light 2.56 seconds after flipping the switch.
These are average times based on the average distance, but the Moon's distance from Earth varies between 357,000 kilometers at its closest and 406,000 kilometers at its furthest, so the time to light the bulb on the Moon would depend on where in its orbit the Moon is located. Generally, the time for current propagation would be between 1.19 seconds to 1.35 seconds. This assumes a straight line, with the wire not bending, sagging, or folding.
What kind of wire would be needed from Earth to the Moon, how thick would it need to be, and what voltage would be required to light a bulb there?
Resistance is a physical quantity that indicates how much a conductor impedes the flow of current. The greater the resistance, the more it obstructs the current. Resistance is typically denoted by "R."
The formula to calculate resistance is:
where:
- is the resistivity of the conductor,
- is the length of the conductor,
- is the cross-sectional area of the conductor.
To address the resistance issue in long-distance power transmission, extensive power transmission and transformation projects are necessary to increase voltage.
According to the formula, if we use a 2.5 square millimeter copper wire as the conductor, at 20°C, the resistance per kilometer is 6.88 Ω (ohms). Therefore, for a distance of 400,000 kilometers to the Moon, the total resistance would be 2,752,000 Ω. To provide a lamp on the Moon with 220V/0.5A (approximately 110 watts), the voltage on Earth would need to be raised to 1,376,000 volts.
China leads the world in ultra-high voltage (UHV) transmission technology. The Huadong-Anhui South UHV power transmission project is the world's highest voltage level, largest capacity, longest distance, and most technologically advanced UHV project, with a voltage of 1,100,000 volts. However, this voltage falls short by 270,000 volts to light up a bulb on the Moon from Earth.
However, this resistance value is calculated at a standard temperature of 20°C. Resistance decreases as temperature drops, and in the space between Earth and the Moon, temperatures can reach several hundred degrees below zero, making the resistance extremely low. Therefore, I believe that with China's current ultra-high voltage technology, powering the Moon is more than feasible.
One potential scenario where power might be sent to the vicinity of the Moon involves the future use of a space elevator.
Many countries have entertained the idea of constructing a space elevator.
This elevator would also require electricity, so having a wire travel with the elevator to the Moon isn't entirely out of the question. Future wires might not be made of copper; resistance could be reduced with new technologies, such as achieving superconductivity, which would mean no resistance at all, making it possible to transmit Earth's power to the Moon.
However, I think while it's feasible to power a space elevator from Earth, sending electricity to the Moon in this labor-intensive and costly manner doesn't seem necessary. I believe scientists, being much smarter than I am, would avoid such impractical endeavors. Even for a space elevator, the power might not come from Earth but could be generated on the elevator itself, using solar energy or other methods.
Sending electricity from Earth to the Moon is entirely a case of going out of one's way for something unnecessary and costly. There's absolutely no need to do something so foolish, because the Moon has at least two power generation advantages over Earth: solar energy and nuclear fusion.
On the Moon, due to the lack of atmosphere, solar energy reaches the surface without any obstruction, causing temperatures to rise to 130°C wherever the sun shines. Moreover, the Moon rotates once every 27.32 Earth days, meaning the sun moves very slowly across the lunar surface, allowing one spot to be exposed to sunlight for nearly 14 days before turning into night. This makes solar energy on the Moon significantly more potent than on Earth, so why not utilize it?
Calculations show that even with the most ordinary solar power devices, the energy generation on the Moon could reach 2.7 kW/h (kilowatt-hours) per square meter. If a solar photovoltaic power plant of 1,000 square meters were built, it could produce 2,700 kW of electricity every hour.
Research and experiments on controlled nuclear fusion have been ongoing for decades, with significant breakthroughs already achieved, predicting commercial application within the next few decades. The primary fuels for nuclear fusion are heavy hydrogen, namely deuterium and tritium, but an even better fuel is helium-3. Helium-3 is considered an exceptionally clean, safe, and efficient fuel for fusion power generation, often called the "perfect energy" by scientists.
However, helium-3 is extremely rare on Earth, with the entire planet's reserves amounting to only about 500 kilograms. In contrast, the Moon has an abundant supply, conservatively estimated at over one million tons. If helium-3 were used as fuel for fusion power, just 100 tons could meet the energy needs of all humanity for a year. Therefore, lunar helium-3 represents a future solution to Earth's fuel problems. Why go through the trouble of sending power from Earth when we could harness helium-3 for electricity on the Moon?
Therefore, no matter how the future unfolds, directly transmitting electricity from Earth to the Moon via wires is unnecessary. It's not a question of whether technology can solve the issue, but rather it's about the impracticality and inefficiency of such an approach.