With the Pentagon recently releasing a second batch of previously classified photos and videos of UFOs, the discussion about whether extraterrestrial life has visited the Earth has once again become the focus of attention from all walks of life. This cultural change, which began with the U.S. Congressional hearings in July 2023, has gradually evolved UFO reports from folk rumors into serious issues that the government and scientific community need to take seriously. But is this newfound legitimacy worthy of the name?Kai James, a professor of aerospace engineering at the Georgia Institute of Technology, recently wrote an article using mathematics, physics and engineering principles to provide an in-depth analysis of the numerous technological obstacles that alien spacecraft must overcome to reach the Earth.
Professor James pointed out that the first obstacle to assessing the possibility of extraterrestrial visitors is the "tyranny of distance". There is currently no evidence of intelligent life in the solar system, which means any alien visitors would have to travel across the vastness of interstellar space. Take Proxima Centauri, the closest star to the sun, for example. Its distance of 4.25 light-years, in a macroscopic model, is equivalent to the long journey from New York to Sydney when the Earth is reduced to the size of a pea. Since stars harboring intelligent life are extremely rare, actual alien civilizations will only be further away from us. In order to avoid the increasing risk of system failures and catastrophic accidents during the long journey, the spacecraft must fly as fast as possible. Although the speed of light is an insurmountable upper limit, the scientific community generally agrees that 10% of the speed of light (i.e. 19,000 miles or 30,000 kilometers per second) is a more realistic cruising speed. Even at this speed, a 10-light-year journey would take a full century.
How to accelerate a spacecraft to such an amazing cruising speed is the core challenge facing all alien explorers. Due to the lack of atmosphere in interstellar space, although the spacecraft does not have to worry about air resistance and can glide by inertia, it also means that it cannot use the atmosphere to slow down. Therefore, an ideal propulsion system must take into account both starting acceleration and terminal deceleration. Several current theoretical propulsion strategies have their own advantages and disadvantages: Although the solution of using high-power laser beams to propel the light sail can free the spacecraft from the burden of carrying its own fuel, it requires the construction of an extremely large energy infrastructure on the home star and lacks a self-deceleration mechanism; due to the extremely low energy conversion rate of chemical propulsion rockets that humans are currently familiar with, if they want to reach 10% of the speed of light, the total mass of fuel required will even exceed the total mass of the entire observable universe, which is completely unfeasible in reality.
Among the more imaginative technologies, the theoretically most effective antimatter propulsion can achieve 100% mass-to-energy conversion and requires less than a quarter of the total mass of fuel to reach the target speed. However, antimatter is extremely unstable and extremely difficult to manufacture. The total amount of antimatter produced by humans so far is less than 20 billionths of a gram, and its lifespan is extremely short and the cost is high. In contrast, nuclear fusion propulsion, which emulates the principles of the sun, has become a more feasible alternative, which can theoretically produce 10 million times the energy per kilogram of chemical rockets. Although NASA and other agencies have been working on the development of related technologies, calculations show that a fusion-powered spacecraft reaching 10% of the speed of light would still require up to 150 times the mass of the spacecraft itself.
In addition to the problems of the propulsion system, the design of the spacecraft structure also faces delicate balances and extreme trade-offs. Interstellar space seems empty, but in fact, tiny cosmic dust and hydrogen atoms are sparsely distributed. As the spacecraft travels at 19,000 miles per second, tiny dust particles impact with the intensity of bullets, while bombardments of hydrogen atoms create radiation cascades powerful enough to eat away at the strongest materials. In order to survive such a fierce attack, the spacecraft must be built like a "flying fortress" equipped with a complex magnetic shielding system, which will inevitably significantly increase the total mass of the spacecraft, thus creating a vicious cycle of "more fuel is needed to carry defensive armor, and more fuel is needed to carry fuel." This conflicting requirement to be both structurally lightweight and extremely durable often brings the intersection of all engineering solutions down to zero.
Professor James emphasized in his summary that although there is no law of physics that explicitly prohibits interstellar travel, hundreds of extreme and conflicting engineering requirements are intertwined, which may impose a "death sentence" on interstellar travel in physical reality. Any potential extraterrestrial visitor must not only possess cognitive capabilities, technological sophistication, and material resources that exceed human imagination, but must also solve these inevitable engineering challenges during technological evolution. When an alien spacecraft really lands on Earth intact, compared to "who are they" or "what do they want?", perhaps the trillion-level scientific question that humans should urgently ask should be: "How did they overcome these engineering desperations and get here?"