Since the manned moon landing mission by the United States in the previous century, humans have never truly ventured beyond Earth’s atmosphere. However, human spaceflights and explorations continue, with unmanned spacecraft delving into every corner of the solar system, providing us with a broader understanding of its conditions. In recent years, the topic of human space travel, involving manned missions back to the Moon and potential landings on Mars, has gained momentum, with plans for colonization on Mars becoming a part of discussion.
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Yet, there is a lingering uncertainty: Why the need for interstellar migration? Why does Mars become the first stop? Earth still possesses many untapped resources and uninhabited areas. Even deserts, icy rivers, and other challenging environments on Earth are considered more habitable than Mars. The substantial funds invested in transforming Earth might be better used to sustain more people. So, why are we aiming for human settlement on Mars?
These questions often surface in forums and social media platforms among science and exploration enthusiasts. Today, let’s delve into this subject and attempt to clarify it. Human colonization of distant planets is not driven by the urge to explore virgin territories; instead, it stems from the deeper purpose of finding a “refuge” for humanity. Are we fleeing from something?
In reality, it’s evident that human society is progressing rapidly, thanks to the ever-advancing science and technology facilitated by handheld mobile devices, internet browsing, airplanes, and high-speed railways.
However, when we scrutinize the issue of human society, we must consider two points. First, our modern civilized and content life owes much to the continuous progress in science and technology. Second, our current stable and happy life is a short-term perspective; it might last for our generation and a few generations to come, but it might not be sustainable in the long run. This timeline could be either quite long or very short.
Another reality is the environmental destruction caused by human over-exploitation of resources since the industrial era, as well as the greenhouse gas emissions from burning fossil fuels, which have intensified Earth’s greenhouse effect—the average temperature has risen over 1 degree Celsius since pre-industrial times. It is believed that a further temperature increase of 3 degrees Celsius could trigger the collapse of Earth’s self-regulating climate system, resulting in unstoppable catastrophes.
For this reason, scientists have issued numerous warnings and studies, suggesting that without restraint and improvement, humanity may face ecological catastrophe and extinction by the end of this century.
Therefore, humans also need to be wary of inevitable natural catastrophes both on our planet and in outer space, such as drastic solar changes and collisions of celestial bodies with Earth—events beyond human coping capability. Presently, the entire human population resides on a single planet—Earth. When confronted with extinction-level catastrophes, humanity might vanish permanently within this universe.
Furthermore, the process of interplanetary migration is not a task to be accomplished overnight but requires efforts of multiple generations, even decades, or possibly centuries. If efforts are not initiated now, the likelihood of human extinction becomes higher. This fundamental reason is why some scientists, entrepreneurs, and policymakers with foresight continue to explore outer space and seek new habitats for humanity.
Scientists have discovered eight planets in the solar system, along with several dwarf planets and moons. When comparing the planetary correlation within the solar system, apart from Earth, Mars stands as the planet most suitable for human habitation. This does not imply that the environments of Mars and Earth are the same; Mars’ environment is still more favorable than those of the remaining six planets
Mercury is positioned too close to the Sun and possesses a small mass and volume, making it a rocky and solid planet. Its rotation has been locked due to solar tides, with one face always directed towards the Sun and the other in the opposite direction. Consequently, the environment on this planet is harsh, with temperatures reaching up to 428 degrees Celsius on the side facing the Sun and as low as -193 degrees Celsius on the side away from the Sun.
Venus is slightly smaller than Earth, with 88% of Earth’s volume and 81.6% of its mass. However, Venus’ rotation speed is unsuitable for human life, and its atmosphere primarily consists of carbon dioxide greenhouse gas, leading to a surface temperature of up to 500 degrees Celsius. The surface pressure on Venus is 90 times that of Earth, frequent occurrences of dense sulfuric acid rain take place, and no liquid water or oxygen is present.
The remaining four planets—Mercury, Mars, Jupiter, and Saturn—are all gas giants. The smallest, Jupiter, has a mass 14.5 times that of Earth, while the largest, Saturn, has a mass 318 times that of Earth. None of these gas planets have solid surfaces. The highest surface temperature on Saturn is -148 degrees Celsius, and the lowest on Neptune goes down to -200 degrees Celsius. Hence, they are entirely unsuitable for human colonization.
In comparison, Mars’ environment is much milder among these six planets. Despite Mars having a thinner atmosphere than Earth, it still maintains one. While the temperature can be harsh, it remains tolerable. Though liquid water is absent, frozen water exists underground and at the poles. Its rotation cycle lasts 24 hours, 37 minutes, and 22.7 seconds—similar to Earth’s. The orbital period is 686.971 days, nearly equal to two Earth years. Importantly, due to its axial tilt of 25.19 degrees—similar to Earth’s 23.26 degrees—the equator resembles Earth’s, with direct sunlight and distinct seasonal changes throughout the year.
This analysis reveals that, except for Earth and Mars, the environments of the other six planets are comparable to infernos.
Some believe that certain moons within the solar system might be suitable for human migration, such as Titan. However, deeper exploration and research indicate that Titan’s atmosphere comprises 98.66% nitrogen, with a surface temperature of -179.15 degrees Celsius. Absence of liquid water, lakes, and seas consists of hydroxylmethane liquids, rendering it unsuitable for human habitation.
Of course, the Moon could potentially become a human settlement in the future, yet its small size and close proximity to Earth make it vulnerable to Earth’s potential calamities, hindering its role as a reliable backup.
Mars, however, boasts unique conditions. Besides Earth’s Moon and Venus, it’s the closest planet to Earth, just about 57 million kilometers at its nearest position. With our current space travel capabilities, reaching Mars marks the limit of our potential. Our farthest unmanned probe, Voyager 1, has been flying for over 40 years and covered more than 23 billion kilometers. Although it has left the solar system, traveling at around 17 km/s, it would still take over 17,000 years to escape the Sun’s gravitational radius of one light-year (9.47 trillion kilometers).
Hence, the possibility of human departure from the solar system currently seems implausible. However, as long as human extinction is avoided, humanity will eventually leave the solar system and venture into the depths of space. This inevitability arises from the Sun’s predicted destruction in 5 billion years (or possibly sooner), rendering the solar system uninhabitable.
Currently, even though humans have discovered thousands of exoplanets—planets outside the solar system—dozens of terrestrial planets are within the habitable zone and potentially possess liquid water suitable for human habitation. However, humans can merely gaze at these planets. Due to various factors, Mars has become the primary choice for human colonization.