SECTION TWO: THREATS FROM THE SOLAR SYSTEM
In the vast universe, the solar system only takes up one small corner. Nevertheless, it is the homeland of humans; the earth we live on is one small planet within the sun’s star system.
Standing above the sun’s north pole, we can see that all eight planets of the solar system rotate around the sun in the same counterclockwise direction. Their orbits almost align onto one plane, called the ecliptic. The orbit of the planets is described as oval, but in reality it is almost round.
The composition of these eight planets vary greatly, but they can be divide into two categories. The terrestrial planets—Mercury, Venus, Earth, and Mars—are solid planets composed of rock and metal. They are dense, they rotate slowly, and they have few satellites, similar to Earth. The Jovian planets—Jupiter, Saturn, Uranus, and Neptune—are bulky, heavy, low-density planets composed mainly of liquid hydrogen, liquid helium, and other substances. They wander space like “water balls,” rotate fast, and have many satellites and rings.
The satellites of each planet are also important celestial bodies in the solar system. There are over 150 confirmed satellites in the solar system, and many more await confirmation. Although the earth is a relatively small planet, its satellite—the moon—is a large satellite, ranking fifth among the solar system satellites.
There are also many smaller celestial bodies in the solar system, such as dwarf planets, asteroids, comets, and meteoroids; they are too numerous to count. In addition, interstellar dust and interstellar rays are also components of the solar system.
As the third planet in the solar system, the earth is 150 million kilometers from the sun; the furthest planet, Neptune, is 3.5 billion kilometers away. That is far from the boundaries of the solar system. Dwarf planets (such as Pluto), many smaller celestial bodies, and interstellar material all exist beyond Neptune.
One: Threats from the Sun
1. The Sun Turns into a Red Giant
The sun’s mass is 2×1030 kilograms; its diameter is 1.39 million kilometers, and it is composed of 71 percent hydrogen, 27 percent helium, and 2 percent carbon, oxygen, silicon, iron, and other elements. Humans’ survival depends primarily on the sun; its light shines upon Earth and warms us. Without the sun’s light and heat, the earth would be a cold, dead planet. If the sun went through any big change, it would be a catastrophe for Earth and mankind.
The sun relies on nuclear energy to emit light and heat; the energy it pro- duces every second is equivalent to that of twelve billion tons of coal. Earth only receives less than 2.2 billion of that light and heat, but it is enough to maintain the earth’s ecology and turn it into a beautiful and pleasant planet. The energy source of the sun has always been a topic of concern to scientists.
Since ancient times, humans’ experience with ignition has never strayed from the concept of chemical combustion. Whether it is coal, oil, or trees, burning has always been generated through the atomic shift of chemical energy. According to calculations done with the highest combustion value fuels, if the sun continues to burn, it cannot last more than thousands of years; the most optimistic estimates do not exceed hundreds of thousands of years. On this basis, people have come to many erroneous conclusions, such as the belief that Earth’s history as well as human and biological history are both significantly shorter than their actual length. The further inference from that views the future of mankind extremely pessimistically. Since the sun’s fate determines the fate of Earth and mankind, if the sun were to burn up in a few thousand years, humans would be extinct in a few thousand years as well.
However, the study of the earth’s crust as well as the study of paleontology showed that both Earth and life on Earth have existed for far longer than people imagine. We also learned through astronomical observations that the actual history of stars was very different from past understandings, so people began to doubt the sun’s energy source.
As early as the 1860s, scientists learned from optical analysis that the sun’s main component was hydrogen. With the discovery of elemental radioactivity at the end of the nineteenth century, scientists recognized that there was a previously unknown energy in nature—nuclear energy. Significant breakthroughs in the understanding of nuclear energy followed quickly; in particular, Einstein’s famous mass-energy equation theoretically proved the existence of nuclear energy, as well as the relationship between energy and mass. Further observations and studies show that the sun’s internal energy is over ten million degrees, which means that in the extreme temperatures of the sun’s core, the violent movement of the nuclei can break through the electromagnetic force. Scientists finally concluded that there was thermonuclear reaction happening inside the sun’s interior and that the sun’s light and heat were provided by nuclear energy. Moreover, the energy of other stars is also derived from nuclear energy.
Today, our understanding of the sun has reached a very high level, enough to ascertain the following: Our sun was born as a star about five billion years ago; its predecessor was a large hot air mass. We can confidently determine that this hot air mass was the remains of a second- or third-generation large star exploding in the universe. Through hundreds of millions of years of evolution, this hot air mass formed its own mass-intensive central area through gravity, then formed a primitive planet that continued to absorb the material around it through strong gravitational force, ultimately igniting the hydro- gen atoms at the core. This was the birth of the sun as a star.
The sun can burn hydrogen elements for about ten billion years; at present, it has burned five billion years with five billion more remaining. This is the stable and mild period of the sun; however, in five billion years, the helium atoms inside the sun will ignite and the sun will become a huge red giant. Helium will continue to burn for one billion years, but once helium is exhausted, the sun will quietly become a white dwarf. Although this white dwarf will have some remnant heat, there will be no nuclear combustion inside of it and it will naturally cool with time. Once the sun evolves into a red giant, the diameter of the new sun will be more than one hundred times that of the original sun, so it will quickly swallow Mercury, Venus, and eventually Earth. Once Earth ceases to exist, the homeland of humanity will disappear into the universe.
Standing on a billion-year perspective to consider the external forces that threaten the survival of mankind, the evolution of the sun into a red giant in five billion years is undoubtedly a threat. In fact, the sun will no longer be stable a few hundreds of millions of years before this. In this transition period, the Earth will suffer constant pressure from the sun.
When the sun evolves into a red giant, its flames will spread out. Not only will Earth be swallowed, but Mars will also become inhabitable. Humans will need to migrate even farther out. At this time, Jupiter or one or more of Saturn’s satellites might be transformed into a place for human habitation, but the external environment will be abysmal.
Once the sun finally evolves into a white dwarf, it will no longer be possible for humans to survive in the solar system. Even though a hot white dwarf can radiate light and heat during its cooling process, its radiation energy will be extremely limited. Perhaps Mercury’s position today would be close enough to enjoy such light and heat, but Mercury would have been swallowed when the sun turned into a red giant. Unless humans could move a planet closer to the white dwarf or live on an artificial celestial body, we would have to move out of the solar system. Moreover, white dwarf planets are not reliable long term, as they will slowly cool until all light and heat are completely lost.
Humans would not be able to survive in the solar system once the transition period began. During this period, the sun would be very unstable as it constantly underwent dramatic change, meaning that it would be impossible to accurately determine its movement pattern. One violent change would be enough to destroy humanity.
2. Threats from Solar Activity
Can we fully trust the sun in the billions of years before it becomes a red giant? Will it suddenly malfunction one day and greatly endanger humans? Do we really understand the sun and have a basis to say that there is no problem there?
The sun is a gas planet divided into the core, the radiation zone, the convection zone, and its atmosphere from the inside out. This atmosphere can be divided into the photosphere, the chromosphere, and the corona. These three layers are not cleanly divided but infiltrate each other. The sun’s convection zone and what lies beneath cannot be directly seen through telescopes; their properties can only be determined through observational data and related theoretical calculations. The most direct impact the sun has on the earth comes from its surface activity; these solar phenomena mainly include sunspots, sun flares, prominences, and solar winds.
Even without the use of observational instruments we can observe that black spots appear frequently on the sun. These black spots are sunspots. Sunspots often exist in pairs and rotate around the sun from west to east in the same direction as solar rotation. They last a few days or as long as a few weeks from formation to disappearance. The central temperature of sunspots is about 4,500 K–1,200 K less than the surface of the photosphere, making them appear black in comparison. Sunspots are generally oval in shape; small sunspots are a few kilometers in diameter while larger ones can reach a diameter of tens of thousands of kilometers. Sometimes they appear in groups that span several hundred thousand kilometers. It is generally believed that the appearance of sunspots is a result of the sun’s magnetic field. The activities of sunspots have obvious cycles; sometimes they appear frequently, and other times they rarely occur. The average cycle is eleven years.
Solar flares are sudden flashes of increased brightness on the chromo- sphere; they are concentrated outbreaks of solar energy within a very short period of time. When a solar flare breaks out, it can throw out a large number of charged particles in a very short time, and it can accelerate solar winds a hundredfold.
Prominences are strong flows of hydrogen that burst out from the chr- mosphere. These bursts of hydrogen ignite into red flames and surge up hundreds of thousands of kilometers. It is generally believed that prominences are a result of sudden changes in the sun’s magnetic field or are produced by the constant fluctuation of hydrogen flows. The activities of solar flares and prominences are closely related to the activity of sunspots; the movement of sunspots is considered to be the main sign in determining the strength of solar activity.
Earth’s ecology is closely related to solar activity. When solar flares break out, strong solar winds interfere greatly with the earth’s magnetic fields; these are known as magnetic storms. Shortwave communication on Earth relies on the ionosphere fifty or sixty kilometers above surface to reflect and disseminate information. Once magnetic storms occur, the ionosphere’s degree of dissociation increases and electromagnetic waves are absorbed or fail to reflect normally, thus weakening the signal and interrupting short- wave communication.
Magnetic storms also affect the chemical structure and dynamics of the earth’s upper atmosphere. Prolonged exposure to magnetic storms may greatly affect the earth’s climate and cause floods or droughts. According to global climate analysis, the climate change cycle is twenty-two years, which is consistent with the sun’s magnetic cycle. Solar activity is also associated with earthquakes. Through analysis of global seismicity cycles over the years, we have determined the earth’s seismic cycle to be eleven years—completely consistent with sunspot activity cycles. Some scientists believe that the energy impact of solar winds on Earth increases during solar activity peak years, leading Earth’s rock layer to discharge under pressure. In addition, the rock layer also stretches and vibrates under the alternating electromagnetic field, causing the rocks that had already accumulated tension to fracture and dislocate, leading to earthquakes.
In the study of old trees, we have learned that when solar activity is frequent, the growth rings of tress are wider, meaning they grow faster. Contrarily, in slower solar activity years, the growth rings of trees tend to be narrower, indicating slower growth. This confirms the impact of solar activity on Earth’s biology. According to historical statistics, the growth of crops also conforms to this pattern.
Solar activity is also closely related to human health. For example, during solar activity, ultraviolet light is significantly enhanced, and the earth’s magnetic field experiences strong disturbance, which can affect cardiovascular functions. During solar activity peak years, bacteria also breeds faster, causing the flu, diphtheria, and other epidemics to occur at higher rates. According to Russian scientists, the biggest cholera pandemics in history generally took place during solar activity peak years.
While it is certain that solar activity influences the earth’s ecology, and that the sun can both nurture and harm life on Earth, none of the above factors can endanger the overall survival of mankind. This conclusion is not deduced by decades or centuries of observation, nor is it summarized through thousands or tens of thousands of years’ experience, but it is proven by five billion years of history. In the past five billion years, the sun has transformed the earth from a barren planet into a beautiful and pleasant planet. It woke the earth from its dead silence and enabled the first batch of life to be hatched 4.28 billion years ago; those were the simplest microbes. From then on, life evolved, continually basked in the glory of the sun, and never stopped until large complex life formed in the ocean 530 million years ago. Then, four hundred million years ago, life moved on land. Apes entered the threshold of man more than four million years ago, and humans completed their evolution nearly 100,000 years ago.
In five billion years the sun has never forsaken us, which is why we have every reason to believe that it will continue to nurture us for the next five billion years of its main star sequence. This conclusion can be confirmed by astronomical observations of other stars similar to the sun in the universe, and by existing scientific theories.
Two: Extraterrestrial Body Collision
In July of 1994, astronomy enthusiasts from all over the world witnessed the astronomical marvel through their telescopes of a comet colliding with Jupiter. While over Jupiter, comet Shoemaker-Levy 9 was torn into twenty- one pieces by Jupiter’s huge gravitational force. These pieces crashed into Jupiter at a speed of sixty kilometers per second, exploding into a huge fireball and flashes of light while also producing a series of dark spots in Jupiter’s atmosphere. The impact of such a collision is equivalent in strength to 100,000 nuclear bombs. If it happened on Earth, our entire ecology would suffer great damage, and humans’ survival would be seriously threatened.
The 1994 Jupiter collision is not the largest impact Earth has suffered. Many people consider the extinction of Earth’s dinosaurs 6,500 years ago to have been caused by an asteroid colliding with Earth. It is believed that an asteroid fifteen kilometers in diameter hit what is today Mexico’s Yucatan Peninsula. Scientists have long been studying an impact pit buried deep under the Yucatan Peninsula; it is estimated to be 180 kilometers in diameter and 900 meters deep. If humans had lived in that era, we would have suffered a huge catastrophe. In fact, a large enough extraterrestrial body would be more than enough to destroy humanity. Therefore, we must study extraterrestrial body collisions in order to research the overall survival of mankind.
Within the solar system, celestial bodies that may collide with us include asteroids, comets, meteorites, and meteors. Since meteorites and meteors are too small to pose a threat to humans, we will only discuss the collision of asteroids and comets here.
1. Asteroid Collision
Asteroids revolve around the sun, just like Earth, but they are merely much smaller in size. There are many asteroids in the solar system, concentrated mainly in two areas. One is the vicinity of Pluto’s Kuiper belt, and the other is the asteroid belt between Mars and Jupiter. As the Kuiper belt is far away from us, the asteroids there are not a threat to Earth. When studying the threat of asteroids, scientist generally do not consider the asteroids of the Kuiper belt.
The total number of asteroids in the solar system is estimated to be more than 500,000 (excluding the Kuiper belt and its outer asteroids). They are mostly small in size, and though they exist in great numbers, their total mass does not reach five ten thousandths that of Earth. Most of these asteroids are located on the asteroid belt in an area that measures roughly 2.17–3.64 astronomical units. (An astronomical unit refers to the distance between the earth and the sun; one astronomical unit is about 150 million kilometers.) The asteroids in the asteroid belt are very far away from us and generally do not pose a threat; however, due to their small size, light mass, and vulnerability to planetary influence, asteroids are highly probable to change trajectory, requiring us to give the distant asteroid belt considerable attention.
Many scientists believe that most of the asteroids in the solar system are concentrated between Mars and Jupiter because Jupiter’s gravity attracted the asteroids originally located in the inner ring. Since Jupiter is the largest planet in the solar system, its gravity is much greater than the planets of the inner ring.
There are also a few very unique asteroids that have strayed from the asteroid belt and approached the inner orbit of Earth, while some others have progressed to the outer side of Saturn’s orbit. Since these asteroids are very close to Earth, we call them near-Earth asteroids. These are the asteroids that truly concern us, as they pose a much bigger threat to Earth than do the asteroids in the asteroid belt.
Although there is no written record of an asteroid hitting Earth, the number of asteroids around us, as well as our observational experience of celestial bodies, make asteroids a very real threat. Compared to threats from the universe and the threat of the sun evolving into a red giant, the threat of asteroids seems much more immediate. The numerous records of meteorites impacting Earth are a warning themselves, seeing as meteorites are just smaller versions of asteroids.
On February 15, 2013, a meteorite penetrated the atmosphere above Russia’s Chelyabinsk and exploded into pieces, forming meteorite rain. The shock waves produced by friction between the meteorite and the atmosphere caused many buildings’ windows to explode, injuring more than a thousand people. This event was filmed by a number of people.
If a large enough asteroid were to hit Earth, it could destroy humanity; even a relatively large asteroid colliding with Earth could greatly damage humanity. Due to this, several major countries have devoted considerable effort into observing and studying asteroids. For example, the US congress requires NASA to record and classify any asteroids that are more than one kilometer in diameter.
At present, we have discovered close to thirty thousand near-Earth asteroids, 1,100 of those are larger than one kilometer in diameter, and the largest of them all is the famous 433 Eros asteroid. Eros’ orbit lies between Earth and Mars; its diameter is twenty-two kilometers. If an asteroid this size hit Earth, it would greatly affect global ecology; many species would go extinct, humans would suffer great destruction, and human cultural achievements would be damaged significantly. Fortunately, we have a very thorough under- standing of this asteroid. The NEAR-Shoemaker unmanned probe launched by NASA on February 14, 2000, successfully entered the orbit of Eros and landed on it after a year of close-up survey on February 12, 2001, conducting very fruitful research of the asteroid.
However, we do not know all the near-Earth asteroids as well as we do Eros, and we have especially inadequate understanding of the smaller asteroids. For example, on March 18, 2004, the small celestial body 2004 FH flew over Earth’s surface at a distance of 43,000 kilometers, but scientists did not discover it until three days before the flight. If the thirty-meter-diameter celestial body had hit a medium-sized city, that city would have been demolished. Of course, such demolition is far from enough to destroy humanity.
Some of the closer near-Earth asteroids could bring great disaster if they collided with Earth. According to observation and analysis, the asteroid (29075) 1950DA will fly very low over Earth’s surface in 2880. If its orbit changes even a little, it may hit Earth. If this asteroid, which is 1.4 kilometer in diameter, collided with Earth, the disaster it would bring could affect Earth’s ecology on a global scale. Huge numbers of creatures (including humans) in a tens of thousands square kilometers range would be largely destroyed. Fortunately, there are more than eight hundred years left for us to re-evaluate the orbit of this asteroid and deal with its possible impact.
2. Comet Collision
Comets are composed of rock, frozen water, carbon dioxide, dust, and various impurities; they are celestial bodies of relatively small mass. If all the material in a larger comet were compressed together, its diameter would not exceed tens of kilometers.
The core of the comet is called the nucleus, and the outer cloudy layer encircling the core is called the coma. When the comet approaches the sun, strong solar winds and radiation pressure from the sun pushes the coma into a long tail. Comet tails range from tens of thousands of kilometers to hundreds of millions of kilometers in length. The comet tail usually has very sparse material; its density is only one quadrillionth of the earth’s surface atmosphere.
The path of a comet is very hard to predict, as their orbits can be elliptical, parabolic, or even hyperbolic. Moreover, the orbit of a comet is susceptible to influence from the planets it passes, and even the stars in the distance. As a result, some comets’ paths change constantly, other comets disappear into space, and new comets arrive inexplicably to the solar system.
Many comets exist in the solar system, but only 1,600 have been observed by scientists, and very few of their orbits have been grasped. The orbit cycle of comets also differs greatly. Short ones last a few years or more than one hundred days, while longer ones may reach thousands of years or even tens of thousands of years.
Comets themselves are very unstable. Every time they pass the sun, some of their material is blown away into space by solar winds, so a comet’s mass is always shrinking until only the nucleus is left. Some comets composed exclusively of ice and dust may eventually disappear completely.
Comets may also be disintegrated by the gravitational force of the sun or other planets. The famous Bella comet is one such example; its revolution cycle around the sun was 6.6 years. On January 13, 1846, the Bella comet suddenly split into two. When the two halves returned, they returned simultaneously as a pair of comets and disappeared after 1859. Large meteor showers occurred later where their orbit intersected with Earth’s orbit, indicating that the Bella comet had completely disintegrated.
When bigger comets collide with Earth, the threat the humanity is obvious. So far, the largest recorded comet collision happened on June 30,1908, when a violent explosion occurred atop Tunguska in Siberia, Russia. The huge fireball explosion could be seen and heard from more than one thousand kilometers away, and the shock waves from the explosion tore down and burned hundreds of square kilometers of forest. All the animals in the forest died, including a large group of reindeer that were grazing in the area. Fortunately, the area was uninhabited, so there were no human casualties. In later investigations, scientists found only burnt land and animal corpses with no trace of meteorites or craters. It is thus inferred that this was a comet com- posed completely of water. When the comet entered Earth’s atmosphere, the high temperatures produced by friction caused violent evaporation, which led to the huge explosion ten kilometers above ground.
Compared to large asteroids, the impact of comets has slightly less catastrophic influence on humanity; however, a large enough comet impacting Earth could still cause devastating losses to humanity—it would just not be enough to cause the total destruction of humanity.
Near-Earth comets are much smaller than near-Earth asteroids, and they generally have long tails that are easy to observe. At present, we have already observed about fifty near-Earth comets. This number is significantly less than that of near-Earth asteroids, seemingly indicating that the possibility of comets colliding with Earth is much smaller—that is not the case. The most prominent feature of comets is that they are extremely susceptible to disturbances from the sun and large planets. Comets at the very edge of the solar system may inexplicably travel to the inner side of the solar system sometimes. In short, the possibility of comets crashing into Earth is higher than that of asteroids colliding with Earth, and they are much harder to take precautions against; therefore, in recent years, scientist have paid more and more attention on comet research.
3. Comprehensive Analysis of Extraterrestrial Body Collision
The possibility of large extraterrestrial bodies colliding with Earth is very small, but a large enough asteroid hitting Earth could destroy humanity once and for all. That is why we cannot ignore the issue just because of its low probability. One chance encounter and humanity may never recover; more- over, such collisions can occur.
Exactly what kind of threat do extraterrestrial bodies pose to humanity? Many scientists have conducted in-depth research on this topic, but the views differ greatly. Different people have reached different conclusions. After considering a variety of different views, the following conclusions can be drawn:
a. The larger the celestial body, the less likely it will collide. As the large celestial objects of the solar system (including Earth) each orbit on their own trajectories over and over, the impact frequency will only decrease.
b. The following can be estimated concerning the impact frequency and perils of different sized celestial bodies:
i. A celestial body eighty meters in diameter may collide every one hundred years. Such a collision could destroy a large amount of life within an area of a few hundred square kilometers, and it would influence the ecological environment tens of thousands of square kilometers around it.
ii. A celestial body eight hundred meters in diameter may collide every two thousand years. Such a collision may result in the destruction of most life within tens of thousands of square kilometers and influence the ecological environment for millions of square kilometers.
iii. A celestial body three kilometers in diameter may collide every ten million years. This impact could destroy most life within one or two hundred thousand square kilometers, and the ecology of the entire world would be affected.
iv. A celestial body over ten kilometers in diameter may collide with Earth every seventy million years. This collision could cause the destruction of many species around the world and seriously damage global ecology; it would take many years for the earth to recover.
v. A celestial body over one hundred kilometers in diameter may collide once every hundred million years; it could destroy global ecology, humanity, and human civilization in one swoop.
c. The most terrifying and relatively realistic celestial collision would be a collision with the asteroid Eros. Eros is the largest near-Earth asteroid, and as a twenty-two-kilometer asteroid, it would cause massive casualties, devastate Earth’s ecology, and ravage human civilization upon impact. However, collision from Eros would not lead to human extinction. On top of that, Eros shows no sign of hitting Earth at the moment, and we employ very reliable tracking and observation of this asteroid as a cautionary measure.
In reality, we have now developed a considerable ability to guard against extraterrestrial body collisions. The development of space observation tech- nology has enabled us to develop a more detailed understanding of how celestial bodies operate and are distributed in the solar system. We are getting better at monitoring the larger celestial bodies that constitute a threat to the survival of mankind. As space technology continues to progress, we are increasingly capable of traveling in space. Humans have already landed on the moon, and consecutive Mars or other planet landings are fully achievable in this century. As for the explosion of nuclear warheads, they are not only capable of destroying large celestial bodies, but they can also change the trajectories of such bodies. Moreover, we not only have spacecrafts that can accurately send these nuclear warheads into the depth of space, but also intercontinental missiles that can send warheads into the space near Earth. With these conditions, it is becoming increasingly possible that humans will be able to intercept any celestial bodies that may collide with Earth.
Scientists have optimistically estimated that as long as we can predict possible celestial body collisions a few years ahead of time, we can change their trajectory and send them away from Earth. If we could predict such celestial body collisions a few decades in advance, we could use spacecrafts to send ordinary explosives into space, slightly changing the orbit of the celestial body and protecting Earth from impact. Therefore, as long as humans further improve their observation abilities, avoiding the impact of small celestial bodies is a very real possibility.
In fact, scientists possess a variety of suggestions for coping with celestial body impact. Some propose the use of lasers to slow down the asteroid and change its trajectory; some have suggested that a smaller asteroid can be propelled to collide with the asteroid coming towards Earth, changing its direction. Scientists have also proposed a number of options to prevent asteroid impact, like launching missiles or spacecrafts to crash into asteroids and change their trajectory; launching gravitational spacecrafts to approach and change the direction of asteroids through gravity; launching spacecrafts to land on asteroids and gradually change their orbit with electric motors; or fitting asteroids with solar sails to change their track.
According to reports, the United States is studying the use of the Yarkovsky effect to change the orbit of asteroids. This is accomplished by unmanned probes hovering over asteroids to spray their surface and change their sunlight reflectivity, thereby changing their absorption of sunlight and heat, which in turn changes their orbit.