The Future of Humanity

by Michio Kaku

  Jupiter is surrounded by a huge, deadly band of radiation, which is the source of much of the static you hear on the radio and TV. (A small fraction of that static comes from the Big Bang itself.) Astronauts traveling near Jupiter would need to be shielded from the radiation and would find communication difficult due to all the interference.

  Another hazard is its enormous gravitational field, which can capture or slingshot into outer space any unwitting passersby that stray too close, including moons or planets. This frightening possibility actually worked to our advantage billions of years ago. The early solar system was full of cosmic debris that constantly rained down on the Earth. Fortunately, the gravitational field of Jupiter acted as a vacuum cleaner, either absorbing it or flinging it away. Computer simulations show that, without Jupiter, the Earth even today would be bombarded with giant meteors, which would make life impossible. In the future, when considering solar systems to colonize, it would be better to look for those that have their own Jupiter, big enough to tidy up the debris.

  Life as we know it probably cannot exist on the gas giants. None of them have a solid surface on which organisms can evolve. They lack liquid water and the elements necessary to produce hydrocarbons and organic chemicals. Billions of miles from the sun, they are also freezing cold.

  THE MOONS OF THE GAS GIANTS

  More interesting than Jupiter and Saturn in terms of the potential to support life are their moons, of which there are at least sixty-nine and sixty-two, respectively. Astronomers had assumed that the moons of Jupiter would all be the same: frozen and desolate like our moon. They were completely surprised, then, when they found that each moon had its own distinct characteristics. This information brought about a paradigm shift in how scientists viewed life in the universe.

  Perhaps the most intriguing of all is Europa, one of the original moons discovered by Galileo. Europa, like some of the other moons of the gas giants, is covered with a thick layer of ice. One theory is that water vapor from the early volcanoes on Europa condensed into ancient oceans, which froze as the moon cooled. This may explain the curious fact that Europa is one of the smoothest moons in the solar system. Although it was heavily hit by asteroids, its oceans probably froze after most of the bombardment took place, thereby covering over the scars. From outer space, Europa appears to resemble a ping-pong ball, with almost no surface features—no volcanoes, mountain ranges, or meteor impact craters. The only visible feature is a network of cracks.

  Astronomers were thrilled when they discovered that underneath the ice on Europa could be an ocean of liquid water. It is estimated to be two or three times the volume of Earth’s oceans—our oceans only lie on the surface, while the oceans of Europa make up most of the interior.

  While journalists often say, “Follow the money,” astronomers say, “Follow the water,” because water is fundamental to the formation of life as we know it. They were shocked to think that liquid water could exist in the realm of the gas giants. Its presence on Europa introduced a mystery: Where did the heat come from to melt the ice? The situation seemed to defy conventional wisdom. We had long assumed that the sun was the only source of heat in the solar system and that a planet would have to be within the Goldilocks zone to be habitable, but Jupiter was far outside this band. We had neglected to contemplate, however, another potential source of energy: tidal forces. The gravity of Jupiter is so great that it can pull and squeeze Europa. As it orbits around the planet, it tumbles and rotates around its axis, so that its tidal bulge is constantly moving. This squeezing and pulling can cause intense friction in the core of the moon as rock is compressed against rock, and the heat generated by this friction is sufficient to melt much of the ice cover.

  With the discovery of liquid water on Europa astronomers realized that there is a source of energy that can make life possible even in the darkest regions of space. As a result, all the astronomy textbooks had to be rewritten.

  EUROPA CLIPPER

  The Europa Clipper is scheduled for launch sometime around 2022. Costing approximately $2 billion, its purpose is to analyze the ice cover of Europa and the composition and nature of its ocean for signs of organic chemicals.

  Engineers face a delicate problem in mapping out the trajectory of the Clipper. Because Europa lies within the fierce radiation band surrounding Jupiter, a probe placed in orbit around the moon might be fried after only a few months. To circumvent this threat and extend the lifetime of the mission, they decided that the Clipper should be sent around Jupiter in an orbit largely outside the radiation belt. Then its path can be modified so that it edges closer to Jupiter and makes forty-five brief flybys of Europa.

  One of the goals of the flybys is to examine, and perhaps fly through, the geysers of water vapor rising from Europa that have been observed by the Hubble Space Telescope. The Clipper may also release mini probes into the geysers in an effort to obtain a sample. Since the Clipper will not land on the moon itself, studying the water vapor is our best chance at this time of gaining insight into the ocean. If the Clipper is successful, future missions may strive to land on Europa, drill into the ice cover, and send a submarine into the ocean.

  Europa is not the only moon, however, that we are seriously scrutinizing for the presence of organic chemicals and microbial life. Geysers of water have also been seen erupting from the surface of Enceladus, a moon of Saturn, indicating that there is an ocean underneath the ice there as well.

  SATURN’S RINGS

  Astronomers now realize that the most important forces shaping the evolution of these moons are tidal forces. It is important, therefore, to study how strong these forces are and how they act. Tidal forces may also give us the answer to one of the oldest mysteries concerning the gas giants: the origin of the beautiful rings of Saturn. In the future, when astronauts visit other planets, astronomers believe that many of the gas giants will have rings around them, as in our solar system. This in turn will help astronomers to determine precisely how strong tidal forces are and whether they are powerful enough to tear entire moons apart.

  The splendor of these rings, which are made of particles of rock and ice, has enchanted generations of artists and dreamers. In science fiction, taking a spin around them in a spaceship is practically a rite of passage for every space cadet in training. Our space probes have discovered that all of the gas giants have rings, though none are as large or quite as beautiful as the ones circling Saturn.

  Many hypotheses have been offered to explain them, but perhaps the most compelling is the one involving tidal forces. The gravitational pull of Saturn, like that of Jupiter, is enough to make an orbiting moon slightly oblong, or football shaped. The closer the moon comes to Saturn, the more it is stretched. Eventually, the tidal forces stretching the moon balance the gravitational force holding the moon together. This is the tipping point. If the moon comes any closer, it is literally torn apart by the gravity of Saturn.

  Using Newton’s laws, astronomers can calculate the distance of the tipping point, which is called the Roche limit. When we analyze the rings not just of Saturn but of the other gas giants, we find that they are almost always within the Roche limit for each planet. All the moons we see orbiting the gas giants are outside the Roche limit. This evidence supports, though does not definitively prove, the theory that the rings of Saturn were formed when a moon wandered too close to the planet and was ripped apart.

  In the future, when we visit planets orbiting other stars, we can probably expect to find rings around the gas giants within the Roche limit. And by studying the strength of these tidal forces, which can potentially rip entire moons apart, one can begin to calculate the strength of tidal forces acting on moons like Europa.

  A HOME ON TITAN?

  Titan, one of the moons of Saturn, is another candidate for human exploration, although settlements there will probably not be as populous as those on Mars. Titan is the second-biggest moon in the solar system, next to Jupiter’s Ganymede, and is the only one to have a thick atmosphere. Unli
ke the thin atmospheres on other moons, its atmosphere is so dense that early photographs of Titan were disappointing. It resembled a fuzzy tennis ball without any surface features.

  The Cassini spacecraft that orbited Saturn, before finally crashing into the planet in 2017, revealed the true nature of Titan. Cassini used radar to penetrate the cloud cover and map the surface. It also launched the Huygens probe, which actually landed on Titan in 2005 and radioed back the first close-up photographs of its terrain. They showed signs of a complex network of ponds, lakes, ice sheets, and landmasses.

  From the data collected by Cassini and Huygens, scientists have pieced together a new picture of what lies beneath the cloud cover. Titan’s atmosphere, like that of the Earth, consists mainly of nitrogen. Surprisingly, its surface is covered with lakes of ethane and methane. Since methane can be ignited with the slightest spark, one might think that the moon could easily burst into flames. But since the atmosphere has no oxygen and is extremely cold at -180 degrees Celsius, an explosion is impossible. These findings present the tantalizing possibility that astronauts may be able to harvest some of the ice on Titan, separate the oxygen and hydrogen, and then combine the oxygen with the methane to create a nearly inexhaustible supply of usable energy—perhaps enough to light up and warm pioneer communities.

  While energy may not be a problem, terraforming Titan is likely out of the question. It is probably impossible to generate a self-sustaining greenhouse effect at such a great distance from the sun. And because the atmosphere already contains large quantities of methane, introducing more of it to initiate such an effect would be futile.

  One might well wonder whether Titan can be colonized. On the one hand, it is the only moon with an appreciable atmosphere, the pressure of which is 45 percent greater than Earth’s. It is one of the few known destinations in space where we would not die soon after we took off our space suits. We would still need oxygen masks, but our blood would not boil, and we would not be crushed.

  On the other hand, Titan is perpetually cold and dark. An astronaut on its surface would receive 0.1 percent of the sunlight that illuminates Earth. Solar energy would be inefficient as a power source, so all light and heat would depend on generators, which would have to run endlessly. In addition, Titan’s surface is frozen, and its atmosphere lacks significant quantities of oxygen or carbon dioxide to sustain plant and animal life. Agriculture would be extremely difficult, and any crops would have to be grown indoors or underground. The food supply would be limited, and with it, the number of colonists who could survive.

  Communication with the home planet also would be inconvenient, as it would take many hours for a radio message to travel between Titan and Earth. And since the gravity on Titan is only about 15 percent of that on Earth, people living on Titan would have to exercise constantly to prevent muscle and bone loss. They might eventually refuse to return to Earth, where they would be weaklings. In time, settlers on Titan might begin to feel emotionally and physically distinct from their earthbound counterparts and might even prefer to sever all social ties.

  So living on Titan permanently might be possible, but it would be uncomfortable and would come with many downsides. Large-scale habitation seems unlikely. However, Titan may prove valuable as a refueling base and as a stockpile of resources. Its methane could be harvested and shipped to Mars to accelerate terraforming efforts or could be used to create unlimited quantities of rocket fuel for deep space missions. Its ice could be purified into drinking water and oxygen or processed into more rocket fuel. Its low gravitational pull would make travel to and from the moon relatively simple and efficient. Titan could become an important gas station in space.

  To create a self-sustaining colony on Titan, one might consider mining the surface for valuable minerals and ores. At present, our space probes have not yielded much information about the mineral composition of Titan, but, like many of the asteroids, it may contain valuable metals that are crucial if it is to become a refueling and resupply station. However, it likely would be impractical to ship ores mined from Titan back to Earth because of the enormous distances and cost. Instead, raw materials would be used to create infrastructure on Titan itself.

  OORT CLOUD OF COMETS

  Beyond the gas giants, at the outer reaches of our solar system, lies yet another realm, the world of comets—perhaps trillions of them. These comets may become our stepping stones to other stars.

  The distance to the stars can seem unfathomably immense. Physicist Freeman Dyson at Princeton suggests that, to reach them, we might learn something from the voyages of the Polynesians thousands of years ago. Instead of trying to make one extended journey across the Pacific, which would likely have ended in disaster, they went island hopping, spreading across the ocean’s landmasses one at a time. Each time they reached an island, they would create a permanent settlement and then move on to the next island. He posits that we might create intermediate colonies in deep space in the same way. The key to this strategy would be the comets, which, along with rogue planets that have somehow been ejected from their solar systems, might litter the path to the stars.

  Comets have been objects of speculation, mythmaking, and fear for many millennia. Unlike meteors, which streak across the night sky in a matter of seconds and disappear, comets can remain overhead for prolonged periods of time. They were once thought to be harbingers of doom and have even influenced the destiny of nations. In the year 1066, a comet appeared over England and was interpreted as an omen that King Harold’s troops would be defeated at the Battle of Hastings by the invading forces of William of Normandy, establishing a new dynasty. The magnificent Bayeux Tapestry records these events and shows terrified peasants and soldiers gazing up at the comet.

  More than six hundred years later, in 1682, that same comet sailed over England again. Everyone, from beggars to emperors, was fascinated by it, and Isaac Newton decided to solve this ancient mystery. He had just invented a new, more powerful type of telescope, which used a mirror to collect starlight. With his new reflecting telescope, he documented the trajectories of several comets and compared them with predictions he had made according to his recently developed theory of universal gravitation. The motion of the comets fit his predictions perfectly.

  Given Newton’s propensity for secrecy, his momentous discovery might have been forgotten if it hadn’t been for Edmond Halley, a wealthy gentleman astronomer. Halley visited Cambridge to meet Newton and was flabbergasted to learn that he was not only tracking comets but could predict their future motions—something no one had ever done before. Newton had distilled one of the most baffling phenomena in astronomy, which had fascinated and haunted civilizations for thousands of years, into a series of mathematical formulas.

  Halley instantly understood that this represented one of the most monumental breakthroughs in all of science. He generously offered to pay the full cost of publishing what would become one of the greatest scientific manuscripts of all time, Principia Mathematica. In this masterpiece, Newton had worked out the mechanics of the heavens. Using calculus, the mathematical formalism that he had devised, he could precisely determine the motion of the planets and comets in the solar system. He discovered that comets can travel in ellipses, in which case they might return. And Halley, adopting Newton’s methods, calculated that the comet that sailed over London in 1682 would return every seventy-six years. In fact, he could go back through history and show that the same comet had consistently returned on schedule. He made the daring prediction that it would return in the year 1758, long after his death. Its appearance on Christmas Day that year helped seal Halley’s legacy.

  Today, we know that comets come mainly from two places. The first is the Kuiper Belt, a region outside Neptune that orbits in the same plane as the planets. The comets in the Kuiper Belt, which include Halley’s comet, travel in ellipses around the sun. They are sometimes called short-period comets, because their orbital periods, or the time it takes for them to complete one cycle around the sun, are mea
sured in decades to centuries. Since their periods are known or can be computed, they are predictable and hence we know they are not particularly dangerous.

  Much farther out, there is the Oort Cloud, a sphere of comets surrounding our entire solar system. Many of them are so far from the sun—up to a few light-years away—that they are largely stationary. Once in a while, these comets are hurled into the inner solar system by a passing star or random collision. They are called long-period comets, since their orbital periods might be measured in tens, even hundreds of thousands of years, if they return at all. They are almost impossible to forecast and therefore potentially more hazardous to the Earth than short-period comets.

  New discoveries are being made every year about the Kuiper Belt and the Oort Cloud. In 2016, it was announced that a ninth planet, about the size of Neptune, might exist deep in the Kuiper Belt. This object was identified not by direct observation through a telescope but by using computers to solve Newton’s equations. Although its presence is not yet confirmed, many astronomers believe that the data is very convincing, and this situation has its precedents. In the nineteenth century, it was pointed out that the planet Uranus deviated slightly from predictions derived from Newton’s laws. Either Newton was wrong or there was a remote body tugging on Uranus. Scientists calculated the position of this hypothetical planet and found it after just a few hours of observation in 1846. They called it Neptune. (In another case, astronomers noticed that Mercury, too, strayed from its anticipated path. They conjectured that a planet, which they dubbed Vulcan, existed within the orbit of Mercury. But after repeated efforts, no planet Vulcan was found. Albert Einstein, recognizing that Newton’s laws could be flawed, showed that Mercury’s orbit could be explained by an entirely new effect, the warping of space-time according to his theory of relativity.) Today, high-speed computers armed with these laws could reveal the presence of ever more denizens of the Kuiper Belt and the Oort Cloud.