From the Center of Uranus to the Singularity of a Black Hole: 19 "What If" Journeys Through the Solar System and the Universe

Uranus is about 3 billion km from Earth. It is the seventh planet from the Sun and the third largest in the solar system. Its rotational axis is tilted 98 degrees relative to its orbital axis, so it travels around the Sun as if rolling on its side. The uppermost clouds contain hydrogen sulfide, which the video describes as "smelling like farts." In 1986, Voyager 2 passed about 80,000 km from the upper layers in just six hours, collecting a large amount of data on the planet, its moons and its rings. Combined with observations from the Hubble Space Telescope and ground-based telescopes, this advanced our understanding of its internal structure.

On the way down, 13 thin rings made of rocks the size of golf balls or larger come into view. Inside the upper clouds of methane ice, the temperature is below minus 220 degrees Celsius and the pressure is about 0.5 atmospheres. Uranus's gravity is slightly weaker than Earth's, so at first the fall proceeds at a speed close to free fall on Earth. In the dark hydrogen sulfide ice clouds below, winds reach 900 km/h. About 130 km further down, the pressure rises to about 10 atmospheres. Beyond that lies a cloud layer of water ice, where lightning flashes. Near the bottom of this layer, the pressure exceeds 60 atmospheres and the temperature reaches about 50 degrees Celsius; by then more than an hour has passed since the fall began, and the depth has reached 240 km.

Below that is the "mantle." It is an almost bottomless ocean more than 2,000 times as deep as the Pacific, thought to make up more than 60% of Uranus. Water mixed with methane and ammonia has been compressed by high pressure into a hot, dense fluid. Eventually the density of the body matches that of its surroundings, and the fall stops. In this region, where pressure reaches thousands of atmospheres, carbon is thought to fall as diamond rain, with the diamonds said to reach millions of carats. After sinking thousands of kilometers over several weeks, one reaches a small core made of rock, iron and unfamiliar forms of ice.

Next comes Neptune, about four times Earth's diameter—slightly smaller than Uranus but more massive. It has been visited by a spacecraft only once, by Voyager 2 in August 1989, and most research relies on ground-based telescopes, so much about its interior remains a mystery. It appears deep blue because methane in the upper atmosphere absorbs red light. It has five rings, made mainly of dust. Its gravity is only 14% stronger than Earth's, so the falling speed is not very different from that on Earth. The upper clouds of methane ice are at 0.5 atmospheres and below minus 200 degrees. About 50 km down, one enters clouds of ammonia and hydrogen sulfide at about 5 atmospheres. Here blow the fastest winds in the solar system, at 2,400 km/h, about twice the speed of sound. In the water clouds about 160 km down, enormous lightning flashes, and the pressure is well above 50 atmospheres. After sinking about 6,400 km, one reaches the mantle, where the suit must withstand more than 10,000 atmospheres. Here too, as on Uranus, diamond rain is thought to fall. The core, thousands of kilometers further down, exceeds 5,000 degrees Celsius and 7 million atmospheres. It consists of rock, iron and ice, and the video says there may be a layer of accumulated diamonds.

Pluto became a dwarf planet in 2006 when the definition of a planet changed. Its diameter is less than one-fifth of Earth's and only about two-thirds of the Moon's. Almost nothing was known about it until New Horizons visited in 2015, and most of our current knowledge comes from that spacecraft's data. Against Pluto's diameter of about 2,400 km, its moon Charon measures about 1,200 km across. As a result, the two orbit each other around a common center of mass, and some call them a "double planet." Pluto and Charon always keep the same faces toward each other, so Charon cannot be seen from Pluto's far side.

Pluto's atmospheric pressure is extremely low. A small amount of methane in the atmosphere is thought to react with ultraviolet light from the Sun and other sources to form substances called tholins, which create a haze. The haze begins at an altitude of about 200 km, where it is about minus 193 degrees, but at altitudes of 20 to 30 km the temperature rises to a maximum of minus 173 degrees. The video attributes this to the greenhouse effect of methane. The average surface temperature is minus 231 degrees, and gravity is weak: a person weighing 60 kg would feel a weight of only about 4 kg, it explains.

The surface scenery varies greatly depending on where one lands. In the heart-shaped region, glaciers of solid nitrogen and mountain ranges of water ice more than 3,000 m high can be seen. Next to it, Cthulhu Macula is dark brown from accumulated tholins. Pluto's orbital period is 248 years, and its rotational axis is tilted 120 degrees, so some places go without seeing the Sun for decades. Even at its closest approach, Pluto is about 30 times as far from the Sun as Earth is, so even at the brightest time of day it is only about as bright as Earth just after sunset. Charon appears about eight times as large as the Moon does from Earth. Meanwhile, Pluto's sky is blue because methane and other gases scatter light. The video presents this as a discovery that no one knew about until a few years ago.

However, there are reports that this atmosphere is rapidly being lost. Pluto last made its closest approach to the Sun in 1989 and has been moving away ever since. As the surface temperature drops, nitrogen, the main component of the atmosphere, freezes and accumulates like snow. One study reportedly found that the atmosphere will be completely gone by 2030. The narrator tells the protagonist that if they wait about 90 more years, they will be able to see a solar eclipse caused by Charon, and then leaves them behind.

Venus is sometimes called Earth's twin because its size, mass and internal structure are similar to Earth's, but the narrator says that's where the similarities end. In the 1950s, some scientists even predicted an environment like Earth's tropics. The reality is far harsher: even the Soviet Venera 13, the probe that operated longest on the surface, lasted only 127 minutes. In visible light, Venus appears white. The yellow images commonly seen are either radar observation data or ultraviolet images with artificial coloring.

At an altitude of 150 km, the temperature is 0 degrees, but there is almost no pressure. Removing the suit would mean losing consciousness within 15 seconds and dying within three minutes. At 90 km, it is minus 100 degrees. From around 70 km, clouds thought to be more than 75% sulfuric acid spread out, and storms with wind speeds exceeding 100 m/s blow. At 60 km, it is 0 degrees, and from there down to about 45 km, the temperature does not exceed 100 degrees, and the pressure is only slightly above 1 atmosphere. For this reason, the region around 50 km altitude is considered one of the leading candidates for human settlement. The thick clouds block radiation, and oxygen and water can be produced from sulfuric acid and carbon dioxide, so there are proposals to float colonies resembling giant balloons there. The video goes so far as to say that, under certain conditions, it could surpass Mars.

On the possibility of life, the video presents both sides. In 2020, a signal of phosphine—a gas thought not to exist in large quantities in an atmosphere except through biological activity—was reported, attracting attention. However, the prevailing view now is that the analysis method was flawed and the signal was actually sulfur dioxide. Even so, there is something unidentified around 50 km altitude that absorbs ultraviolet light, and hopes for further exploration continue. On the other hand, there are counterarguments: research suggesting there is too little water for life, and the fact that no microorganism capable of withstanding both high temperatures and strong acidity has been found on Earth.

Below the clouds, around 40 km altitude, sulfuric acid rain falls, but it evaporates around 30 km and never reaches the surface. The surface is at 460 degrees and 90 atmospheres, and the atmosphere is 96.5% carbon dioxide and 3.5% nitrogen. Even lead and zinc melt, and substances such as lead sulfide that evaporate from the surface accumulate like snow on cooler mountaintops. Surface winds are at most about 2 m/s, but because the atmosphere's density is about 6.5% that of water, one might not be able to walk into the wind. A day lasts 117 days, even daytime is only about as bright as an overcast day, and the temperature barely drops at night. Theories for why Venus became this way include one holding that, lacking a magnetic field, it could not protect its water from the solar wind, and another holding that it received so much energy from the Sun that the greenhouse effect of water vapor ran away. Earth, too, is expected to become like Venus in somewhat more than a billion years as the Sun grows brighter, and the narrator remarks sarcastically that humanity's carbon dioxide emissions might hasten that. There was also an event 300 million to 500 million years ago that resurfaced the entire planet, and one theory holds that the loss of plate tectonics led to the loss of the magnetic field.

The narrator introduces Mercury as "a planet with no presence," then proceeds to overturn that assessment. With Pluto excluded, Mercury is now the smallest planet in the solar system. Its radius is about 2,400 km, smaller than the moons Ganymede (about 2,600 km) and Titan. On the other hand, its average density is 5.4 g per cubic centimeter, second only to Earth among the planets, and its mass is more than twice that of those moons. Only three spacecraft have ever visited it: Mariner 10, MESSENGER and BepiColombo. The video explains that the reason is not low priority but the difficulty of getting there. Approaching the Sun increases a spacecraft's speed, so without intervention it would simply fly past Mercury. Repeated gravity assists are therefore necessary; BepiColombo is scheduled to enter orbit seven years after launch, following one flyby of Earth, two of Venus and six of Mercury.

Because there is almost no atmosphere, craters billions of years old remain without weathering. Near the equator, the temperature reaches 427 degrees when Mercury is closest to the Sun and the Sun is at its highest. Even so, this falls short of Venus's average of 464 degrees, because without an atmosphere the heat escapes into space. During the night, which lasts 88 days, the temperature drops below minus 160 degrees, making Mercury the planet with the largest surface temperature range in the solar system. Its orbital eccentricity is the largest of all the planets, and its distance from the Sun varies by a factor of 1.5, from 46 million km to 70 million km. The energy it receives varies by a factor of 2.25, and the maximum equatorial temperature when it is farthest away is thought to be about 280 degrees. Its axial tilt is only 0.03 degrees, so the poles receive almost no energy from the Sun. It is known that in places that have not seen sunlight for billions of years, there is ice exceeding the volume of water in Lake Biwa by a wide margin.

Furthermore, one study claims that digging about 1 m underground greatly stabilizes the temperature, and that the subsurface at mid-latitudes stays around 22 degrees, ideal for humans. Gravity is 38% of Earth's, almost the same as Mars. The ice can be used for agricultural water or broken down for fuel and air. Solar power is also available, and past asteroid impacts may have left gold, platinum, cobalt and nickel near the surface. For these reasons, the video says that Mercury's subsurface, like the skies of Venus, could become a candidate that outshines Mars under certain conditions.

Mars's atmospheric pressure is less than 1% of Earth's, and temperatures range from a high of 20 degrees to a low of minus 150 degrees, averaging around minus 60 degrees. Pure liquid water cannot exist; at most, brine with large amounts of dissolved salt can pool in small amounts for brief periods. There is also no magnetic field, so radiation pours down. The video also touches on crewed exploration plans, noting that SpaceX's target has changed from 2026, as previously reported, to 2029 as of May 2022.

What the falling scene emphasizes is how "in-between" the atmosphere is. It is too thin to slow down with parachutes alone, yet thick enough that heating from compression during entry becomes a problem. About half of all Mars missions have failed, many of them due to trouble during landing. InSight, launched in 2018, was equipped with a heat shield able to withstand more than 1,500 degrees and needed retro-rockets just before landing. Spirit, in 2004, cushioned the impact with 24 airbags. The protagonist has neither and is made to hit the ground at about 1,000 km/h. The sky is usually a pale ochre because of dust containing iron oxide, and at sunset only the area around the Sun turns blue—the reverse of the colors on Earth.

The narrator says the chance of life on the surface is low, but the subsurface is another matter. Temperatures underground are stable, and one theory holds that some of the water from former oceans remains in the ground bound to minerals. There is a great deal of ice at depths of 1 to 100 m, and a network of groundwater once extended across the entire planet. A lake of liquid water is known to exist beneath the south polar ice, and one study estimated that aerobic organisms could survive even on the small amount of oxygen in the Martian atmosphere. However, Martian soil contains more than 0.5% perchlorate. California limits its concentration in drinking water to 6 μg per liter or less, and Martian soil exceeds that by a factor of tens of thousands or more. Having started digging a hole, the protagonist becomes unable to return to Earth due to fears of contamination and unknown bacteria.

The video explains that Mars 100 million years ago was a red planet much as it is today. One difference is that small-scale volcanic activity may have continued on Olympus Mons, which is more than 20 km high. In addition, calcium perchlorate lowers the freezing point of water to around minus 75 degrees. There are landforms on the surface that look like traces of liquid brine, and whether such phenomena still actually occur today has yet to be determined. There are also reports of minerals altered by liquid water within the last 170 million years found in meteorites from Mars. For this reason, 100 million years ago there may have been water in amounts as fleeting as a mirage.

Even 2 billion to 3 billion years ago, the pressure was low, but water deep underground burst through the surface due to volcanic activity, causing massive floods. Their flow rate reached up to 1 billion cubic meters per second, said to be more than 3,000 times the Amazon River's maximum of about 300,000 cubic meters. Between 3.7 billion and 4.3 billion years ago, an ocean covered almost the entire northern hemisphere, with its deepest point more than 1.6 km deep. One study claims there was an atmosphere of at least 0.5 atmospheres 4 billion years ago, and there may have been three blue planets, along with Venus and Earth.

The cause of the change was the solar wind. Particles collided with atmospheric molecules and gave them energy, allowing the atmosphere to escape into space. As pressure dropped, water evaporated, and the water vapor was split into hydrogen and oxygen by ultraviolet light. The hydrogen escaped because Mars's gravity could not hold it, and the remaining oxygen reacted with iron to form red rust. This change is thought to have taken place between 3.9 billion and 3 billion years ago. The magnetic field that protected Earth is maintained by convection in the outer core, driven by buoyancy from light components pushed out as the inner core grows. Mars, with about half Earth's diameter and about one-tenth its mass, cooled easily, and a thin liquid layer covering the core like a blanket also reduced the temperature difference between inside and outside. As a result, its magnetic field is thought to have shut down a few hundred million years after forming, about 3.9 billion years ago. However, the video notes that the conditions under which planets form magnetic fields are not themselves clearly understood.

Space colonies are presented as a hedge against Earth's risks—as precarious as "castles built on sand"—such as supervolcanic eruptions, asteroid impacts and nuclear war. Because "colony" evokes imperialism in the English-speaking world, some experts recommend terms like "space settlement." Compared with bases on the Moon or Mars, they have two advantages: there is no night, so solar power is always available (a lunar night lasts two weeks), and gravity is low, making it easy to come and go. On the other hand, microgravity is a serious problem, causing astronauts on the International Space Station to lose bone density and muscle even with training. For this reason, most designs reproduce gravity with centrifugal force from rotation. The Bernal sphere of 1929 is 500 m in diameter and would provide Earth-like gravity at its equator by rotating 1.9 times per minute. The O'Neill cylinder is 8 km in diameter and up to 32 km long, rotates about 0.5 times per minute, and requires less material per unit of living space. The Stanford torus is a doughnut shape 1.8 km in diameter that rotates once per minute; it requires more material, but it is easier to include a non-rotating section for docking.

The biggest obstacle is procuring materials. Even to low orbit, a single launch can carry at most 140 t, and the International Space Station, though only about 100 m long, weighs more than 400 t and took 10 years to become operational. One approach being considered is to set up bases on the Moon or asteroids and send resources from there. The colony's interior would be divided into residential areas and food production areas, with materials fully recycled. Water can also serve as walls shielding against radiation. Since the climate can be controlled, there would be no typhoons or earthquakes. Non-rotating sections could be used for tourist attractions like zero-gravity pools, or for research producing large protein crystals with few defects. The narrator predicts that residents will eventually come to see themselves as distinct from Earthlings, yet frames colonies as a necessary step for humanity to survive beyond the lifespan of the Sun.

A space elevator is a structure in which a cable is extended from an ocean platform to a station in geostationary orbit, with a counterweight placed beyond it, and climbers travel up and down. It works through a balance between gravity and centrifugal force, and because it doesn't burn through fuel the way rockets do, it is thought to greatly reduce transportation costs. However, there are many challenges. Steel and aramid fibers aren't strong enough, and even with promising carbon nanotubes, the longest produced as of 2022 is only tens of centimeters. Geostationary orbit is about 36,000 km away. There is data showing that a defect of a single atom reduces strength to 70%, and research suggesting that realistic strength is about 30% of the theoretical value. Considering wind, rain, lightning, meteoroids and radiation, a safety margin of several times is needed, and whether that is achievable is completely unknown. Other issues cited include the need for a counterweight of more than 10,000 t even when placed 10,000 km above geostationary orbit, the need for solar panels several hundred meters on a side to power the climbers, and the shrinking advantage over rockets as private-sector entry since the 2010s has lowered launch costs.

Even so, once completed, it would make space travel accessible to ordinary people and could carry precision equipment that is vulnerable to vibration. As an example, the video cites the large amount of time and money the James Webb Space Telescope spent on vibration testing. Since the geostationary station is already moving at about 3 km/s, there are also proposals to extend the cable further and launch payloads directly toward Mars or Jupiter. That research claims it would shorten travel time to Mars and eliminate restrictions on launch windows. There is also a view that elevators could be built on Mars or the Moon even with current technology.

We observe light that took about 13.8 billion years to reach us as the edge of the universe. However, because the universe is expanding, the place that emitted that light is now about 46.5 billion light-years away. This light is the cosmic microwave background, from about 370,000 years after the Big Bang, when the temperature dropped enough for electrons and atomic nuclei to combine and light could travel freely. Even if one traveled there in the suit, the view would hardly differ from that seen from Earth, because the visible universe is only part of the whole and its "edge" is not a special place.

The video says the current consensus is that one would never reach an edge, no matter how far one travels in one direction. If the curvature of space is zero or less, the universe extends infinitely; if it is greater than zero, it is finite like the surface of a sphere, but one would return to the starting point. In either case, there is no edge. The apparent size of the hotter and cooler regions seen in the background radiation almost exactly matches their theoretical size. If space were curved, light would bend too, and the sizes would be off, so the curvature is known to be nearly zero. Why this is so has not been explained.

Voids are regions with extremely few galaxies that stretch between galactic filaments. The Milky Way is about 100,000 light-years across, the Laniakea Supercluster about 500 million light-years, and the filament it belongs to about 1.6 billion light-years. Most voids are 30 million to 300 million light-years across, and some approach 1 billion light-years. Whereas the air in a cup contains about 2.5×10^25 molecules per cubic meter, space within a galaxy is estimated to contain 100,000 to 1 million particles, and space outside galaxies fewer than one. The Boötes Void is 250 million light-years across, and only 60 galaxies have been found in it; applying the density around the Milky Way, there could be around 10,000. A 2014 study estimated that voids make up 77% of the universe's volume but only about 15% of its mass.

The video explains that the galaxies visible to the naked eye lie within a few million light-years at most, since the Andromeda Galaxy (about 2.5 million light-years) is easily visible and the Triangulum Galaxy (about 3 million light-years) is near the limit. At the center of a void, it is likely pitch-black in every direction. However, because there is no clear boundary between voids and other space, every announced discovery sparks controversy. One study even claims that the Milky Way itself lies near the center of the KBC Void, 2 billion light-years across, though there are opposing views.

The center of the Earth is thought to reach 5,200 degrees Celsius and 3.6 million atmospheres. On the premise that a 13,000 km hole runs through it, the video addresses the physical problems one by one. The first is the Coriolis force. Tokyo moves east at about 1,350 km/h, but the deeper inside the Earth, the smaller the radius of rotation and the slower the speed, so a falling person would be pressed against the eastern wall at more than 100 km/h. The video "solves" this by aligning the Earth's rotational axis with the hole.

The second is air resistance. If the air pressure inside the hole were the same as at the surface, terminal velocity would be only about 200 km/h, and traveling 13,000 km would take more than 60 hours. Moreover, the closer one gets to the center, the more mass lies overhead, weakening the downward pull of gravity until one becomes weightless at the center, after which one is pulled back in the opposite direction. As a result, one would swing back and forth like a damped pendulum and end up stopped at the center.

If the hole were a vacuum, one would reach 35,000 km/h at the center and slow to exactly zero at the surface in Brazil. Taking into account changes in density inside the Earth, the trip would take about 38 minutes. Even with an angled hole, without friction, the trip between any two points would take about 40 minutes. The shorter the distance, the closer the direction of gravity comes to being perpendicular to the hole, reducing acceleration, so the two effects cancel out. Finally, the narrator wraps up by listing "four simple steps": dig the hole, change the rotational axis, create a vacuum, and build a heat- and pressure-resistant suit.

The origin of Saturn's rings is unknown. One theory holds that they formed together with Saturn more than 4 billion years ago, and another that they formed within the last 150 million years; neither is conclusive. Either way, the leading view is that they are the remains of a body that came too close and was torn apart. The fact that such events were common in the early solar system supports the ancient-origin theory. On the other hand, the ice in the rings is relatively clean, whereas after billions of years it should be darker and covered in more dust. At the extreme, one study predicts they formed about 15 million years ago. Ring particles are at most about 10 m in size, and the rings are losing 400 to 3,000 kg per second due to micrometeoroid impacts and to ionization by the solar wind followed by capture by the magnetic field. They may be gone in about 300 million years.

Even if Saturn's rings (from the inner radius of the innermost D ring, about 67,000 km, to the outer radius of the E ring, about 480,000 km) were moved to Earth as is, they would quickly disappear for two reasons. One is that the boundary within which water ice can remain solid lies between Mars and Jupiter, so at Earth's position the ice would evaporate. The other is that Earth's gravity is weak, so at that distance the particles would clump together into moons. In fact, the Moon is thought to have formed from a ring created by a giant impact. A realistic ring around Earth would be made of rock, with an inner radius of at most about 10,000 km and an outer radius of at most about 18,000 km, and the video says the outer part would have a structure whose density varies periodically due to the Moon's gravity.

With rings, the night would never be completely dark. This would be a major problem for corals, which time their spawning by moonlight, and for dung beetles, which navigate by the starry sky—though if the rings had been there from the start, such creatures might never have evolved in the first place. For humanity, ground-based astronomical observation would become difficult, and launches would need to avoid particles flying at several kilometers per second. It's debatable whether the spectacular view would make up for that, and the narrator concludes that it might be best to leave rings as Saturn's privilege.

The Jurassic period lasted from about 200 million to 145 million years ago, and its name comes from the strata of the Jura Mountains. Its beginning and end are marked by mass extinctions, and birds are thought to have first appeared during this period. Tyrannosaurus lived about 68 million years ago, closer to the present than to the Jurassic. The creature to watch out for in the Jurassic is Allosaurus, 10 m long and, by some estimates, able to run at 55 km/h. Temperatures were 5 to 10 degrees higher than today, and there were almost no ice sheets. Meanwhile, one estimate puts carbon dioxide levels at 1,000 to 2,000 ppm, compared with about 400 ppm today, and some research suggests that even around 1,000 ppm can impair cognitive function and cause dizziness and headaches. However, no firm conclusion has yet been reached on these effects. The biggest problem is food. Flowering plants such as rice, wheat, corn, potatoes and fruit barely existed, and since farming would be difficult with only gymnosperms and ferns, the only option would be to hunt small mammals and insects.

The Cretaceous period 100 million years ago was more comfortable than the Jurassic, with oxygen levels the same as or somewhat higher than today and carbon dioxide at several hundred to 1,000 ppm. India was an island, and Australia and Antarctica were connected. One theory holds that because Antarctica was not isolated, it was not locked in ice, and sea levels were as much as 200 m higher than today. A shallow sea was beginning to form in the middle of North America. One extreme estimate puts the average temperature around 30 degrees, and volcanic activity was intense. Stegosaurus had already gone extinct, and Tyrannosaurus and Triceratops had not yet appeared. Instead, there were Spinosaurus, 14 m long with a 1.5 m sail on its back; Giganotosaurus, which may have been larger than Tyrannosaurus; and Argentinosaurus, 30 to 35 m long. The video also introduces oddly shaped creatures such as Pterodaustro, a pterosaur with a flamingo-like beak; Stomatosuchus, a crocodilian with a pelican-like lower jaw; and Nigersaurus, with a mouth like a nail clipper. This was a time when bees and butterflies were beginning to take on pollination and flowering plants, previously a minority, were spreading; grasslands of grasses did not yet exist. It is also thought that about 60% of the world's oil was formed during the Cretaceous.

At the K-Pg boundary about 66 million years ago, about 75% of living species are said to have gone extinct. Iridium, found in strata around the world, is nearly absent on Earth's surface but abundant in asteroids, making it evidence of an impact. The impact site was off the Yucatán Peninsula. Bones of paddlefish fossils from the impact layer have growth patterns like tree rings, and their analysis shows that the impact occurred in spring in the Northern Hemisphere. On the other hand, estimates of the asteroid's size range from 10 km to several tens of kilometers, and its energy, in TNT equivalent, is not precisely known, ranging from tens of millions to 100 million megatons. The Hiroshima atomic bomb was 0.015 megatons, and even the Tsar Bomba, the largest nuclear bomb in history, was 50 megatons.

At the moment of impact, a fireball several times brighter than the Sun appeared, and temperatures are predicted to have exceeded 100 degrees even 1,800 km away. Tsunamis are said to have reached up to 1,500 m, traveling more than 100 km inland, and one estimate says they exceeded 10 m even along Pacific coasts. Debris thrown up into space reentered the atmosphere and released heat, causing temperatures to spike worldwide, and in some regions plants are thought to have spontaneously ignited. Massive wildfires broke out in North America and, on the opposite side of the globe, in India and Australia. Furthermore, the asteroid struck at a shallow angle of 45 to 60 degrees into a shallow seabed rich in gypsum (calcium sulfate). The video says this was the worst possible combination of conditions. Massive amounts of sulfur and soot blocked sunlight; by one account, 100 billion t of sulfur was released, temperatures dropped by more than 20 degrees, and extreme weather lasted about 30 years. Photosynthesis stopped, and the food chain collapsed.

There is no clear answer as to why only the dinosaurs went extinct. One theory holds that they were large warm-blooded animals that needed a lot of food; another holds that because they hatched from eggs, adults and young differed greatly in size and occupied different ecological roles, making them vulnerable. Theories for why birds and mammals survived include their ability to eat plant seeds, and the strength of mammalian immunity against fungi that proliferated on carcasses. The narrator quotes the writer Larry Niven to the effect that "the dinosaurs became extinct because they didn't have a space program," and also mentions an estimate putting the probability of a 10 km-class impact at 1 in 10,000 per century. The narrator then points out that humanity currently lacks the technology to deflect a 10 km asteroid.

In the near future, the Moon's influence will slow Earth's rotation until even leap seconds can no longer keep up, making it difficult to keep time standardized. In about 15,000 years, precession will reverse the orientation of Earth's rotational axis, swapping the Northern Hemisphere's distance from the Sun in summer and winter. The difference between seasons will grow, and it is estimated that the Sahara Desert will become savanna, as it was about 10,000 years ago. In 20,000 years, the entire area contaminated by the 1986 Chernobyl nuclear disaster will have settled to radiation levels that pose no problem. However, a 2017 study suggests that supervolcanic eruptions capable of collapsing civilization occur on cycles of 5,200 to 48,000 years. An ice age is predicted to arrive in 50,000 years, though one estimate says carbon dioxide emissions will delay it by another 50,000 years. In about 64,000 years, Sirius will occupy a position like that of a south pole star.

Sometime between 10,000 and 1 million years from now, Betelgeuse or Antares may go supernova, shining as brightly as a full moon for several months. Effects on life are said to occur within about 30 light-years, and these two stars are several hundred light-years away. However, it has been pointed out that WR 104, 8,400 light-years away, could explode within 1 million years and that its gamma-ray burst could hit Earth directly. The video adds the caveat that the mechanism behind such bursts is poorly understood and that the chance of being hit is low. In about 1.29 million years, Gliese 710 will approach to within 0.17 light-years, becoming the brightest star in the sky. It could disturb the orbits of comets in the Oort cloud, sending some inward and potentially into collision with Earth.

In 10 million years, East Africa will split apart, dividing the continent in two. Predictions beyond that vary greatly by model, and even the theory that Japan and Hawaii will collide cannot be stated with certainty, since plate movements may reverse. According to one model, Europe and Africa will collide in 50 million years, eliminating the Mediterranean and creating a Himalaya-like mountain range extending to where the Persian Gulf is now. The Antarctic ice sheet, along with Greenland's, will melt and raise sea levels by 90 m; in 80 million years, the current Hawaiian Islands will have sunk; and in 250 million years, the supercontinent Pangaea Ultima is expected to form. The Earth will be swallowed by the Sun, having become a red giant, in about 5 billion years, and one study estimates that life can survive until 2.8 billion years from now.

The Komarov and Challenger accidents occurred within the atmosphere; the only people to have died above 100 km are the three crew members of Soyuz 11 in 1971. During reentry, a malfunctioning device caused the cabin air to escape at an altitude of 168 km, and all three suffocated. On Voskhod 2 in 1965, a spacesuit ballooned so much that the cosmonaut could not fit through the hatch; he thought quickly, let out air from the suit, and made it back. A fire and a collision on Mir, and water leaking into a helmet in 2013, are also cited as close calls.

NASA officially states that it has no manual for a death in space. However, training for missions to the International Space Station includes an exercise simulating the death of a crew member. While trainees are discussing how to handle the body and how to return it, new information is added—"the accident has been posted on social media, and reporters are calling"—forcing them to handle everything up to official announcements and contacting the family. This training is reportedly well received by those involved. Inside the station, a body decomposes just as on the ground, and pathogens and odor threaten the crew. It would therefore most likely be placed in a sealed bag and stored in the cargo area until a return flight. Disposing of it into space would add to debris, so this is considered out of the question except in emergencies.

If someone went outside without a spacesuit, even though space is minus 270 degrees, there is no material to conduct heat, so body heat is lost only through radiation. One estimate says it would take about 18 hours for body temperature to drop below 0 degrees. Since liquid water cannot exist stably, the body would eventually become something like a frozen mummy and drift semi-permanently. What to do if someone dies during an interstellar voyage lasting decades is a question no one has yet answered, not only technically but also legally and ethically. The narrator says that until this problem is solved, humanity may not truly be a spacefaring civilization.

Black holes cannot be observed directly, but their existence is considered certain based on things like celestial bodies torn apart by something invisible and gravitational waves from collisions between invisible objects. At a sufficient distance, a black hole is no different from any other celestial body; if the Sun were replaced by a black hole of the same mass, the planets' orbits would not change.

For a non-rotating black hole with five times the mass of the Sun, the radius of the event horizon is about 15 km—only large enough to contain Tokyo's 23 special wards. Even 1 million km away, gravity is about 70 times that at Earth's surface. The problem is the difference in gravity between head and feet: for a 170 cm person falling headfirst, that difference exceeds 10 times Earth's gravity at a distance of 2,800 km. This is considered the limit of human survival. At 20 km from the center, a difference of just 1 mm amounts to 17,000 times, and the body is stretched thin. This is "spaghettification."

On the other hand, for Sagittarius A* at the center of the Milky Way, with 4.3 million times the mass of the Sun, the event horizon lies about 12 million km from the center, and the difference in gravity between head and feet there is only about 0.01%. As one falls, an accretion disk can be seen circling the black sphere, with disks also visible above and below the sphere. This is light from the far side of the disk that has been bent by gravity on its way to the viewer. The disk reaches thousands of degrees and emits ultraviolet and blue light. For a non-rotating black hole, the innermost stable circular orbit lies 36 million km from the center, and matter cannot orbit stably inside it. Further in, at the photon sphere, light leaving the back of one's own head could theoretically circle the black hole once and enter one's eyes. Near the horizon, time passes extremely slowly, so to an outside observer the falling person appears to grow redder and dimmer, seem to freeze at the horizon, and then fade away.

For rotating black holes, there is a theory that the horizon harbors a high-energy region called a "firewall." Inside the horizon, according to the equations of general relativity, the roles of time and space switch, and heading toward the central singularity becomes unavoidable. Trying to escape only gets you there sooner, the video says. Mathematically, the singularity has infinite density, but many scientists regard this as an artifact of the calculations and believe that a different theory replacing general relativity will reveal its true nature. Modern physics has no answer to what the singularity is actually like. The video takes the protagonist to that unknown place and ends the journey by having them torn apart through spaghettification.