Earth doesn’t always behave as we expect. If that perturbs you, it may be easier to leave the planet than you think.
The Sun may rise and set at different times, but there are always exactly 24 hours in a day. Nothing could be simpler, right? Barring the odd ‘leap second’ here and there, this is true in terms of the human definition of a day. We have decreed that the day shall be divided up into 24 periods of 60 minutes, and thus it is made so. The petulant Earth doesn’t quite behave according to our wishes.
Astronomically speaking, a ‘day’ is the time required for a planet (or other body) to complete one turn on its axis. Most objects in the Universe rotate. It would be odd if they did not. Anything in space that’s been pummelled or punched at some point in its history – which is to say, everything – will have been knocked into a spin. In space, where there’s no drag or friction to slow things down, objects just keep spinning and spinning.
Most of the planets round our Sun spin in the same direction. What we’re seeing here is leftover momentum from the creation of the Solar System. The Earth and other planets formed from a disc of gas, dust and ice grains spiralling inwards towards the infant Sun. Over time, these particles were attracted together by gravity and electrical charges. Over even more time, the so-called protoplanetary disc coalesced into the worlds we see today.
The laws of physics require that the angular momentum of the system must be preserved. What does that mean? Imagine flinging yourself off a children’s roundabout at high speed. You won’t stop dead. Your angular momentum will see you fly through the air in a curved path until you hit the ground. At that point, you’ll probably stumble, fall, and roll along for a short while, as friction with the ground robs you of your angular momentum. Now let’s go back to the protoplanetary disc and imagine it as a colossal roundabout. As the gas and dust coalesce, they must retain their own angular momentum. There’s no convenient tarmac up here to suck up all the energy, so the forming bodies go into a spin to conserve momentum. In other words, our planet was set spinning right back in the earliest days when it first formed. It has taken many big knocks since, which will have altered its spin rate, but most of the momentum comes from that ancient protoplanetary disc.
Phew. And so back to the periodicity of that spin. Our insistence on a 24-hour day is very convenient for timekeeping. Twenty-four can be divided into all kinds of handy portions: 12 hours (a half-day), eight hours (a typical work shift), six hours (a quarter-day), four hours (a common period between doses of medicine), three hours (a good drinking session), two hours (a play or film), one hour (a lunch break).
Imagine how much would get thrown out of sync if we suddenly stopped ignoring the truth: that the Earth actually takes, on average, 23 hours, 56 minutes and 4 seconds to rotate about its axis. That’s right, your day is four minutes shorter than you thought it was. This more precise time is what you get if you measure the Earth’s rotation against the stars. In other words, you start the stopwatch when a star is in a certain point in the sky, and stop the watch when it returns to the same position the following night. This definition is called a sidereal day.
You get a different result if you instead measure your day against the Sun. Actually, you get many different results. The Earth’s orbit is elliptical. Our planet is also tilted so that the poles do not point directly ‘up’ and ‘down’ relative to the plane of our orbit. Thanks to these factors, a day measured by the position of the Sun (a solar day) can be anywhere between 21 seconds less or 29 seconds more than the 24 hours we all think of as a day.
Why does the sidereal day differ from the solar day by as much as four minutes? As well as turning on its axis, the Earth is also moving in orbit around the Sun. Every day, the Earth moves about one degree further round its orbit. So our planet must rotate one extra degree to bring the Sun to the same apparent position in the sky as the day before. There are 1,440 minutes in a day and 360 degrees in a rotation. Divide minutes by degrees to get the time for one degree of rotation and, hey presto, there are our missing four minutes.
All of this seems very complicated, and you’re probably wishing you never got started on this whole confusing business. And yet there are still further factors that can affect the Earth’s rotation, and therefore its day length. Tidal interactions with the Moon, a wobble in the axis of rotation known as precession, and even strong earthquakes can all have small effects on the precise spin rate. Thanks to its interactions with the Moon, the Earth is gradually slowing down. The dinosaurs enjoyed a day that was appreciably shorter than ours – something like 23 hours. Their body clocks would have run to a slightly different rhythm. Our distant descendants, 200 million years from now, will know 25-hour days. They’ll probably have five-hour drinking sessions, the lucky blighters.
Far from a dependable, reliable quantity, a day on Earth is actually a squirmsome beast to pin down. Our daily cycle is set to 24 hours only by human definition. The reality is a chronological tangle.
Nicolaus Copernicus (1473–1543) was the mover of worlds. The Renaissance astronomer shifted around our understanding of the Solar System. Until his time, it was widely believed that the Earth was the centre of all things, and that the Sun, the planets and even the stars revolved around our world. This notion is termed a geocentric model, or the Ptolemaic system after one of its ancient champions, Claudius Ptolemy (c.100–168CE). Copernicus suggested a simple switcheroo: put the Sun at the centre of everything, and let the planets, including Earth, circle around that. It was revolutionary stuff, in more than one sense.
Copernicus formulated his ideas in his 30s, but withheld publication for decades, fearing criticism. His thesis was finally published in 1543, around the time of his death. One unverifiable legend even has Copernicus receiving his newly printed masterwork while on his deathbed, the last thing he would ever see. Heliocentrism found early fans, but didn’t make the astronomical mainstream for the best part of a century. Nor was the Catholic church particularly annoyed at the idea’s contradiction of scripture. That would come much later, when Galileo Galilei, taking up Copernicus’s ideas, would be branded ‘vehemently suspect of heresy’. Eventually, the evidence became overwhelming that Earth was not at the centre of the Universe, until few could doubt it.
Copernicus rightly occupies a celebrated position in the history of astronomy. Yet his heliocentric ideas had been mooted before – almost 2,000 years before, in fact. The Greek astronomer Aristarchus of Samos (c.310BCE–c.230BCE) was the first known person to contradict the intuitive view of a centrally placed Earth. In a long-lost book, fortunately quoted by others, he claimed that the planets revolve around the Sun. Further, he believed that the stars in the sky were similar to the Sun, only much further away. He used geometric methods to support his conclusions.
It’s sometimes written that Aristarchus was persecuted for his far-out views and that heliocentrism was quashed by the ancients as sacrilegious. Geocentrism then became the default view and lasted throughout the medieval period until Copernicus opened everyone’s eyes. This account is not completely accurate. Although ancient astronomers certainly favoured an Earth-centric view of the cosmos, Aristarchus was not alone in entertaining the other possibility. Books by other astronomers championing the heliocentric viewpoint are thought to have once existed, but are now lost.
There are few images in the entire history of our species more arresting than ‘Earthrise’. The awe-inspiring photograph of our blue planet hanging above the cratered lunar surface was taken on Apollo 8, the first manned mission around the Moon. The Earth looks delicate, a fragile bauble wandering through illimitable space. It also looks small. Very little detail can be seen in that original Apollo 8 photo*. You can just about make out the difference between the land and the ocean, but certainly not any artificial structures like the Great Wall of China.
This is true of every photograph of the Earth taken from the Moon. All you can see is land, sea and clouds. It’s not entirely clear whether lights could be seen on the night portion of Earth. The Apollo astronauts did not report any. The eye would be hindered by glare from the Sun, which was always in the sky during the Apollo landings. The cities of the world are much bigger and brighter than they were in the time of Apollo, so it is at least possible that a glow could be detected from the Moon. The Great Wall of China, though, is not particularly illuminated. Night or day, it cannot be seen from lunar distances.
Where did the myth come from? Surprisingly, it’s one of the oldest in the book. Writing in 1754 about Hadrian’s Wall – an ancient Roman fortification near the border of England and Scotland – the English antiquarian William Stukeley (1687–1765) noted:
‘This mighty wall of four score miles in length is only exceeded by the Chinese wall, which makes a considerable figure upon the terrestrial globe, and may be discerned at the moon.’
He couldn’t have known. This was two centuries before the Space Age. Stukeley, though, was an influential writer. His speculations may well have been taken seriously. Who, after all, was going to prove him wrong? One way or another, the idea got passed down to our own times, and is still widely believed despite evidence to the contrary.
Sometimes the myth appears in a slightly meeker form, that the Great Wall of China is the only artificial object visible from space. This is also incorrect. Astronauts on the International Space Station (ISS), for example, can spy many structures on the planet below – from large cities, to dams, to long straight roads, to airports. The Great Wall, by contrast, seems to be at the limits of perception. Chinese ‘taikonaut’ Yang Liwei was unable to see the national monument, which is a similar colour to the surrounding terrain. Others believe they have seen it.
* FOOTNOTE Contrary to many accounts, the Apollo 8 photographs were not the earliest to show the Earth above a moonscape. In August 1966, the Lunar Orbiter 1 probe from the USA sent back the first such images. It was an impressive feat for the time, but it’s hard to work up much enthusiasm for the grainy black-and-white images. It should also be noted that ‘Earthrise’ is a slightly misleading term. The Earth would not rise on the Moon in the same way that the Moon rises when viewed from Earth. The Moon always keeps the same face pointed at our planet. If you’re on that side, you would always see the Earth in the sky; if you’re on the far side, you would never see the Earth. There would be no rising or setting. The Apollo astronauts saw an artificial Earthrise thanks to the motion of their capsule around the Moon.
It might be dubbed the final frontier, but space isn’t really all that far away. The internationally agreed boundary is easy to remember at exactly 100km (just over 62 miles) above sea level. You could comfortably drive it in an hour, if only you had a magic car.
A distance of 100km might sound like an arbitrarily round number, but it does coincide with a discernible boundary. At this height, the atmosphere is so tenuous that a winged aircraft could not possibly operate. A little beneath it, and you could theoretically pilot an aircraft if it were going at colossal speed with specialist aerodynamics. A little above, and the speed required for lift would be so great that you’d instead find yourself orbiting.
This 100km boundary is known as the Kármán Line, after Theodore von Kármán (1881–1963), the Hungarian-American engineer who first calculated the limit. It is the internationally agreed boundary of space, as set by the Fédération Aéronautique Internationale (FAI), the body which keeps an eye on aeronautical and astronautical achievements.
By ‘internationally agreed’, I mean ‘almost internationally agreed’. The US Air Force has its own definition of where space begins. It plumps for 50 miles up (80km). This marks the boundary between two layers of the atmosphere – the mesosphere and the thermosphere. Happily, this definition allowed test pilots to claim astronaut wings. During the 1960s, eight pilots reached this height in the experimental X-15 hypersonic rocket-powered aircraft*.
NASA goes along with the FAI definition like everyone else, but bends the boundary depending on circumstance. During the space-shuttle era, for example, controllers would consider 122.3km (76 miles) as the limit, as this was the point on re-entry when the orbiter would stop using its thrusters and rely on its aerodynamic surfaces.
* FOOTNOTE One of whom, Joe Walker, became the first person to reach space twice, on 22 August 1963.
Putting something into space is a cinch. You can do it right now. Simply go outside and shine a torch straight up. Much of the beam will be scattered or absorbed by the atmosphere and clouds, but some will make it through. Actually, it’s even easier. Stand outside on a sunny day and your body will reflect small amounts of light back out into space. That’s how spy satellites are able to see people and objects on the ground. Just by heading outside, you’re deflecting millions of particles into space every second. You’re a walking space programme.
Photons of light are easy enough to shift, but what if we want to send something heavier, like a human, into space? Rockets are the simplest and most familiar means of doing this. The technology to fire rockets short distances has been around since at least the 11th century, and the idea of sending them up to the stars is probably just as old. But it was only in the mid-20th century that technology caught up with the imagination, allowing the first spaceflights.
Rockets remain the only viable way of sending payloads into space. That’s not to say that there aren’t other possibilities. Most remain impractical with current technology, but there is impetus to keep trying. Rockets are costly, inefficient and environmentally choking. Much of their weight is from the fuel needed to power through the dense lower atmosphere. We’ve yet to engineer a better way of solving this, but there have been no shortage of suggestions.
Writers, philosophers and crackpots have dreamt of spaceflight for centuries. One of the earliest, and most demented approaches, can be found in The Man in the Moone (1638) by Francis Godwin. Its protagonist is pulled to the Moon by a flock of tethered geese. His anserine chariot is greeted by a tribe of musical Christians, who live in a lunar paradise. The story may not have been based on actual events.
A still more remarkable work appeared a couple of decades later. Cyrano de Bergerac’s posthumous story Comical History of the States and Empires of the Moon (1657) bursts with invention. Its narrator’s efforts to conquer space include two of the oddest solutions imaginable. Our would-be astronaut first attaches a ‘great many Glasses full of Dew’ to his person. When the Sun’s rays cause the liquid to evaporate, our man is also drawn up into the heavens. He later attempts to launch a kind of jerry-rigged space plane from a rock. It fails miserably as a flying machine, leaving our poor hero to anoint his bruised body with beef marrow. The vehicle eventually gets off the ground when fireworks are strapped to its frame. The pilot continues all the way to the Moon guided by a mysterious force that tugs at beef marrow*.
All of these ancient strategies are pure fantasy. Yet one technology familiar to Godwin and Bergerac could be turned to space travel – the gun. Fire a pistol into the air and the bullet will travel a few kilometres vertically before returning to Earth. Scale things up, and there’s no reason why you can’t launch an object much higher and reach the realms of space.
Schemes like this are among the earliest serious proposals for spaceflight. A giant gun sets up the titular journey in Jules Verne’s From the Earth to the Moon (1865). The major advantage is that you need carry no fuel with you – all the energy comes from the initial blast of the gun. The rather dispiriting downside is that any passengers would be crushed to death by the swift acceleration – you'd be compressed by the G-force. Nevertheless, several experimental guns have been constructed to test the idea for payloads. One even worked. In the 1960s the USA and Canada teamed up on a project called HARP (High Altitude Research Project) which involved using a series of gigantic guns, based in Barbados and Arizona. The two nations wanted to test the re-entry systems of ballistic missiles. A reliable space gun would be cheaper and have faster turnaround than expendable rockets. The ensuing guns were capable of lofting projectiles to impressive heights. The record, which still stands, came in 1966 when an 83.9kg (185lb) shell was fired to a height of 179km (111 miles). In other words, a gun that can fire into space – though not orbit – is a demonstrated technology.
The only other proven way of entering space without rocketing through the dense, lower atmosphere is to go for a compromise. A payload is carried to the upper stratosphere by a conventional aeroplane or balloon. With the thicker part of the skies beneath it, a rocket is then released to power its own way into space. Pegasus rockets work in this way. Since 1990, they have placed dozens of satellites into orbit. Launching from altitude cuts down on the amount of rocket fuel needed, and also does away with expensive infrastructure, like a blast-proof launchpad. People have made it into space along similar lines. The US Air Force’s X-15 rocket planes were also dropped from carrier aircraft, and could just about dip their noses into space. Private companies such as Virgin Galactic are now attempting to commercialize these systems. Their craft will allow paying passengers to experience a few minutes of weightlessness at the edge of space, but they won’t have anything like enough power to get into orbit.
At the more hypothetical end of the scale is the space elevator. Imagine a cable stretching up into the skies as far as can be seen. Small capsules ride up and down the cable, taking passengers and equipment to unimaginable heights. There are only two stops on this vertical highway: the ground, and a space station high above the atmosphere. Such a structure would drastically cut the cost of getting into orbit. Rockets – expensive, polluting and dangerous – would be made obsolete. The space elevator would be the ultimate way of getting from Earth to space. It would work on other moons and planets, too.
The space elevator had its genesis way back in the 19th century, long before any rocket left the atmosphere. Russian scientist Konstantin Tsiolkovsky (1857–1935), whom we met in the rocket-science section, was the first to suggest a tower reaching up to geostationary orbit. His concept called for a structure like a superextended Eiffel Tower. In effect, a building. No known material, then or now, could support its own weight to such a height.
The archetypal space elevator (there are many variations) instead works by balancing the downward force of gravity with a chunky counterweight – perhaps a captured asteroid – way out beyond geostationary orbit of 36,000km (22,000 miles). The competing forces would keep the cable taut and stable. Loads could then be lifted up the tether like riding a giant elevator to space. The physics is all worked out, but current materials are not strong enough to handle the stresses. A space elevator would also require unprecedented capital spending and political clout to get off the ground. We’re unlikely to see one for many decades if not centuries.
Various other schemes to bypass the lower atmosphere have been suggested. These include the use of a long maglev (magnetic) track for acceleration, and even riding nuclear blasts into space. Perhaps the most likely to succeed in the short term is a hybrid craft that performs like an aeroplane in the lower atmosphere, but switches to rocket power to accelerate into space. People have been working on this ‘single stage to orbit’ challenge for decades, but it’s a difficult nut to crack. A new type of engine known as SABRE (Synergistic Air-Breathing Rocket Engine), currently under development, may finally overcome the problems sometime in the 2020s. It could revolutionize travel, offering journeys halfway around the world in just an hour.
To return to the flippant tone that opened this section, we should also note that rockets aren’t even the commonest means of transferring mass from Earth to space. Nature does it all the time by a process called atmospheric escape. Every minute, an estimated 181kg (400lb) of hydrogen leaves the atmosphere, and 3kg (6½lb) of helium goes the same way, along with smaller quantities of nitrogen, oxygen and other gases. The border between Earth and space is more porous than you might imagine.
* FOOTNOTE A medieval concept that has more of gravy than gravity about it. One wonders if this is the origin of the cow who jumped over the Moon in the nursery rhyme ‘Hey Diddle Diddle’.
Here are three headlines, all from July 2017:
KFC Launches Chicken Sandwich into Space
Watch: Toy Frog Launched 19 Miles into Space
£1,500 Watch Being Blasted into Space From Derbyshire Today
The three stories all concern high-altitude balloon launches. For as little as £500 ($650), a company will send any small object you like up into space. Your sandwich, teddy bear or sex doll (a porn site actually did this) will ascend into the heavens by helium balloon. As it climbs to maximum altitude, your object of choice will be filmed with the curve of the Earth as a spectacular backdrop.
Missions like these hit the press many times each month. They usually fall into one of two categories: school science projects and PR stunts. The toy frog, for example, was a laudable way to teach primary-school children about the atmosphere and our planet. KFC’s chicken stunt, meanwhile, clocked up tens of thousands of viewers on social media as it live-streamed the cosmic adventures of a Zinger burger. The £1,500 watch reached 35.4km (22 miles) before landing ‘north of Doncaster in the area between Barnsley and Pontefract’. This truly is the dawn of a new Space Age.
Scale up the balloon, and humans can also take a ride. Felix Baumgartner and Alan Eustace both leapt from high-altitude balloons, in 2012 and 2014 respectively. Widespread media coverage allowed us to watch these death-defying missions. Both men wore impressive spacesuits. The TV pictures showed a curved horizon, as though the adventurers were well above the atmosphere. Had they really taken balloons into space, or was the media telling fibs?
A balloon rises when the gasses inside are lighter than those outside. Above the stratosphere (50km/31 miles), the atmosphere is so tenuous that it’s near impossible to fill a membrane with anything lighter. Consequently, the highest altitude ever reached by a balloon – and one untroubled by plastic frogs or chicken sandwiches – is 53km (33 miles), set in Japan in 2002. That is barely halfway to the 100-km/62-mile internationally agreed limit of space. As for balloons with human payloads, the altitude record set by Eustace is a shade over 41km (25½ miles). His was an impressive feat, but to call it a spaceflight is akin to claiming Everest when you’ve actually only reached base camp.
None of this will deter the media. ‘Someone Actually Sent a Bonsai Tree into Space’ makes a far more alluring headline than ‘Company Paid to Send Shrub One-fifth of the Way into Space’. Basically, if you see the words ‘balloon’ and ‘spaceflight’ in the same headline, then you’re being misled – albeit in a light-hearted way.
At the time of writing, that £1,500 space watch lies unclaimed somewhere between Barnsley and Pontefract.