Surviving the Next Doomsday Asteroid Impact

One day, as has happened many times before, a large asteroid thousands of metres in diameter will hit our planet.

The result of the impact will rain hot debris across the entire Earth, heating the atmosphere to an oven-like temperature and shrouding it in dust clouds. Fires will burn for years adding billions of tonnes of soot to the atmosphere, and then the global temperature will fall dramatically. Photosynthesis will grind to a halt. Almost all species of plants and animals will perish. The only survivors will be those lifeforms that are able to eat the remains of long dead life, such a cockroaches and deep sea creatures.

The collision of a large asteroid thousands of metres in diameter with our planet will almost certainly result in the extinction of most lifeforms. It is unlikely that humans would survive.

If such an impact happened now humans would not survive. The extinction of our species would be certain. Essentially, life on Earth would be reset to the point just after the last massive impact event 66 million years ago (the Cretaceous-Paleogene extinction event that caused of the extinction of the dinosaurs).

By the next century we are likely to have permanently inhabited colonies on the Moon, Mars and possibly even beyond, but there will be a few hundred inhabitants at best. And most crucially those colonies will not be self-sufficient. If Earth is rendered uninhabitable and our civilisation destroyed then those colonies will die soon after. It will be a couple of centuries until self-sufficient colonies with tens of thousands (preferably millions) of people exist away from our planet. Until then we will need sanctuaries on Earth where a significant human population can survive should a massive asteroid impact occur. And by 'significant' I mean several million people at least.

The cost of building such underground sanctuaries would be extreme, and it would take decades or longer before they were ready for habitation. Hundreds of billions of dollars would be needed each year, and a workforce of millions. But it is actually affordable and achievable, if only the world's governments could be less paranoid and divert some of their defence spending to the project. The total spending each year by NATO members (29 European and North American countries) is more than one trillion dollars. If just a quarter of that budget could be redirected then the underground sanctuaries for North America and Europe could begin construction. If the likes of Russia, China, Japan and their neighbours did the same then sanctuaries in their region of the world could begin construction, too.

After a century of construction and expansion, the main chamber of a vast underground sanctuary, deep beneath mountains in Europe, is home to a thriving city of a million people. Powered by geothermal energy, the sanctuary is a self-sufficient haven for humans, one of several spread around the Earth. Even the impact of a large asteroid, such as the one that resulted in the extinction of the dinosaurs and many other species, would not lead to human extinction if such facilities exist.

Each sanctuary would need to be hundreds of metres underground, with locations beneath mountains the most preferable. As well as hundreds of kilometres of tunnels and smaller chambers there would need to be huge areas for agriculture, and even larger caverns to create the feeling of space and distance we have evolved to need, if only to maintain the sanity of individuals. Power could be generated using geothermal technology.  Energy generation using geothermal power stations currently provides 30 percent of the electricity requirements of Iceland, with the Philippines not far behind that. It is a relatively clean and renewable source of energy and would be ideal for subterranean habitats. New power generation technology, such as nuclear fusion, will hopefully be perfected over the next decades, which could provide almost unlimited power capabilities.

One of the many farming chambers that surrounds the main city of each of the underground sanctuaries. Agricultural facilities such as this, powered by geothermal energy, or even fusion energy, provide food for millions for thousands of years in the event of a catastrophic incident up on the Earth's surface.

Once created the sanctuaries would need to be permanently occupied to a high capacity: at least 50 percent. This is necessary to ensure that the sanctuaries are fully functional and under constant maintenance. They should be regularly assessed to identify areas of improvement. They would need to be totally self-sufficient if the need for them arises. There would hopefully be many months, if not years, of notice before a large impact would occur, giving plenty of time for people to migrate into the sanctuaries.

There are ethical and moral issues with regards to who would be selected to migrate into the sanctuaries should an extinction level impact be confirmed. The choice of certain sections of the population is obvious: there needs to be specialists in all areas of science and technology (engineering, medical, computing, utilities, agriculture etc) and also educators; it will be essential that education levels are maintained for the generations that will live in the sanctuaries, so schools and universities would be required.  The majority of inhabitants, however, will be ordinary citizens with ordinary levels of education and skills. Whatever method is used to choose which of those individuals and families are chosen - a lottery, or genetics (to maintain the diversity of the gene pool) - there would be objections, protests and even wars fought no matter who is chosen (there would be nations of the world without any access to sanctuaries of their own). Great care would need to be taken to ensure that the security of the sanctuaries is maintained and the chance of sabotage is kept to a minimum.

The selection of who is to migrate to the sanctuaries, and how to prevent attacks on them as they travel there, will perhaps be a more troubled process than the construction of the actual sanctuaries themselves.

Of course, it would be best if a cataclysmic asteroid collision was prevented altogether. There are numerous proposals to deflect or destroy asteroids that are identified to be on an eventual collision course with Earth. None of them has yet been tried, and all of them required many years, or even decades, of warning. Research into such methods should be intensified and an effective asteroid defence system should be implemented as soon as possible. The underground sanctuaries would allow a small percentage of our human population to survive the aftermath, but they should be considered a last resort. The need for then should ideally never arise.

Developing the ability to deflect or destroy asteroids on a collision course with Earth is essential to prevent the extinction of our species. A joint NASA and ESA mission in 2020-2021 will perform the first test of such technology.

If we construct several underground sanctuaries around the world, and deploy a system capable of destroying or deflecting asteroids, we stand an excellent chance of surviving as a species even if an object many tens of kilometres wide collides with our planet, and of eventually repopulating the surface once its ecosystem has recovered.

But, before the creation of the underground sanctuaries can happen we need to cultivate a will to change our thinking and work together for the greater good and the very preservation of our species and the wider ecosystem on which we depend. If we fail to do so we will fail all of humanity, and our extinction will be assured.

The Next Mass Extinction Has Begun

There have been five major mass extinction events in Earth's past. During each one more than 70 percent of all species were wiped out.  One of them, the Permian-Triassic event 252 million tears ago, actually killed off about 96 percent of all species.

Most of the events were caused by a significant climate change brought on by a combination of volcanic activity, asteroid impacts, sea level changes and drops in oxygen levels. The most recent mass extinction - the Cretaceous-Paleogene event 66 million years ago - is thought to have been almost entirely due to a massive asteroid strike known as the Chicxulub Impact, on Mexico's Yucatan Peninsula. It has become the most well-known of all extinction events, primarily because it caused the extinction of the non-avian dinosaurs. The dust cloud created by the impact reduced global temperatures by several degrees for decades. After that there was a period of rapid warming due the huge increase in carbon-dioxide in the atmosphere. For up to 100,000 years the global temperature was an average of five degrees higher. There was little time for most species to adapt.

An artist's impression of the Chucxulub impact crater a few hundred years after the asteroid hit. The crater is about 150 kilometres in diameter, and up to 20 kilometres deep.

There will be another mass extinction event - a sixth massive reduction in the number of species living on this planet. That extinction event, known as the Holocene extinction, is already under way, and its cause seems to be exclusively linked to human activity.

Humans have been responsible for extinctions for many thousands of years. The mammoth, which had been in decline as a species since the end of the last ice age more than 11,000 years ago, is one of the first examples. It was finally hunted to extinction 5,000 years ago. This was the start of our destruction of our planet's ecosystem.

Over the last century our rapid advance into a global technological civilisation, and our massive population growth, has lead to extremes of habitat destruction, over hunting, pollution and subsequent climate change. Without a drastic change to our activities most species will be extinct within a few centuries. Humans will have caused the fastest and most extreme mass extinction ever known on this planet. And it is more than possible that our own species will be one of those driven to extinction as the ecosystem we rely on collapses.

To survive as a species we need to do two things. And we need to do them now.

1. Halt our Destruction of Earth's Ecosystem

An extreme level of cooperation between the governments of the world's nations is required if we are to stop, or even just slow down, the climate change caused by our activities. And that cooperation needs to start this year. Unfortunately, as always, governments seem preoccupied with political, economic and territorial disputes. Climate change, and its effect on our very survival as a species, seem to be far down the list of priorities.

Radical policies are needed, with developed countries pushing for the complete replacement of fossil fuels with clean energy generation within a decade. Nuclear energy, despite its controversial nature, is very clean compared to coal and gas burning. In fact, nuclear energy generation produces zero emissions, and very little waste due to the density of nuclear fuel. The land required for a 1,000 megawatt nuclear energy generator is only about one square mile, compared to 75 square miles for the equivalent solar power plant. As well as building many more nuclear power stations, solar and wind power generators should, of course, be expanded rapidly, too. This will accelerate the closure of coal and gas power stations.

The massive and ever increasing human population on Earth needs to be ethically controlled. Developing countries in particular need immediate help to remove the need for people to have more than two children. If population growth is not controlled it is predicted that there will be almost ten billion people living on this planet by 2050 (up from 7.6 billion now). The demand on resources is already unsustainable. With such an increase we are condemning billions to starvation.

2. Build Self-Sufficient Colonies Away From Earth

The creation of self-sustaining colonies away from Earth should have started decades ago, but at least there seems to have been some recent minor progress towards such colonies. NASA have brought forward plans to get humans back to the Moon. It will create a sustainable human presence on the Moon by the middle of the next decade, and the Gateway, a space station in orbit around the Moon, will be a staging post for crewed missions to Mars.

NASA's space station orbiting the Moon. Construction is expected to start as early as 2022. It will be used as a staging post for Moon landings, and as a stepping stone to Mars.

While NASA does not seem to have much of a timeline beyond for the crewed missions to Mars (other than 'sometime in the 2030s') the plans for the Moon seem to be quite well defined, and impressively ambitious. The resources of the Moon can and will be used to manufacture the fuel for future missions to Mars.

This relatively sudden new momentum is encouraging, and must continue.

As soon as a permanent settlement is established on the Moon the missions to build one on Mars must begin immediately. No time can be wasted. And as the Mars colony grows in the 2030s there must then be a push to establish colonies further afield, such as on Jupiter's moon, Callisto (which orbits outside of Jupiter's harsh radiation belt - see 'Humans on Callisto within 15 Years'), and on Saturn's moon, Titan. Despite the extreme cold Titan is particularly suitable for a very large human colony - see my article, 'Human Colony on Titan' for more on that subject. In fact, it seems to be one of the best places to create a very long term human presence.

There must also be space-born colonies: vast habitats that do not rely on a planetary body and that can exist in their own independent orbits around stars and in interstellar space. They would obtain all the resources they need from small asteroids and comets, found abundantly for example in the Kuiper belt and the Oort cloud.

Will this century be the last time we'll be able to look down on Earth from space? Is the damage to the environment and climate caused by human activity already beyond recovery? If so, our technological civilisation will soon end.

If we do nothing to limit the effects of the current extinction event then we certainly deserve our fate. If our species becomes extinct then at least our destructive activities will cease. Earth will begin a slow recovery and a new climate equilibrium will allow a host of new species to flourish. Over many millions of years the Earth will become a thriving and lush haven for life, at least until another highly intelligent species starts to dominate and abuse its environment.

Is it possible that some of the many smaller spikes of extinction and climate change in Earth's past were caused by a dominant intelligent species that brought about their own demise? We could simply be continuing a natural cycle of events that has already played out on this world and millions of others. That cycle needs to be broken right now, before we doom yet another of Earth's civilisations to oblivion.

Nuclear War - Our Biggest Immediate Threat

Climate change is considered the main threat to our civilisation. Over the life time of the human generation being born right now our planet will almost certainly experience a significant detrimental effect from the amount of greenhouse gases that have been pumped into the atmosphere by our  activities. The increase in the number of forest fires in North America, flooding in Asia, and longer and more intense heatwaves in Europe is just the beginning.

It is not impossible to slow down or even halt such effects, but the huge political, economic and social changes necessary make it very unlikely that the nations of the world can come to any sustainable agreement on what to do.

As the effects of climate change intensify over the next few decades there will be severe pressure on the countries worst effected. Ultimately wars will break out as failing harvests, flooding and desertification force desperate nations to fight for the resources that other less effected nations are holding.

An artist's impression of how Miami would look with the relatively small sea level rise expected with a 1.5 - 2.0 Celcius temperature increase. It is now expected that a 3 Celcius increase is likely by the end of the century without drastic action. Such a sea level rise will displace hundreds of millions of people from coastal areas and leave major cities abandoned. Wars are inevitable.

The recent report by an Australian think-tank that civilisation is likely to end by 2050 is not at all far-fetched, but civilisation is likely to end much sooner if the political situation in many leading countries continues to radicalise. There will be little attention given to climate change by the leaders of such countries, which will be disastrous as some of them are the very ones with the ability to actually do the things necessary to limit the effects of such change. There is an ever-increasing chance of a major war between those nations. And such a war is likely to quickly escalate out of control. There could well be an exchange of nuclear weapons, which will have an immediate and devastating effect on our climate, far and above that of the climate change expected from our normal activities. Even the use of just a few low yield nuclear weapons would have a worldwide effect.

One likely scenario is between India and Pakistan: both nuclear powers. Pakistan, one of the most 'water stressed' nations on Earth, relies on water flowing from India. If India diverts that water (which it has already threatened to do in part, in response to a terrorist attack in Kashmir) and Pakistan suffers droughts then it may become desperate enough to destroy India's dams. India would retaliate. And then Pakistan, desperate and pushed to the brink as its citizens protest and riot, may feel it has no choice but to launch a nuclear attack. India will certainly respond in kind.

A long range nuclear missile launching

As well as destroying large parts of both countries and killing hundreds of thousands almost immediately, the effects of such a regional nuclear conflict would be felt across the world. Even conservative estimates predict that five million tonnes of smoke would be released into the atmosphere and trigger a nuclear winter of two to three decades, reducing global temperatures by two to four degrees and decimating the protective ozone layer (removing most of our natural protection from UV radiation). There would be widespread famine as food production is significantly reduced. Skin cancer would become common place.

Of course, the threat of a major nuclear war has been present since the 1950s, but since the end of the cold war the strict controls and mutual understanding and respect of the nations with nuclear weapons seems to have eroded. The threat of such a war breaking out is increasing day by day, and the rhetoric and threats of the major nuclear powers recently does nothing but exacerbate the situation. Russia has recently exited the INF (Intermediate Nuclear Forces) treaty following the exit of the USA a few months earlier. Nuclear arsenals are being modernised and upgraded., and a new type of missile that can travel at hypersonic speeds (greater than Mach 5) is in rapid development by Russia, China and the USA (and France recently became the first European country to announce a desire for such a weapon).

A hypersonic missile seconds after launch. It would only take it a few minutes to travel hundreds of kilometres, and it would be unstoppable with current defences. It is in development by the major world powers and is a highly destabilising weapon. 

Hypersonic missiles, against which there is currently no effective defence, can hit a target in minutes at an incredible speed. They are designed primarily to carry conventional weapons (but could carry small nuclear warheads if required), and one White House official has even dubbed them 'instant leader-killers'. This suggests one use for them: a means to assassinate the leader of a nation virtually without warning and with almost no risk of the missile being intercepted. Such an ability is incredibly destabilising.

The amount of money spent on military defence around the world is staggering. In 2018 the world's governments spent more than $1.8 trillion. The USA alone accounted for just over a third of that. We are spending about 28 times more on military hardware and development that we are on tackling climate change. To put it simply: we seem to be more concerned with securing our self-inflicted annihilation than securing our survival. Imagine what could be achieved if we could divert even just ten percent of that spending from defence to renewable energy production, for example?

One of the most chilling explanations of the Fermi Paradox (which refers to the contradiction of the lack of evidence for intelligent extra-terrestrial life when observations suggest it should be quite common) is that civilisations destroy themselves soon after developing radio or spaceflight technology (and with it the technology to wreak mass destruction). We now have that capability. It is quite likely that a nuclear war will end our technological civilisation, and the subsequent immediate climate change will almost certainly result in our near or actual extinction. And if we manage to avoid such a war, it seems that we may well be too late in trying to tackle the climate change that is happening now.

As I've discussed before, our survival instinct, which has served us well up until now, continues to ensure that we will develop more and more powerful weapons to defend ourselves, and allow us to forcefully take what we need to survive. Ironically this instinct now seems to be taking us closer to extinction. Our natural instinct to be suspicious of others seems almost impossible to overcome. We are unlikely as nations, and even as individuals, to cooperate enough, and to put aside our differences, to work together and sacrifice some of our comforts in order to stop climate change or the risk of a nuclear war.

If we can survive the next few decades, and work together instead of feeling the need to build ever more destructive weapons with which to destroy each other, our civilisation will be well on the way to expanding beyond Earth and creating colonies on and around other planets. Pictured here is the impressively conceived Venus cloud colony.

If it's true that almost all the intelligent civilisations in our galaxy destroyed themselves soon after they developed the ability to do so, then it is likely that we will too if there is not a radical change in our behaviour. We must strive to be one of the exceptions. We need to grow beyond our blind sprint towards our own destruction. It is the only way we can avoid an eventual nuclear war. And it is the only way we can tackle climate change and save our planet. If, against all odds, we manage that then we will be well on the way to expanding our civilisation beyond the Earth. Our long term survival as a species will then be assured, and we will take our place among the select few civilisations that overcame their self-destructive tendencies.

There is still time. There is still hope. But both are dwindling fast.

Saving Planet Earth - Forever

Human-induced climate change is the biggest short term threat to our planet and our species. Neutralising that threat should be our top priority. But ultimately, the natural warming of our sun will change our climate to such a degree that one day the Earth will uninhabitable for even the hardiest and most basic life.

The warming of the sun is something that we can have no control over.

No matter how we balance our ecology with sustainable energy generation, recycling, pollution and population control, in a few hundred million years our world will warm up to a point that will render our species and all other higher life forms extinct. And a billion years after that the Earth will be nothing but a hot and sterile planet - wiped clean of billions of years of evolution, experience, culture and history. It will be a hellish world not unlike how Venus is at the moment.

This image of Venus, taken by Japan's Akatsuki probe, is how Earth could look in a few hundred million years time. The natural warming of the sun has caused the oceans to boil away, and the uncontrollable greenhouse effect that followed has rendered the planet uninhabitable. 

Earth, the cradle of our species and civilisation, will become a dead world. No matter how far humans have spread throughout the Solar-System and beyond, it would be a sad and poignant day when our planet of origin is finally abandoned by humanity to face its natural fate.

There are, however, things we could do to preserve our home world's habitability for much longer, and possibly forever:

1. Block Some of the Sunlight

There is a point between the Earth and the Sun known as a Lagrangian point (specifically L1 - there are four others). It is a stable point where the gravitational pull of the Earth cancels out that of the Sun. In other words, any small object positioned there will stay there, directly between the Earth and the Sun.

It is the ideal position to place a filter - something that can reduce the amount of light and energy from the Sun. Unfortunately L1 is 1.5 million kilometres away, which means the filter would need to cover tens of millions of square kilometres. This would be unfeasible for a single object which would need to be many thousand of kilometres in diameter, but a formation of very small spacecraft, each just a metre or so across, would be quite possible.

A swarm of hundreds of millions of smart probes positioned at the stable L1 point between the Earth and Sun would be able to filter out a few percent of the Sun's energy. This would stop global warming, allowing the reversal efforts on the Earth to have time to work. In the longer term this would allow the climate on Earth, and life, to be less affected by the gradual warming of the Sun.

The objects would need to be very light - just a couple of grams - and would consist of a very thin stretched panel to deflect sunlight. Each spacecraft would be intelligent enough to maintain its position within the formation, and use its panel as a solar-sail to move away from the Sun, and ion engines to move in other directions.

Such a swarm of spacecraft would need regular renewal as a certain number of them fail each day. Ideally a permanent production facility away from Earth would be needed, perhaps on the Moon, to enable a constant supply of replacements.

The swarm would only need to reduce the amount of energy reaching Earth by two or three percent to halt global warming. And it could even be used to cool certain areas of the Earth at crucial times, such as over the oceans to lower the intensity of hurricanes and typhoons, and even prevent them altogether.

2. Move Earth to a Higher Orbit

As the Sun slowly warms up the effectiveness of the filter at L1 would reduce. The next step is to alter the Earth's orbit, increasing its distance from the Sun. The most efficient way of doing this is to cause a relatively massive object, such as an asteroid, to repeatedly pass very close to our planet, pulling it into a slightly higher orbit each time.

The larger asteroids in the asteroid belt, such as Vesta or Pallas, would be ideal candidates, although changing their orbits, and directing them into the inner Solar-System on the correct course is a mammoth undertaking. It would take many millennia to get those objects on the right trajectory so that they pass by Earth at just the right distance to have the desired effect.

Vesta, one of the largest asteroids in the asteroid belt between Mars and Jupiter. Such an asteroid would be able to 'pull' Earth into a higher orbit around the Sun if it were sent on the correct trajectory to pass very close to Earth.

There is, of course, an immense risk to such a strategy. Any miscalculation could send the asteroid on a collision course with the Earth, or the Moon, which would be truly catastrophic. Fortunately the Sun's warming is a very slow and steady process, so as long as the L1 filter is in place we would have plenty of time to overcome the technical challenges and design in all the necessary safeguards.

Once the process has begun the asteroid would be directed to pass the Earth regularly, possibly every couple of centuries, to carefully nudge our planet's orbit away from the Sun. Our climate and the life it supports would remain stable and comfortable.

There is a limit to how far the Earth's orbit could be altered. We could only move our planet so far before we get too close to the orbit of Mars. We could, of course, start moving Mars as well, but then eventually the asteroid belt would become a hazard to both worlds (and Mars will be far too important to the humans living there to be placed in such danger). We would end up having to alter the orbits of all the significant bodies in the asteroid belt as well, which would then put at risk the colonies we will have set up on Jupiter's moons, or in the inner Solar-System. It would be an impossible task to manage.

3. Build a Shell Around Earth

As the Sun's relentless warming continues there is something else that we could do to further protect our climate: build a shell around the Earth. Just like altering Earth's orbit, this will be a massive undertaking, and one that could take millennia to build. But it is possible, and there will be plenty of time to complete it.

A huge amount of material will be needed, which would make mining it all from the Earth almost impossible. The Moon and the asteroid belt would be able to provide all the resources necessary. The supports would be built first, evenly placed around the whole planet. These would ideally rise up to the Karman line, the officially recognised edge of the atmosphere at an altitude of 100 kilometres.

The view from the Karman line, a hundred kilometres above the Earth's surface. The planet's protective shell would be built at this altitude.

Next the shell would be built, spreading out from each support until it joined together with other shell segments. On the inside surface of the shell there would be artificial lighting, powered initially by the Sun beyond. Once completed the shell would ensure that the intensity of light and energy reaching the Earth's surface is fully under our control. Excess heat would be radiated up the supports and out into space from the shell's outer surface. It's interesting, and even sad, to think that life on Earth would never again see the Sun, Moon or stars in the sky, but as the construction of the shell could take a hundred human generations or more, those alive to see the shell completed would not really have seen them and so would not miss the experience.

Having such a shell around the Earth would have other benefits besides climate control. It would provide another surface on which to construct manufacturing, accommodation and space launch facilities. It would be the idea place for astronomical research. Additional outer shells are likely to be constructed, with the spaces between the shells used for any number of functions. Such spaces could be hundreds of metres high in some cases, which could be pressurised and provide vast areas for agricultural activities that would cultivate and breed enough food to feed billions of people on Earth, and for export to colonies around the Solar-System.

After thousands of years of construction, millions, even billions, of humans and animals could end up living on the plains and cities constructed in those vast spaces between shells way above Earth's natural surface.

Eventually the very outer shell, one of hundreds, could have a diameter many thousands of kilometres greater than that of the Earth that is hidden and protected far below.

4. Move Earth to Another Star System

There will come a time, more than a billion years from now, when the Sun starts to swell. Slowly but surely it will expand and engulf the inner planets Mercury and Venus, and then, no more than five billion years from now, it will engulf the Earth, and possibly Mars, too. The magnificent multi-layered shell that we will have constructed around the Earth will no longer be enough protection. It will be vaporised along with our planet.

The inner Solar-System will have become a hellish place long before that. Without a protective shell, the Earth would become uninhabitable for humans in a couple of hundred million years. Within six hundred million years plant life would most likely die off completely bringing the food chain to a halt. And a hundred million years after that the oceans will have boiled away.

Within 200 million years we will need to be preparing for the next stage of Earth's survival. It will involve much more than just moving to a higher orbit in the outer Solar-System (although that could be an interim stage). We will need to move the Earth to another star system.

This would be a truly momentous task, but we would have millions of years to get the Earth and its all encompassing shells ready. A means of propulsion will need to be developed. It would have to be gentle, with a barely noticeable gee force, but able to be sustained for years due to the incredible mass it would need to move. Over many centuries the Earth's orbit would need to be changed to a more and more elliptical one, taking it far out to the orbits of the outer planets, and then back in to the inner Solar-System. Eventually, when the destination star has been chosen, the Earth can be directed on a course that would take it very close to the Sun.  With its propulsion system pushing as hard as it safely can, the Earth would head in to the inner Solar-System and sling-shot past the Sun, picking up enough extra velocity to escape from our planetary system and head out into interstellar space. At this point the Earth and its shell would be travelling at many hundreds of kilometres per second, and this would be maintained by the propulsion system until the planet left the influence of the Sun's gravity. The Earth and its shell would now essentially be a massive generation star ship: one with a huge amount of comfortable and well-protected living space for billions of people.

There would be many options with regard to the destination star, and the obvious choice would be a star very similar to the Sun, but younger. The disadvantage of that would be the need to move to another star system in a few billion years time. A better choice would be a different kind of star: a smaller and cooler star type that just happens to be the most common type of all - a red dwarf. Red dwarf stars are very stable and very long lived. At a minimum they can live ten times longer than the Sun, and many will exist in a stable condition for a trillion years or more - hundreds of times longer.

As red dwarf stars are much smaller and cooler than the Sun, the Earth would need to be in an orbit much closer to it to receive the same amount of energy that we do at the moment: so close that a year would last just a few days. Of course, Earth's extensive system of shells and energy generation would allow it to have a much more varied choice of orbits. If a red dwarf is chosen as the destination star there would be no need to move the Earth to another star system again, at least in any timescale that we could comprehend.

Earth, safe beneath its worn but still intact outer protective shell, arrives on in the region of a younger star after a voyage of many millennia across interstellar space

Once the Earth is settled and secure in its new star system humans would once again begin exploiting the local resources to enhance their home. More and more layers would be added to the shell, and over many thousands of years there would be tens of billions more people thriving up there. It's not inconceivable that one day the Earth would be no more than just a small part of a vast structure that has grown to something a hundred thousand kilometres in diameter - almost ten times that of the planet within. There's no reason why it could not keep growing and growing.

By this time, hundreds of millions of years from now, there will be other planets that have been terra-formed and had their own shells built to protect them. The Earth will be one of many highly protected planets hosting billions upon billions of humans. Some of those planets will be mobile and able to traverse the vast distances between stars, and some will even be capable of travelling the intergalactic voids between galaxies. Perhaps the Earth, with its ever deepening structure of shells, will become a legend amongst those intergalactic travellers: the unreachable source of all humanity where hundreds of billions still live out their lives.

Hundreds of million years from now there could be trillions of people spread throughout our galaxy, with billions more on their way to other galaxies, safely cocooned within planetary shells.

It's an incredible prospect, and an awe-inspiring concept. But it's not impossible. If this happens then the survival of our species will be secured for trillions of years.