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.