A republication of an older post
The dinosaurs would disagree. After owning the Earth, they were now in a bit of a bother. A major re-arrangement of the Earth had taken place. Pangea had split; Gondwana was broken up. The Indian ocean had formed but not in a clean way: a number of parallel rifts were running through Africa, and the easternmost rift had won the prize and been awarded the Indian Ocean. The other rifts did their best and did succeed in splitting off further bits of Africa. Madagascar went, but did not go far. A large part split off and a sizable landmass had begun to move away from Africa, accelerating as it did so. On its way, India crossed a hot spot in the crust (this is the hot spot currently underneath Reunion Island). It caused havoc. The hot spot melted a substantial part of India’s lower crust, causing the much-thinned sub-continent to begin to move much faster across the Indian Ocean. This finally ended with a major collision as it went at headlong speed into Asia. But before this, the melt found a way through and began to erupt on the surface. The flood basalt formed the Deccan traps, and in the process poisoned the atmosphere. The dinosaurs did not like it.
And then the asteroid hit, while the Deccan trap eruption was still in progress. Left alone, the dinosaurs would have survived the volcanics, and probably have recovered. As you read this post, you would have lifted your tail and thought about how Darwin had proven that humanity had evolved from tree-dwelling reptiles. Instead, the devastation coming from the Yucatan amplified the turmoil, spread worldwide, and caused years of global winter. Not a single dinosaur survived. Sometimes, the Universe just does not play fair.
We, the mammals, came out winners and we would have reason to be delighted about this unexpected course of events. But dinosaurs fascinate us, and their demise is regretted by everyone’s inner child. Both volcanoes and asteroids ended up with a bad press, presented as random killers which are best avoided.
But in this post, I will try to restore their standing. Asteroids may be dangerous, but they can also save us – from a danger that is still far in the future, but which makes the current global warming look like small fry. And fry we will, if nothing is done.
Our lives are shaped by the Sun. We are creatures of the day (teenagers excepted), needing light to live. We also need the warmth of the sun, as we lack a natural insulating skin cover. But it goes further than that. Our world depends on a constant climate. The crops we grow need to be in a certain range of temperature and rain fall. Even a seemingly small change can make our crops vulnerable to diseases and pests, and reduce the yields. In some places we will now need to change crops to cope with climate change. But overall, we have been lucky. Over the past 8000 years, the climate has been remarkably stable. But it has not always been like that, and going back in time there were both ice ages and hot-house ages, when the world was different and habitability was not always guaranteed.
The ultimate driver of our climate is the sun. It is extremely stable: variations are only by 0.1%, too small to have a significant effect on our temperatures. There is an effect from the orbit of the Earth: the ice ages were triggered by a change in the orientation of the Earth, which affected how much sunlight falls on land rather than the oceans. A small change for the Earth – but a big change for us. But the sun itself was not to blame.
But it won’t always be thus. Very, very slowly, the sun is getting brighter. This is not on human time scales. When the Earth was formed, the sun was fainter by 25% than it is now. That was 5 billion years ago. The Earth compensated for the very-slowly brightening sun by reducing the amount of greenhouse gases in the atmosphere, lowering the thermostat a little. But this will not work forever. Eventually, the minimum is reached, and temperatures will begin to increase. Trouble will start in another half a billion year, and a billion year from us, the sun will be 10% brighter than it is now, and temperatures will have become so high that complex life will become impossible. Shortly after the oceans will begin to evaporate. This is still over unimaginable time scales. But scientists are paid for long-range thinking.
Lucky we have plenty of warning, and science has time to devise ways to keep the Earth cool. What can we do? Some blue-sky thinking is required.
To cool a planet
There are a few things we can consider. First, could we devise anti-greenhouse gases? Greenhouse gases let visible sunlight through, but stop the infrared radiation from the Earth from going out. Some gases act in the opposite direction: they stop visible light, but let the Earth’s radiation out. Can we add that to our atmosphere?
Yes, we can, but it is unlikely to help. The main such gas is SO2, which indeed has an anti-greenhouse cooling effect. But it doesn’t last in the atmosphere, and breaks down within years. So we would need to continuously pollute the atmosphere with SO2, and as soon as funding would run out, the Earth would heat almost immediately.
We can also create more clouds, but these have a fairly neutral effect: they cool the days but warm the nights, with little or no net benefit. (Also, creating clouds goes against the principle of blue-sky thinking.)
We can paint parts of the Earth white, to increase reflectivity. This happened during the ice ages, but it would mean keeping the Earth colder than it is now, so that snow occurs continuously at low latitudes, and does so without any break. A better way may be to cover 20% of the tropical oceans with a white material – but not ice.
Less practical would be a solar shield: a fabric floating in space between us and the sun. It would need to be three thousand kilometer in diameter. And we would need to devise a way to keep it aligned. It couldn’t be in orbit around the Earth. Keeping it stable for a billion year may be an unsolvable problem.
Can we move house? How about living on Mars? That is not ideal either. We would need to some drastic planetary engineering to give it an atmosphere that can keep it warm, but the planet is too small to maintain a stable atmosphere. It would need continuous re-engineering. A quick trawl through the estate agents does not reveal other vacant properties in the right location.
This leaves only one option. It is the mobile home solution – we need to move the Earth.
Moving the Earth
We don’t need to move by that much! To counter a 10% increase in solar brightness, we need to move outward by 3%. At the moment we are orbiting the sun at a distance of 150 million kilometer. This needs to be 155 million kilometer. A 5-million-kilometer move must be doable.
There are some consequences. At the moment the year is 365 days long. That will become 380 days – we need to put two extra weeks somewhere in the year, or add a day to each month.
This is just the initial move. As the sun keeps brightening, we will need to keep moving further, and as the increase gets faster, we will need to move faster. But let’s first do the first billion year move.
To move to a different orbit requires changing your velocity. To move 3% further out requires making the Earth move faster. Over the billion year, we would need to accelerate by 500 m/s. For a rocket, that is not hard. But the earth is a lot heavier than a rocket. How can we do this?
In fact using a rocket is not the only way you can pick up extra speed. Long-distance missions try to fly past one or more planets on the way, in order to use a gravitational assist: you let the planet bend your orbit, and in the process you end up go a little faster (going slower is also possible). It is also called a gravitational slingshot. Often, Earth itself is used. It is funny to launch a mission to a faraway place, and after 12 months see it fly past Earth within a thousand kilometer of its launch position. It is not an error in the directions – it is done to go a bit faster than the launch rocket could achieve by itself. Rockets are an incredibly inefficient way to pick up speed, because you need to accelerate the remaining fuel as well, and most of the craft is fuel. So space missions are paired back to the bone regarding their net weight. By using a gravity assist, you can get a few km/s extra velocity, and because you need less fuel, the actual spacecraft, the bit with the instruments, can be made heavier – and bigger. All large missions now use this method.
The ESA mission to Jupiter using five fly-bys to pick up speed, including three of Earth. (This trajectory has since been slightly changed because of the change in launch date.)
But the laws of physics are pretty inflexible, and they do not allow this to go unpunished. As the spacecrafts gains momentum, the planet needs to lose it. The accountants of physics insist on balancing the books. Now the planet is a lot bigger than the spacecraft, and it only needs to change velocity by an unmeasurable amount in order to keep Newton happy. The planet (say Earth) may be 1022 times heavier than the spacecraft, and the velocity change scales with the inverse of mass (in order words, mass times velocity is constant). So no worries – we are not doing anything bad. We are taking a few milli-pennies from the very very rich to pay off the debts of space travel.
In the example above, Earth is used to speed up a spacecraft as it is traveling to the outer solar system. That means Earth slows down, and as you slow down you drift towards the inner solar system. (It also means that as you drift inwards, you pick up speed from the sun’s gravity, and in fact gain more than you lost in the first place. In space, if you slow down you end up going faster. That is rocket science.) Inward, towards the sun, is the wrong way for us. So instead of launching things to the outer solar system, we need to use fly-bys of satellites to send the spacecraft inward, to the inner solar system. They slow down and we speed up, drift outward – and end up going slower. Such is life.
Using an asteroid
Spacecraft are much too small to make a real change to the orbit of the Earth. Even if you had one fly-by every second for the next billion year, the change would still be a factor of 10 million smaller than what we need.
Now do the same thing with an asteroid. Take an asteroid of 60 km radius (that is big – there are not so many in that category). It has a mass one millionth of the Earth. Put it on a fly-by which slows down its own speed by 0.5 km/s. Do this a million times – and the Earth will increase its velocity by the amount we need. After a million fly-bys, the Earth is in its new, up-town orbit. And as we have a billion years for the move, this means we only need one fly-by from the asteroid every 1000 years.
But after the first encounter, the asteroid is drifting towards the inner solar system. How do you get the asteroid to come back to us for the next encounter, for the next fly-by? Initially, no problem, as it will still be on an orbit that intersects ours – only the orbit will now be more elliptical, with closest approach to the sun closer than it used to be. But after a couple of such these gravity pushes, it is getting too close to the sun. How do we solve this problem? It is easy! As the asteroid slows down in its orbit, direct it towards Venus. Now the asteroid can do a Venus fly-by in which it picks up its missing speed again. Venus will slow down and move inward, heating up even more than it already has, but Venus is a lost cause anyway.
Venus will redirect the asteroid towards us, and the process starts again.
Health and safety
The risk analysis flags up one awkward possibility. A slight deviation from the intended course could be sufficient to instead of a fly-by, have a direct hit. And hitting the Earth would be bad. An asteroid that size will create a crater 2000 kilometer across, ten times to size of the one that exterminated the dinosaurs. Nothing would survive, and there would be no need to move the Earth any further. Now NASA is on record that there is only a chance of one-in-a-million that a gravitational assist would go wrong and the spacecraft hit the Earth instead. So, since we are planning for a million fly-bys, that makes the chance of a mishap 100%. (Actually, 50%.) That is not quite acceptable.
So, you need to be able to make small course corrections to the asteroid, while it is on the way. The earlier you catch a potential problem, the smaller the correction is that is needed. So, be vigilant, and once you spot a deviation, fix it immediately.
Such small course corrections can be done using a so-called gravity tractor. Put a spacecraft near the orbit of the asteroid, and give it a very small engine, enough to keep its distance to the asteroid but not enough to get away. There will be a tiny gravitation force from the rocket on the asteroid, but over time it adds up to a small change in orbit of the asteroid. And only a small course correction would be needed, so this can be sufficient. Remember, we have a lot of time. In the example of Ida above it has a miniscule moon which could be used.
A safer way
It is still uncomfortable to have a large asteroid flying past only a few thousand km above our heads. The margin for error is not large. Can we do better?
Yes, if you are willing to make a small sacrifice. Instead of the Earth, we can fly the asteroid past the moon. We will use it to speed up the moon, which will put it into a wider orbit around the Earth. After a number of such fly-bys, the moon will eventually escape from Earth altogether and enter its own orbit around the sun. We may need to fly the asteroid very close to the moon, but this is no problem as there is no atmosphere, so you can get as close as you want. And if you do miss, you only hit the moon and a moon-wide extinction event is also a zero-impact event.
The velocity of the Earth will not be affected to any significant degree by the change to the moon’s orbit. But once the moon is free, it can now be used for fly-bys. Because it is much heavier than an asteroid, you need fewer fly-bys – a few hundred will do. And they can be at a much larger distance.
Once you have moved the Earth to its desirable location, you leave the moon parked in an accessible orbit, for when you need to move Earth again. The Trojan point is best: this is the same orbit as the Earth, but 60 degrees behind it (or in front of it). An object in a Trojan point is in a stable orbit.
This will work for almost the full 5 billion years the sun has left before it becomes a red giant. (By that time, the sun will be twice as bright as it is now.) Eventually, while we are moving the Earth we will get too close to the orbit of Mars and a collision would become likely. That could recreate our moon (the moon formed when a Mars-sized planetoid called Theia (named in hind sight) smashed into the Earth) but that would be the only redeeming feature. We would therefore have to get rid of Mars in some other way first. I am open to suggestions.
Dinosaurs versus humanity
The dinosaurs died out because of bad luck. The Deccan traps was a survivable event. The asteroid impact was also a survivable event, at least at some distance. But the combination was lethal. What was the chance of the asteroid hitting while the Deccan eruption was in progress? It turns out, not that small. Flood basalt events are fairly common. Since the time of the Cambrian explosion, the chance of a dinosaur-extinction-size asteroid hitting during a Deccan-sized eruption is not far from 50%. If not the dinosaurs, it would have hit someone else, but eventually it would have happened.
But we are not dinosaurs. We can know what is coming, and our science can find solutions. All we need is imagination – and an accurate calculator. Physics can do the rest. Moving the Earth is not difficult. It only requires a bit of physics and a lot of time. Asteroids can save us again, as once they saved us from the dinosaurs. Only the dinosaurs would disagree.