Can we use a giant thruster to change Earth’s orbit?

One of the most steady, unchanging properties in our cosmic history is Earth’s orbit. For the past 4.5 billion years, Earth’s orbital path around the sun has remained practically unchanged, even as a whole slew of fantastic events have occurred: giant impacts, the formation of moons, the continued slowing of our planet’s rotation, and the emergence of life. Even taking into account the gravitational influence of all the other objects in our solar system and galaxy, there’s greater than a 99% likelihood that Earth’s orbit will continue to remain unchanged in any appreciable fashion.

Over the long term, this will lead to an unmitigated catastrophe for the entire planet. Even the worst-case scenario for our present battle against global warming, where unchecked rises in the concentrations of greenhouse gases causes a severe temperature rise and the melting of all the polar ice on Earth, pales in comparison to what the sun will eventually cause. If nothing significant changes, the sun’s ever-increasing energy output will boil away all of Earth’s oceans over the next 1 to 2 billion years, likely killing all life on Earth.

Is there any way to save Earth from this fate? Migrating our planet to a different location in the solar system, by changing Earth’s orbit, might be our last best hope. Here’s how a giant thruster at the South Pole could wind up saving the entire planet.

Right now, the sun appears as it does because of its temperature, energy output, and distance from the Earth. As its energy output increases, we must move Earth farther away or the sun’s increased output will boil the oceans away. (Credit: Public domain)

The environmental problem

If you think the global warming we’re presently experiencing is bad, just you wait until you learn what the sun has in store for us. Today, the major cause of Earth’s changing climate and increasing temperatures has nothing to do with the sun, but rather is driven by the atmospheric changes caused by human activity since the dawn of the Industrial Revolution. Between adding greenhouse gases to the atmosphere (mostly carbon dioxide and methane) and feedback-driven changes in the long-term water vapor concentrations, Earth’s energy budget has changed dramatically over the past ~200 years.

Just as piling blankets on top of you when it’s cold helps you better retain your own internal heat before it’s radiated away, adding greenhouse gases to our atmosphere helps the Earth retain heat. As was established more than 50 years ago by new Nobel Laureate Syukuro Manabe, doubling the concentration of CO₂ would increase Earth’s temperature by 2 °C (3.6 °F) or more, with worst-case scenario changes leading to the melting of all the polar ice on Earth within perhaps a few thousand years. An ice-free Earth wouldn’t be unprecedented, but it would be extraordinarily bad for humans on Earth.

Comparisons of the predictions of different greenhouse gas emissions scenarios and the warming they will induce by 2100. Note that the more optimistic scenarios all require a significant and rapid decline in our CO2 emissions: something that is not presently coming to fruition. (Credit: IPCC AR6 and AR5 reports)

But it won’t be nearly as bad as what the sun will gradually do as time goes on. Inside the sun, nuclear fusion occurs inside the core only, where temperatures exceed 4,000,000 K. In the very center of the core, temperatures can reach as high as 15,000,000 K, with the rate of fusion reactions rapidly increasing with temperature. But here’s the problem as time goes on:

1. the sun’s core converts appreciable amounts of hydrogen into helium
2. the helium gathers in the inner core, but cannot fuse any further at present
3. the concentrated helium leads to gravitational contraction and causes the interior of the sun to heat up
4. the inner core’s temperature and expands the “4,000,000 K and above” region to a greater internal extent
5. this leads to a gradual increase in the sun’s rate of fusion, which increases the sun’s overall energy output

With greater amounts of energy reaching the Earth, there are only so many defenses and feedback mechanisms our planet has at its disposal. Once the global average temperatures rise above 100 °C (212 °F), a scenario that will likely take place between 1 to 2 billion years from now, our oceans will boil away. For all intents and purposes, this will mark the inevitable end of the line for complex life on Earth.

The farther away your distance is from a brightness source, the smaller the flux. Brightness has an inverse-squared relationship with distance, as illustrated here. (Credit: E. Siegel/Beyond the Galaxy)

The energy problem

If we can’t prevent the sun from heating up, then perhaps migrating the Earth farther away from the sun could provide the ultimate solution. There’s a simple and straightforward relationship between brightness and distance: Every time you double your distance from a luminous source, the brightness that you experience is quartered. This is excellent news: If the sun’s energy output were to increase by 10%, you’d only have to migrate Earth an additional 4.9% of the distance away from the sun to keep the energy we receive constant.

Given that the sun’s energy output is currently increasing by ~10% with every billion years that passes, this is a long-term problem that we’re going to have to address someday if we want our planet to remain habitable. Changing our orbit by a few percent might not seem like a particularly major task. After all, Earth orbits the sun in an ellipse, with our closest approach to the sun taking us within 147.1 million km (91.4 million miles) and our farthest distance clocking in at 152.1 million km (94.5 million miles). The difference in radiation received is about 6.5%, meaning that if we could simply replace Earth’s current orbit with one that constantly kept us at our aphelion distance, we’d keep Earth’s energy budget from increasing for more than 300 million years.

Although Earth’s orbit undergoes periodic, oscillatory changes on various timescales, there are also very small long-term changes that add up over time. While the changes in the shape of Earth’s orbit are large compared to these long-term changes, the latter are cumulative, and hence, are important. (Credit: NASA/JPL-Caltech)

But that’s more than a major task — it’s an astronomically difficult one. The reason Earth orbits the sun in its present location is because that’s where our kinetic energy, or the energy of Earth’s motion around the sun, balances the gravitational potential energy at our current distance from the sun. If we managed to steal energy away from Earth, we’d lose energy, causing us to sink toward a more Venus-like orbit but with greater speeds. Similarly, if we wanted to rise to a more Mars-like orbit, we’d need to pump energy into Earth, leaving us with a net speed that’s currently smaller than our speed around the sun today.

The concept isn’t difficult, but the amounts of energy involved might seem like a dealbreaker. For example, over the next 2 billion years, we’ll have to push Earth’s mean distance from the sun out from its current value of 149.6 million km (93 million miles) to 164 million km (102 million miles) to keep the energy impacting our planet constant. But recall that Earth is incredibly massive: about 6 septillion kilograms, or 6 × 10²⁴ kg. To move us into a stable orbit that was that much farther away, we’d have to input an extra 4.7 × 10³⁵ joules of energy into our planet: the equivalent of 500,000 times the cumulative energy generated by humanity for all purposes combined, continuously, for 2 billion years.

The planets move in the orbits that they do, stably, because of the conservation of angular momentum. However, an impulse or a thrust could give us that sought-after change we desire, allowing us to migrate the Earth after all. (Credit: NASA/JPL/J. Giorgini)

How a thruster can help

And yet, as tall an order as that seems, it’s possible. There’s enough energy out there for us to collect, coming directly from the sun itself. Remember, the sun emits radiation omnidirectionally, where, at the present Earth-sun distance, every square meter of area receives 1500 W of continuous power, as long as nothing blocks its line of sight to the sun. That’s 1500 joules of energy every second, and we have two billion years (or about 6 × 10¹⁶ seconds) to:

• gather that energy
• convert it into thrust
• use that thrust to change the momentum and kinetic energy of Earth

Gathering the energy is one of the hardest parts of this problem. That’s where the idea of a solar collecting array in space can help tremendously. It might take an array that’s an astounding 5 × 10¹⁵ square meters in size, or about the surface area of 10 Earths, to collect the necessary amount of energy from the sun. But that energy is available. More importantly, from a different point of view, it’s “only” 0.000002% of the sun’s energy that we need to harness: a large, but not impossible, amount.

The concept of space-based solar power has been around for a long time, but no one has ever conceived of an array that’s 5 billion square kilometers in size: the amount required to gather enough energy to migrate Earth to a sufficiently higher orbit. (Credit: NASA)

The other key is to use that energy effectively to raise Earth’s orbit. In physics terms, the task would be the same for any mass in a gravitational field: we have to apply an external force over a certain duration of time, creating an impulse that causes an acceleration and changes the mass’s momentum. The same physics that works for launching a rocket into space would work for launching Earth to a higher orbit. All you’d have to do is apply a thrust that changes Earth’s momentum in a positive direction and it would eventually boost us farther away from the sun.

This requires a thruster: some sort of device where the action (accelerating the Earth) is balanced by an equal and opposite reaction (expulsion of spent fuel) that you put to good use. Ideally, you’d always aim your thruster so that it pushed the Earth forward in the direction it’s already moving. However, that’s very difficult to manage on a rapidly and continuously rotating planet. Instead, a superior strategy would to be to fire your planet-accelerating thruster continuously, assuming you could gather, control, transport, and convert that energy into usable work.

As Earth rotates on its axis, any force we exerted on the surface would alter our planet’s rotation significantly. There are only two locations that wouldn’t: the north and south poles. Given that the north pole is over the ocean and the south pole is over land, choosing the south pole is a no-brainer decision. (Credit: World Meteorological Organization)

Why the South Pole?

That’s literally the reason why you’d choose the South Pole! Once all the ice melts on Earth’s surface, the continent of Antarctica will be exposed. Although it’s currently beneath a massive sheet of ice, there is a vast mass of land that rises far above the ocean; if we were to remove all of the ice from Antarctica today, the South Pole would sit at approximately 9,000 feet (almost 3,000 meters) above sea level. Install your massive thruster there and fire it continuously, and a tremendous number of positive things begin to happen:

1. The Earth begins to accelerate and will be boosted to a higher orbit.
2. All of the thrust will be utilized; none of it will be wasted countering Earth’s current direction of motion.
3. The Earth will be “lifted” out of the current Earth-sun plane, but only slightly. After 2 billion years of thrust, we’ll then be orbiting only a few degrees out of our current plane.

But most importantly, as we increase our kinetic energy through continued thrusting, it helps dig us out of the Sun’s gravitational potential well. That would take us to a greater orbital distance and enable us to slowly decrease the flux of the solar radiation that strikes our planet.

Today on Earth, ocean water only boils, typically, when lava or some other superheated material enters it. But in the far future, the Sun’s energy will be enough to do it, and on a global scale. (Credit: Jennifer Williams/flickr)

As thousands and millions of years pass, we’ll have to begin contending with continental drift. So long as the thruster gets periodically repositioned so that it stays at the south pole and points directly along Earth’s rotational axis, we won’t have to worry about changing Earth’s axial tilt in a catastrophic fashion. This is a huge concern because the total amount of rotational kinetic energy that our planet has is “only” 2 × 10²⁹ joules, or less than one-millionth of the energy we need to transfer to Earth to boost us to a higher orbit. Only by thrusting in line with our axial rotation will we eliminate the risk of messing our planetary rotation up.

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When you think about it, it really would be the ultimate geoengineering feat. We’re not talking about changing the Earth through chemical or feedback processes, but rather through sheer brute force. Over long timescales, the meteor showers we experience will change, as our changing orbit moves us out of the path of certain long-period objects and into the paths of others. But with the right technological developments and investment of resources, we could achieve our ultimate goal of decreasing the amount of solar radiation that strikes our planet and preventing the oceans from boiling due to our sun’s ever-increasing energy output.

As the Sun becomes a true red giant, the Earth itself may be swallowed or engulfed, but will definitely be roasted as never before. However, if we can migrate Earth away from the sun prior to this, not only could we avoid being consumed, but life on our planet could thrive for billions of additional years than if we simply did nothing. (Credit: Wikimedia Commons/Fsgregs)

It’s important to remember that there are some long-term changes that will happen to our planet regardless of human activity. The sun will burn through its fuel, its core will grow and heat up, and its overall energy output will increase. That, in turn, will increase the amount of radiation reaching Earth. These changes will be extremely slow, but the lifetime of stars like our sun is long: we’re already receiving perhaps ~30% more energy than we were some four billion years ago, and that will continue to increase by about 10% with each subsequent billion years.

We cannot stop our sun from running out of hydrogen fuel and eventually entering the red giant stage of its life, but we could potentially buy a few extra billion years for life on our planet by migrating the Earth away from the sun. It would be the grandest project undertaken in the entire history of our world — perhaps in the entire history of the universe, for all we know. It would truly showcase the power of our species, if we chose to use it. The sun will boil Earth’s oceans and end life on our planet, if we do nothing, in just 1 to 2 billion years. But if we develop and implement the right technology, a South Pole thruster could literally be the one and only thing, after the ice melts, that truly saves our planet.