Moving the Earthby Robert Zubrin

In just 500 years we will have enough power available to be able to undertake this project, with only one percent of our total power generating capacity needed to support the job. 
But what can we do? One suggestion might be to modify the Sun, to keep it from heating up. But no one has any idea of how to do that. Fortunately, there is an alternative plan which should be much more practical to implement; move the Earth. Our home planet, is after all, only about 1 millionth the mass of the Sun, much cooler, much closer, and thus, overall, much more readily available for manipulation. Furthermore, since solar heating falls as the square of the distance, to cope with a ten percent solar flux increase, we only have to increase the distance of the Earth by five percent. This will make things much easier.
So let’s see what it would take to move the Earth outward from the Sun by five percent over the next billion years, thereby compensating for increased solar heating. A little bit of fancy math shows that to do this, a velocity change of 1,200 meters per second will need to be imparted to our home planet. That works out to an acceleration rate of 1.2 microns per second per year, or 3.8 x 10^{14} m/s^{2}.
Now the mass of the Earth is 5.97 x10^{24} kilograms. So, force equals mass times acceleration, to get the thrust required to accelerate the Earth at the required rate, we just multiply the above two figures together and obtain a thrust of 2.27 x 10^{11} N, or 227 billion newtons. That’s really not that much, when you think about it: it’s the weight of a cube of water 284 meters on a side.
So what kind of rocket could be used to generate that amount of thrust? A Saturn V had a first stage thrust of 33.4 million newtons, so thrusting together, 6,796 of them could do the job. Making that many rockets should not be a problem: the Germans produced more than 4,500 V2’s during 1944 alone. Unfortunately, however, it’s not so simple. Because the average exhaust velocity of a Saturn V first stage is only about 3,000 meters per second, to generate a velocity change of 1,200 meters per second would require using about a third of the mass of the Earth as propellant—and that’s just for the first billion years of operation! Clearly we need to use a rocket with a higher exhaust velocity.
So let’s consider electric propulsion, which could readily provide an exhaust velocity of 60,000 meters per second. That would reduce the mass requirement for propellant twentyfold, meaning we would only need to sacrifice about two percent of the mass of the Earth every billion years, an amount that most people would hardly notice. With such an exhaust velocity, the propellant mass flow required to feed the rocket system would only be about 3,780 metric tons per second, equivalent to a modest river 120 meters wide by 30 meters deep, following along at a leisurely 1 meter per second. To be certain, the power requirement would be sizable: about 13,620 terawatts, which is to say about 800 times the current power production of the human race. This may sound like a lot, but if we consider that human power production has increased by a factor of ten over the past hundred years, we can see that at our current rate of growth, in just 500 years we will have enough power available to be able to undertake this project, with only one percent of our total power generating capacity needed to support the job.
So how might we engineer this? After all, the Earth in spinning, so that if the rocket were just put in one location it would only occasionally point in the right direction. (NASA will eventually discover this to be a serious problem for their planned Asteroid Redirect Mission, which hopes to use an electric propulsion system to tow a 500ton nodoubttumbling near Earth asteroid to lunar orbit. But I digress.) So I suggest that we use twelve rockets, and put them in geosynchronous orbit, spaced 30 degrees apart, like the numbers on the clock, around the Earth. Each of these rockets would be connected to the Earth by a super strong and heatproof tether, and only fired for a space of time that most of its thrust vector would be in the desired direction. In addition to transferring the thrust from the tow rockets to the Earth, the tethers could also act as skyhook cable systems, facilitating the transport of propellant from the Earth’s surface up to the tow rockets. The materials to create such cables do not currently exist, but believers in nanotechnology assure us that they will, in the relatively near future, and we don’t need to start operations for another 500 years. Thus, there’s really there is not much technical risk involved in the design.
If we wanted to eliminate the problems associated with tethers, upward propellant transport, and geosynchronous rockets, we could put the system on the Moon, which is gravitationally bound to the Earth. We would need to increase the exhaust velocity by another factor of ten, in that case, since otherwise we would use up the mass of the Moon as propellant. This would also require increasing the system power a hundredfold; ten times to maintain thrust at the higher exhaust velocity, and another ten because the Moon would only sitting off be in the correct direction from the Earth for thrusting about onetenth of the time. As a result the start of the project would be delayed another 200 years to provide an adequate power budget, but, given how development schedules work in the space program, we are likely to have the time, regardless.
Of course, we could make things even simpler by using a photon rocket, which has an exhaust velocity of 300,000,000 meters per second. In that case, we could put the population system on the surface of the Earth, and just shine the light upward in the direction opposite to the intended acceleration. This would eliminate the need for any propellant, or orbital propulsion systems, but as a result of the increased exhaust velocity, our power requirement would increase 5,000 times over that of our orbital electric propulsion system baseline. Instead of 13,620 terawatts, we would need 68 million terawatts. But in the year 2914, such power capacity should be well inhand, and for the sake of convenience, spending a little extra on electricity may well prove to be the preferred option.
So take that, ETs! If you want to save your planets, you’ll just have to show yourselves. 
Now, as original as this discussion may seem, it has undoubtedly been entertained before. There are hundreds of millions of habitable planets in our galaxy alone, and the residents of nearly all of them are facing this very same problem. The laws of the universe are the same everywhere. As above, so below. If we are going to need to do this someday, many others elsewhere are probably already doing it now. Might it be possible for us to spot them? What would a 68millionterawatt rocket exhaust look like, if pointed directly at us, by people trying to save their planet located in a star system many light years away?
The power output of our Sun is about 3.85 x 10^{14} terawatts, or about 5.7 million times the power of our planetmoving photon rocket. Of course, the rocket will be focused to point just in one direction, so if we assume a gain of 1,000 in apparent power by such focusing, the photon rocket would be 1/5,700 times as bright as the Sun. That’s about a difference of nine stellar magnitudes. Now, if seen from 10 light years away, the Sun would be about a 2nd magnitude star, so our planet rocket would be 11th magnitude, and readily visible using a good amateur telescope. But there are only a few stellar systems within 10 light years, so we would have to be really lucky to spot one so close.
However, there are over 12,000 stellar systems within 100 light years. That would drop the apparent magnitude of the planetmoving rocket to 16th, about the brightness of Pluto’s moon Charon as seen from the Earth. While beyond the capability of all but the most dedicated and wellequipped amateurs, there are many professionalgrade telescopes that could spot such an object. Of course, this rocket flare would be positioned close to a star, which would make it harder to spot, but it would still be thousands of times brighter than an Earthlike planet or even a Jupiterlike planet as seen from interstellar distances, so if we can spot one of those, we should be able to spot one of these. The trick, however, will be to catch it when it is pointing at us, which will only be for a brief period of time during each orbit, after which we will have to wait a whole planetary year to catch it again and prove reproducibility of the event. But with enough time and effort, it should be possible.
So take that, ETs! If you want to save your planets, you’ll just have to show yourselves.