# Is it worth colonizing a planet that travels near the speed of light?

There is a planet that is orbiting around not a single star, but the core of a galaxy. It is independent of the other solar systems in the galaxy, but is affected by the center of mass of the core. It is orbiting very near the speed of light. It does have a clean orbit, meaning, it does not collide with any other planets or stars in the galaxy while it’s orbiting around the core.

It is very difficult to land a spacecraft on this planet as only a few civilizations in the galaxy can approach a significant fraction of the speed of light, therefore only the most advanced ones can come close to the planet and land on it.

The question is: would it be worth it to colonize this planet for any reason? Politically, economically or from a military standpoint? Or use it as a treasury, or anything? Would we have any use for it? Would it be worth any investment?

(The technology to live in very cold places, far from stars is already developed and established, so even if this planet does not have an atmosphere and very cold, cities could be build on it which can support life with existing technology)

• From the perspective of a very distant place, at the edge of the observable universe, our system is travelling at near the speed of light... – Serban Tanasa Jun 21 '16 at 21:33
• To orbit the galactic center at close to the speed of light, it would have to be really, really close to the central supermassive galactic black hole. – Serban Tanasa Jun 21 '16 at 21:34
• Adding to what @SerbanTanasa said, you're going to have to worry about the Roche limit. For the Milky Way's central black hole, a planet at the Roche limit (assuming Earth-like density) will only be going 0.06c. – Rob Watts Jun 21 '16 at 22:53
• The statement "orbiting very near the speed of light", can ONLY hold true just outside of the Event Horizon. That is one definition of the Event Horizon, the distance where the orbital velocity = c. In which case, the orbit is anything BUT clean. – Aron Jun 22 '16 at 6:39
• @Aron actually Michael Kjörling is right. The region where the orbital velocity is the speed of light is called the photon sphere, and for a Scwarzchild black hole (i.e. non-charged, non-rotating), it is 1.5 times the radius of the event horizon. – Nathaniel Jun 22 '16 at 12:19

My gut instinct is "no: don't colonize." The biggest problem I see with this planet isn't the cold: it's time dilation.

Politically, most civilizations can't even send diplomats there - and with time dilation, I can't imagine it would be a "key player" in intergalactic politics.

Economically relies a lot on how the economy in the rest of the galaxy works. With that said, even if there are a lot of key resources on the planet itself, exporting them would have to outweigh the costs of the ships required to get there in the first place. Furthermore, that time dilation rears its ugly head again: time is money! (Setting up an interest-earning account, going to this fast planet, then returning to collect your earnings may or may not be a viable solution, depending on banks.)

Militarily is a little more interesting. It's moving fast enough that I don't think it would be a key defensive or offensive position. It'd be hard to invade due to its speed... but all someone has to do is move something massive into its orbit and wait to effectively eliminate it. (Launching a kinetic weapon from it might be useful, if you just so happen to be going by someplace interesting at just the right time.)

The only positive feature I can see would be to someone wishing to "time travel to the future" - and even then, it only works if they'll end up close to where they want to be when they want to leave.

Once all that is said and done, however, it might be of interest to scientists who wish to perform experiments - either observing very long-term experiments off-world, or perhaps some relativistic experiments on-world.

• Honestly, even the benefits of time dilation wouldn't necessarily be enough to colonize the planet, since a fast enough ship would give the same effect. And you would have to have one if you were going to get to the planet anyway. Depending on the experiment the same applies for relativistic research as well. Unless you need a very large facility that wouldn't fit on a ship I just don't see it being viable or worth it – D.Spetz Jun 22 '16 at 13:55
• @D.Spetz Without knowing more about the technology in the setting, it is possible you're correct - or it's possible that it is expensive (in terms of fuel/power?) to maintain a ship at such speeds for long periods of time. If the latter, it's "cheaper" to temporarily get a ship to that speed, land, and then proceed with experiments. (If fuel/energy is cheap, then the spaceship is almost certainly the better choice.) – Ghotir Jun 22 '16 at 14:00
• Actually time dialation may not be huge. You can get rather close to the speed of light before time dialation becomes a problem. – PyRulez Jun 22 '16 at 14:11
• @PyRulez Ironically, when getting my degree in physics, one of my final papers was on the visual effects of near light-speed travel - and "rather close" is almost an understatement. Even 0.9c isn't as entertaining as you might think - it isn't until 0.99999c that things start to get truly alien. – Ghotir Jun 22 '16 at 14:55
• @Ghotir I remember once playing a little (Java?) game that illustrates some of the visual effects (though not to scale). Everything was a lot less weird than I expected, right up until everything kinda disappeared. Random side note: just the other day I discovered that that game was created by the father of one of my favorite youtube guys. – Deolater Jun 22 '16 at 15:10

Well, not colonization, but certainly a research station. There is exactly one location in the galaxy which fits your description - close orbit around the super black hole (~4 million solar mass) which inhabits the center of the galaxy. For a Schwarzchild geometry, there exists an unstable orbit at twice the Schwarzchild radius, with an orbital velocity of 0.7 c. The closest stable orbit occurs at about 3 times the radius, but the orbital velocity is a good deal less.

Of course, construction will be a problem, since tidal forces will run several thousand g's per meter of radial distance, so some sort of gravitic control on the part of the advanced races will be a must.

And let's not forget the radiation problem. This close to the black hole, infalling radiation and matter will have achieved very respectable energies.

• Can you expand on what is the time dilation at that close distance? And what is the reference frame for the speeds of orbital velocity? – JDługosz Jun 22 '16 at 14:46
• I'd also add: What would happen if you tried to hit a baseball pitched at 90% the speed of light? You'd have to worry about stray particles and space dust setting off massive explosions. At Newtonian speeds, a 60 to 190 meter asteroid exploded with 10-30 Megatons of force of, flattening over 2,000 square kilometers. At the relativistic speeds in this question, a planet-killer asteroid would be a pebble. – HopelessN00b Jun 22 '16 at 20:24
• @HopelessN00b - Not quite. At 0.9 c a 1 kg mass has energy equivalent to about 100 kT. Nothing to sneeze at, but not a planet-killer, either. – WhatRoughBeast Jun 22 '16 at 20:33
• @WhatRoughBeast I was thinking about higher speeds, per the graph here (doesn't really seem "worth" the effort for such a small time dilation effect unless you add a few nines to the end of that), and accounting for the fact that when you smash particles at those speeds, you get a lot of extra energy released from the fact that you're either causing fusion, or breaking apart the subatomic bonds that hold the atomic nucleus together. – HopelessN00b Jun 22 '16 at 20:44
• My gut feeling is that if you can produce artificial gravity good enough to counteract gravity next to a black hole you have mastered time travel, wormholes etc. en passant already. – Peter - Reinstate Monica Jun 23 '16 at 15:11

While the amount of time dilation isn't as extreme as something going at .9 c, it is still enough to make some significant difference between the planet and objects in the rest of the galaxy.

Since time is money, this could be exploited for various economic effects. The simple example of a person collecting compound interest in the outside galaxy while residing on the planet shows some of what is possible, and eventually many more subtle and complex financial instruments could be devised based on the time differential between the planet and the remainder of the galaxy.

The other effect which comes to mind is the use of the planet for long term storage. Items which have limited lifespans but command a high value could be stored on the planet and have their lifespans artificially extended due to the time dilation effect. Of course, don't expect this to allow you to store a bottle of wine for millennia, the time dilation effect is not all that great at .6 c

Realistically, if there is a desire to use time dilation for political, economic, social or military ends, it would make more sense to either build some sort of construct near the event horizon of a black hole or travel at relativistic speeds in order to take advantage of the Lorentz factor.

• This just makes me think of the ending of Forever War. Without spoiling things, a group travels back and forth in a ship going some fraction of c to make use of the time dilation effect. Which seems much more reasonable then using an entire planet with all of the build costs and engineering necessary – D.Spetz Jun 22 '16 at 13:58
• +1 for talking about Lorentz factor. Depending his definition of very near the speed of light Makes a huge difference. There is a good difference between 0.99 and 0.9999. – Timmy Jun 22 '16 at 18:31

You're going to need to give it handwavium shields.

lets assume 0.9 c as it's speed. Lets assume that it's path is as clean as interstellar space without any gravel or asteroids. The only thing it hits is the fine mist of atoms between the stars.

Lets treat the planet as about the size of earth.

Volume: 1.08321×10^12 km3

mass: 5.972 × 10^24 kg

Surface area: 510.1 trillion m²

Approximate circumference: 40075 km

approximate cross section: 1.28×10^8 km²

using the figures for a cold neutral interstellar medium from wikipedia: 20—50 atoms/cm3

So let's go with 25 atoms/cm3

25000000 atoms per cubic meter.

We can treat the volume of space that the planet passes through as a cylinder with a cross section of 1.28×10^8 km²

Now lets look at how much it hits while traveling, say, 10 light years from an outside reference frame (I think about 4 years planet-time).

I'm going to ignore time dilation because it's hard and I need to maintain my sanity.

Treat it as a cylinder 10 light years long with the diameter of the planet.

This lets us estimate the total number of (almost all hydrogen) atoms in the path of the planet, lets assume they all hit and there's no shockwave effects:

37984965888934182667500000000000/pi m^3 (cubic meters)

so over the course of 10 light years it will impact with 505924000 metric tons of gas atoms.

How much energy will they be carrying?

505924000 metric tons at .9c carry 5.885×10^28 J (joules)

This energy will need to be radiated away by the surface. To do it accurately I would have to take into account time dilation but to err in favor of the planet and to keep myself sane I'm going to ignore it.

Surface area is 510.1 trillion m²

The planet needs to radiate away 6.7180365296 × 10^23 Joules per hour.

Dividing by the surface area each square meter needs to radiate

365.8 kW per meter square

For comparison the sun radiates ~17.53 kW per square meter of it's (approximate) surface

The surface of this planet is going to be glowing hotter than the surface of the sun

You don't need to survive the cold, you need to survive the heat.

• @RobWatts But look at the bright side, you'll die from the heat way before you'll need to worry about deorbiting. Actually, on second thought, don't look at the bright side; it will blind you. – reirab Jun 22 '16 at 20:13
• I would add earth like planet will evaporate in approximately 30000 years that way, probably even faster (much faster). Conclusion naturally forming planet can't exists that way - so as artificial construction it may have some interesting stuff to investigate. – MolbOrg Jun 22 '16 at 20:15
• Even this is pretty optimistic. At relativistic speeds, you're going to have to worry about fusion and atoms being smashed apart as a result of these collisions. At relativistic speeds fast enough to take advantage of time dilation, every atom the planet collides with is going to cause one or the other... might even end up with a cascade fusion reaction. – HopelessN00b Jun 22 '16 at 20:32
• This is assuming that the planet is the only thing travelling at relativistic speeds, and the surrounding gas is at zero velocity, right? But any nearby gas is going to have to be moving at a similar speed to maintain its own orbit, or else it would have fallen into the black hole already. – Jander Jun 23 '16 at 4:31
• I think the point is that ANY celestial body traveling near-C in a stable orbit will have been doing so for a VERY long time, and will already have smashed into innumerable planets, asteroids, moons, gas clouds, dust clouds, maybe even supernova ejecta, right up until it achieved a mass so great that it somehow cleared out an entire orbit around a galaxy. The end result, however it was created, would almost certainly end up becoming a black hole itself and then merging, eventually, with the supermassive black hole at the center of the galaxy. – Drunken Code Monkey Jun 23 '16 at 6:23

As a few of the others have said, due to time dilation, this planet wouldn't be very useful as a colony. Between the speed and the proximity to the SMBH, it would probably be one of those situations where every hour that you spend on the planet, a hundred years go by in the rest of the galaxy*.

But there is one thing that it could be used for, and that's long term storage, especially of data.

Scientists have looked into all kinds of long term data storage methods, trying to find something that could last 1000 years or longer.
But by using the time dilation you no longer have to worry about that at all. Send a library probe to the planet, and 20,000 years is only a few days on the planet.

Updating and retrieving information would be slow, but possible. You simply fly your ship near by and use a laser to make your data request. The library receives the request, the super computers do the lookup and 3 ms later it uses its own communication laser to send the information back.
Back on the ship it's been 3 months, and the reply comes back with the recipe for how to make yum yum sauce that had been lost in the last galactic collapse.

* This is only an example.

• Looking at two other answers, posted earlier than yours, that contain real information, you can see that the time dialation of the closest possible planet to the supermassive black hole in our galaxy is... not 100 years to the hour. Try 1.4 hours to the hour. – JDługosz Jun 22 '16 at 11:23
• If it takes 40000 years to get to the planet (at the speed of light), worrying about storage for 20000 years is not a useful thing. – JDługosz Jun 22 '16 at 11:25
• @JDługosz actually it had nothing to do with the time dilation due to speed, but dilation from gravity. 100y/h is probably high, but it really depends on the mass of the BH. This answer has a lot of relevance to the question. Also, I don't see anything in the question about it taking 40000 years to get there. Stop making stuff up and then complaining about my numbers. I did have a disclaimer for my made up number. – AndyD273 Jun 22 '16 at 12:29
• I'm not saying anything about time dilation due to speed. The BH is in the center of the galaxy. The galaxy is like 100000 light years in diameter. If we had warp drive the question would be moot. So, any of the galactic civilizations is making jaunts on a time scale of tens of thousands of years, just to get from place to place. Follow? From our (typical) position in the galactic suburbs, it would take around 40000 years one-way, using relativistic ships or "beaming". – JDługosz Jun 22 '16 at 12:37
• Ah, right: the speed of the orbit vs. simply being so close to the blackmhole. The chart I read it from above only considered the effect due to speed. The distance of the closest orbit is noted at 2 times the radius, but didn't state the dilation factor. Or, what reference frame the 0.7c is stated in! – JDługosz Jun 22 '16 at 14:45

It occurs to me to wonder how large the radius of the orbit is, and what the centripetal force would have to be to remain in orbit, and what the mass of the primary would have to be to keep an object in orbit at such a speed.

If the radius of the orbit is the same as Earth's distance from the sun and the orbital velocity is 0.8c, then using classical mechanics -- ignoring any relativistic effects --, v^2=GM/r, I calculate the mass of the primary at 1.3e38 kg, or like 100 million times the mass of our sun. The centripetal acceleration, v^2/r, would be 384,000 m/sec^2. Earth's centripetal acceleration is about 6.3 m/sec^2. The force to keep this planet in such an orbit would be huge. I seriously question if it wouldn't be torn apart.

But for the sake of the story let's assume it's possible.

What would be the advantages of colonizing such a planet? Assuming near-light-speed travel is not routine in this society, it might be difficult and expensive to reach. That could make it a good place to store something valuable, the secret plans to the Death Star or whatever. But if you have the technology to reach the planet, presumably you have the technology to launch a spaceship travelling at near-c, in which case you don't need the planet for your high-speed storage. That is, any benefit gained by the planet travelling at such high speed, to get there you have to have a spaceship that can travel that speed. Why not just do whatever on the spaceship, and why do you need the planet? Unless it's something on such a huge scale that you need a planet for it.

Time dilation would mean that time on this planet would pass very slowly relative to the rest of the galaxy. Good if you want to see the future or leave some message for posterity. Maybe a place to keep an archive that will survive for thousands of years of outside time. (But again, why not just put it on a ship?) Not so good if you're worried about keeping up with technological advances. A military base on this planet would be obsolete very quickly. It would also be difficult to keep up with news from outside, as it's happening so fast relative to you. So as a military or political center, it would be a terrible choice.

Might be nice for research on relativity.

Besides that, I'm hard pressed to come up with any advantage.

I suppose the planet might have some valuable resource that has nothing to do with its unusual orbital speed. But that begs the question.

• You can't ignore relativistic effects here. WhatRoughBeast already answered with real details about orbits around a lack hole, which BTW Newton's laws og gravity can't handle. "The force to keep it in orbit is huge" yes, gravity! The orbit matches the attraction to the center mass!! Tearing apart is from tidal effects not centripital force, and is called the Roche Limit. – JDługosz Jun 22 '16 at 11:17
• @JDługosz I didn't intend to say that I thought it was valid to ignore relativistic effects, rather that I hadn't taken the time to figure out just what their significance would be, so I was doing a "rough draft" of the calculation without them. Yes, of course the force that keeps the planet in orbit is gravity. If that needed saying, then cool, you just said it. I'd think the force problem would include but go beyond tidal effects: you have this huge force to overcome inertia. – Jay Jun 22 '16 at 13:40
• @Jay A system in a uniform gravity field is indistinguishable from a system in an inertial reference frame. It's "only" tidal force you have to worry about. – Taemyr Jun 23 '16 at 3:23
• @Taemyr Ok, I was thinking you could detect the force, but no, you can't. Brain freeze. Not sure that tidal effects are the only experiment you could do -- couldn't you detect the difference in orbital velocity from the dark side of the planet vs the light side, e.g.? maybe there's a catch to that. Whatever. – Jay Jun 24 '16 at 18:21

Assuming that such a planet is stable, and assuming you're limited to relativistic travel in your universe, this planet might be extremely valuable as a way station.

To get from one planet to another quickly you have to expend a huge amount of energy getting up to near lightspeed, then expend that same amount of energy again to slow down to synchronize with your destination's orbit.

But this planet is already moving at near lightspeed. So with careful planning to meet up with the planet as it matches the direction of your travel, you could make a pit stop for refuelling, trading, etc. without incurring the energy cost of slowing down.

[EDIT] ...except that I didn't consider that the gravitational attraction of the black hole is going to be accelerating the ship as it comes in. That could be enough to invalidate my whole answer.

That planet could be the galactic Panama Channel. If you want to go from one side of the galaxy to the other, and you either don't want nor can't use shortcus (wormholes), then you have two options: hop from arm to arm, or go through the core. The planet could then also harbor a transportation hub, where ships can ressuply, or where you could unboard one shuttle and board another one.

It could be neutral ground for negotiations. If it's hard to get there due to the delta-V you need to get there (due to its orbital speed), then it should be very costly, probably prohibitively costly to send a large military fleet there. Only relatively "light" crafts may be able to get there, so even if space fighter craft can get there, the carriers, motherships, frigates, destroyers or whatever you'd like to call the bigger ones can't get close.

It could thrive on tourism. Think of the Everest in our own world: people want to reach it its top just because it is hard and it is there, and this keeps an economy going at the base of the mountain. The top of the galactic everest is the very core of the galaxy, with your planet being the place where the galactic sherpas live.

It could be a tax haven. If it's hard to get there, it is implied that it's hard for auditors to get there.

It could be a pirate hideout, just like some islands in the caribbean during the 1700's. Okay, this is not proper colonization, but the pirate population could grow enough to become a society on their own.

And since not everyone has the technology to get there, then some race or civilization who can get there may monopolize transportation to and from that planet, and make a hell lot of money.

Your advanced race could colonize it because of prestige ("the coolness factor" / "because it's there" / "because we can"). Prestige in the difficulty of not only getting onto the planet, but hardening the colony against the radiation, tidal forces, and all the other cons mentioned in other answers. Sort of a galactic "my tech capability is bigger than your tech capability" contest. Similar to the Cold War era "space race" here on earth.

Not sure if that would be very advanced, or very immature, though... but hey, you might just have the need for such an antagonistic race in your story.

A better military option. Storage of troops. Assuming a great enough time dilation, you can land 1 million troops and 1 million support personnel every year. Spend a fraction to support them (only a short time has passed for them) and in a hundred years launch a 100 million person armada. You would need to refit with modern tech, or hope technology did not drastically change.

• 100 milion soldiers armada can beat almost anything even fighting with their hats only... :) – Crowley Jun 23 '16 at 8:54

Your orbit doesn't correspond to any known theory of gravity.

If it's travelling extremely fast, but isn't orbiting any particular object (like a supermassive blackhole), then it will very quickly exit the galactic core.

I'm pretty sure that matter going that fast is extremely unstable. This planet might be glowing hot just from its own movement. I also think it would be shaped like a bent spear just from pure force, being slung around in very small orbits very VERY fast.

Landing would indeed be difficult, because contact with matter going this fast is going to give a hell of a recoil. Your ship needs to be able to take a punch, because this planet has an enormous mass (because it's a planet) and an even bigger force (due to going near the speed of light). If you can't keep up it would be the equivalent of an unstoppable force hitting a fly.

Then there's part three, the difference in time. Time, gravity and speed are very closely related forces, as far as more sciencey people tell me. Even if the landing succeeds and the colonizing mission only takes one day on that planet, when the crew returns home they might discover that two centuries have passed at a safe distance from this black hole.

Is this colonizing mission worth it? I dunno. But something on that superfast planet should be worth a lot if it requires this much effort, because it sort of stretches the "doing it because it can be done" mentality.

• "matter going that fast is extremely unstable. This planet might be glowing hot just from its own movement." huh?! "Bigger force..." what?? Speed is a force? Hmm, maybe there's your problem... – JDługosz Jun 22 '16 at 1:24
• The speed as a force is not a valuable resource if you need to be able to get to that speed it to colonize it in the first place. – Notthegrouch Jun 22 '16 at 7:53
• I do not understand what you said there, @notthegrouch. – JDługosz Jun 22 '16 at 11:00
• I also might've misunderstood what you said, because you just commented with quotes and snarkiness. Not real helpfull comments. – Notthegrouch Jun 22 '16 at 12:25
• I'm not good at snarkiness. "Speed is a force" is not a correct thing. I speculated that this initial mistake is the source of other statements that don't make sense or are wrong. That might be actually correct in a sense: you are not using correct terminology so someone who knows what the words really mean in physics will be very confused. – JDługosz Jun 22 '16 at 12:29

The reasons why to not to do it mention radiation, time dilatation, nonexistence of such planet etc.

Even if such stable planet exists and our civilisation can deal with the radiation and forces in such system there is one point - you have to catch the planet to land on it.

Using Newton's law $F=\frac{dp}{dt}=\dot p$ and relativistic definition $p=\frac{m_0v}{\sqrt{1-\frac{v^2}{c^2}}}$ we can estimate the thrust of the spacecraft to accelerate to the desired speed.

$$F=\dot p=\frac{m_0+\sqrt{1-\frac{v^2}{c^2}}}{1-\frac{v^2}{c^2}}\cdot\dot v$$

It's easy to see that for constant acceleration $\dot v$ the thrust force $F$ diverges to $+\infty$ and for given thrust force the acceleration goes to $0$ when $v\rightarrow c$.

For $v=0.9c$: $F=\dot v\left(5.26m_0+2.3\right)$
for $v=0.99c$: $F=\dot v\left(50.25m_0+7.08\right)$

Assuming there are no in-universe technological reasons to not colonize it, I see the Time Dilation factor as highly useful from an archival standpoint. As humans with only one colonized planet, we have already seen the need to preserve history and things like seeds in vaults that are to be used in the case of disaster, why would a highly intelligent race not do the same things with their history, seeds, and other vital records? Throw those items onto a planet where time is passing more slowly, and you have a preserved, living record. This helps avoid the "Dark Age of Technology" problem in failing civilizations.

• Because space dust will obliterate it given the speed difference and the records will fly off, never to be seen again. It's very hard to get stuff there and harder to return. – Donald Hobson Jul 5 '16 at 22:34
• @DonaldHobson if the planet still exists in any form, you can assume there's sufficient gravity and other forces to prevent things from flying off it and something keeping the dust from obliterating it. Otherwise it wouldn't exist. – Marshall Tigerus Jul 6 '16 at 13:18

Militarily, I could see one use for such a planet. Assuming that your civilization is advanced enough to land on it, they are advanced enough to move a galactic body close enough to gravitationally nudge the planet out of orbit. With nothing to slow it down, the planet would be a near light speed rocket which would destroy most anything it hit. Such a civilization would likely be able to make the calculations necessary to direct it to where they want it to go.

• Go where? If it is lifted out of orbit it loses that kenetic energy. – JDługosz Jun 22 '16 at 1:25
• I would disagree. Energy wouldn't be destroyed. Kinetic energy is lost by objects on earth because they are acted upon by friction which isn't nearly as relevant in space. – HighSage Jun 22 '16 at 1:53
• The speed is due to orbital velocity, not because it's heading somewhere. The sum of gravitational potential energy and kenetic energy is constant. If you send it straight away from the BH it will rise until the speed is zero and then fall back in, as that was still not enough to escape. It's physics. I was not talking about friction, but orbital mechanics. – JDługosz Jun 22 '16 at 2:14