Deflecting an incoming star, railgun style

Question

A friend of mine, ɹɐqooɟ, lives in a large globular cluster where stellar encounters and collisions are very common. His civilization is a Type 1.8 civilization, having built half of a Dyson Sphere around their star.

Astronomers on his planet (a super-Earth of $7M_\oplus$ orbiting a F5V star) have discovered a red dwarf star that will pass less than $0.8\text{AU}$ from their planet in $6733 \pm 50$ years. This will cause their planet to be ejected out of their solar system, drastically altering the lives of the inhabitants of the planet.

As ɹɐqooɟ's civilization has not yet developed stellar engines, they have decided on launching some massive object (not a black hole, they haven't found a way to create micro-black holes that can be stabilized yet) at $70\%$ the speed of light into the red dwarf star, which is $1.050510$ light years away. The impact needs to either destroy the star or redirect it enough so that it will completely miss their star system, causing minimal damage.

What should they do? (Please answer before 3/7/6600 at 16:42 UTC-6)

Answer 1

You've got this the wrong way around.

You're trying to deflect an entire star, which engineers refer to as very very heavy.

You're doing this to save a planet, which those engineers might refer to as merely very heavy.

Any technology capable of measurably changing the trajectory of an entire star would be better put towards adjusting your home planet's own orbit, current or future, to ensure that it remains in the neighbourhood that the current occupants like.

If you don't have the technology to change your planet's orbit, you don't have a whelk's chance in a supernova of deflecting a star.

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Lets have a slightly less sarcastic look at an existing proposal for moving stars around, though.

A Shkadov Thruster needs similar construction techniques to a Dyson swarm, but basically involves making one giant statite and using it as a solar sail, bound to the star by its own gravity. Upside: your friend might be able to build it, assuming that

• they can get to the problem star in good time
• there's enough mass available at the destination star
• they can get building pretty swiftly

Unfortunately, the thrust of one of these things is rather low, defined by

$$F_A = \frac{L_s}{2c} (1 - \cos{\Psi})$$

where $L_s$ is the luminosity of the star and $\Psi$ is the mirror rim angle. If we model our star on Barnard's star we have a luminosity of ~1.22x10²⁴W, and if we make the statite a nice hemisphere we have a rim angle of 90° and a thrust of ~2.05x10¹⁵N, giving a maximum lateral velocity change of about 1.4mm per second after six thousand years of operation.

You should probably start work at least a million years ago, preferably last galactic year.

I am now wondering if you could speed things up using your own Nicoll-Dyson laser to warm up the target star a little...

If you reconfigured your own dyson elements to make a Shkadov thruster around the much hotter F5V star you'd get 3 orders of magnitude more thrust for only 1 order of magnitude increase in mass... that's a whole 28cm per second velocity after 6000 years! It isn't enough to get you out of the way in time, as you're still only going to have moved less than a quarter of an AU by the time the other star arrives, and it means you can't use your dyson elements to harvest power for other mitigation techniques.

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As a slightly less implausible solution, PcMan suggested a good way to move a planet is with the use of a gravity tug, which basically involves bringing a large object close to the homeworld in question and using rockets to stop it crashing into the planet (and a clever orbit to stop the rocket plumes impinging on the planet's surface or atmosphere). Their mutual gravitational attraction will cause the planet to change orbit over time.

(PcMan doesn't seem to be writing an answer of their own on this, so I'll tack my take on it down here, but I'll pop it off again if they change their mind)

An acceleration of 1m/s/yr will add up to a good 6km/s over time, which should help push it into a position where its year length and location when the star flies by is, if not necessary safe, might be safer. Moreover, it might give a way to correct a surprise escape trajectory into something more comet like, given the occupants time to sort out a more permanent solution.

To get this acceleration on a planet of mass $M$ ~4.18x10²⁵kg, we will need a force $F$ of ~1.32x10¹⁸N. If the tug is placed at radius $R$ 15000km from the planet's barycenter, it needs to mass $\frac{FR^2}{GM}$ ~1.07x10¹⁷kg. That's in the same ballpark as real-life moon of Saturn, Hyperion. This is a potatoid rock about 360x260x160km in size. Hyperion is a bit bigger than we need by a factor of about 4, so you can slice it up and use the leftovers for reaction mass.

Gravity will pull your tug towards the planet with the same force as it is tugging the planet. You will therefore need an engine capable of exerting similar force to keep your tug in space. Having ownership of a partial dyson sphere will give you all the energy you need (50% of the output of an F5V is going to be of the order of 1.4x10²⁷W, which is quite a lot. Even if only 1% of that was captured, you're still doing well) and you should have no problem powering a suitable mass driver firing ground-up moon to keep your asteroid in place.

The biggest problem you'll have in the future is the fate of your dyson components. They're very valuable, and very hard to replace, as they'll represent a non-trivial proportion of the easily available mass in your planetary system, and the rest of the mass is going to be flung into deep space when your visitor arrives. Moving them into safer locations inevitably entails reducing the power supply available to your tug. You might need to build an absolutely vast fusion plant on it, and grind up a few ice moons to feed it for the final stages of the visitation...

Answer 2

TL;DR don't move the red dwarf - move your planet.

During those six thousand years, adjust the planet's orbit so that, during the encounter - which will last no more than about two months - the planet is on the opposite side of its primary, farthest from the incoming star. Then, prepare and deploy appropriate measures - for example, massive asteroids and moons in slingshot trajectories - so that the newcomer's attraction's effects are neutralized overall.

(That, of course, assuming the stars aren't gonna smash together).

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Previous answer

You don't have very many workable options here. Nothing available to a K1.8 civilization will be able to destroy a star, and "redirecting" it requires either kinetic energy or momentum in quantities, again, not directly available.

Accelerating a massive object 70% of the speed of light is not enough to influence a star unless the object is many times a planet's mass -- but, then, you'd have no trouble in relocating your own planet at a much more leisurely pace.

It might be possible to alter preemptively your planet's orbit so that the passing of the star will either not influence it at all, or influence it so as to put it back in its original orbit. This, plus the ability of partially directing the motion of a puny planet for the period of the closest encounter, might be enough.

You could also deploy the "massive object" in such a way as to alter your own star's motion - something of the kind appears in Asimov's Nemesis. But the target star is too close for that - there's no time.

This could be supplemented by the alternative plan of building a lensing statite (or Bowl of Heaven after Benford's idea) around the incoming star. And maybe also your star.

The statite will reflect part of the solar radiation at a single target into the star, causing a controlled flare. This flare will be magnetically focused to impart thrust to the parent star, curving its orbit. Magnetics do not effect work, so you can theoretically focus several hundred thousands tons of matter per second at stellar wind velocities with little trouble once the necessary megacoils have been built. That effectively turns the star into a stellar engine.

The thrust, however, is not enough to work in the available timeframe.

Concentrating the sunlight from your star into a beam would supply a comparatively tiny thrust to your star; at that distance, focusing on the target star is unfeasible, so "pushing it away" is again not an option.

Answer 3

I'm going to do some rounding and assumptions. Also usually for orbital mechanics there is a whole lot of mathing for approaches, but for stars approaching each other on an orbit around a galaxy center the approach is practically a straight line.

Red dwarves have masses of up to 0.6 M☉. Let's assume this one is 0.5 M☉. That's 9.945 × 10²⁹ kg.

1.050510 ly is 66,435.38389 AU. Let's round it up to 66,435.8.

66,435.8 - 0.8 means the star will cover 66,435 AU in 6,733 years. That's ~46,800 m/s, slow enough that we can play with this scenario without involving relativity much. That is, the relativistic mass of the star is practically its rest mass.

At that speed, the momentum of the star relative to you is 4.65426 × 10³⁴ kgm/s.

I could stop here, and let you figure out what kinda mass you need to use to zero that out. But I'll give you an example. ɹɐqooɟ's own planet being launched at 0.7c would have a momentum of 1.22822642 × 10³⁴ mkg/s (considering relativity), which is about a quarter the amount of momentum you need to stop the star. So maybe throw a gas giant instead.

Also don't hit the star with a single solid object much smaller than the star itself. At that speed it might penetrate the star and go out the other side. The star would be deformed for a while but would have got back to shape before it hit you. Instead, pulverize your missile so that the shrapnel covers a lot of area. This way you might be able to stop the star.

Answer 4

A few comments:

The question implies that the civilization has just discovered the red dwarf star and is frantically asking for suggestions from far more primitive civilizations.

It also states that the approaching red dwarf star is 1.050510 light years away.

So I have to ask why did it take them so long to discover the position and velocity of the red dwarf star?

This table:

https://en.wikipedia.org/wiki/List_of_nearest_stars_and_brown_dwarfs#Distant_future_and_past_encounters[1]

And this graph:

https://en.wikipedia.org/wiki/List_of_nearest_stars_and_brown_dwarfs#/media/File:Near-stars-past-future-en.svg[2]

Depict past and future close encounters of various stars with our solar system. Note that many of those calculated encounters are many times 6,733 years in the past or future. So why hasn't the much more advanced alien civiliation discovered the approaching star tens of thousands of years ago?

It seems to me that a civilzation that advanced will have no trouble with their planet being ejected from their solar system into outer space.

Many members of their species should live in enclosed space colonies on various hostile worlds or in artifical space habitats. It is quite possible that their communities on their home planet also have their own self contained ecosystems, to avoid interacting with and damaging what is left of their planet's natural ecosystems. If they already live in enclosed space colony like structures on their homeworld, it won't affect them much if their homeworld takes a different orbit.

And to preserve the natural habitat and organisms of their homeworld they can build a giant satellite with fusion powered lights to shine at the surface of their planet to similate the natural light and heat of their star.

And if their space travel technology is advanced enough to consider diverting an incoming star, they should be able to move their entire population off their homeworld to other places in their solar system or in nearby solar systems long before the red dwarf star passes in 6,7333 years. And if their planet is hurled out of the solar system it should remain close enough for easy space travel (using their incredibly advanced technology) to other planets and space habitats in their system for many thousands of years. So people who want to move between the ejected planet and any colonies remaining in the star system will have a lot of time to do so.

And what about their one half Dyson sphere? If it will not be disrupted by the passing star, it will be a good place for any dispaced persons to move to, since i assume that moving much of the population there was the original plan.

Answer 5

If ɹɐqooɟ lives in a dense cluster, this is a problem that will need to be solved periodically. That means there must be a better solution.

Fortunately, if ɹɐqooɟ's civilization has built half a Dyson Sphere, they have built a stellar engine. They just need to deform the hemisphere enough to form an oblate ellipsoid (so most of the reflected energy misses their own star) and jack up the albedo of the interior surface as far as possible (ideally, they'd make it specular rather than diffuse, but making it bright first is more important). Let the hemi-ellipsoid act as a solar sail to support its own weight, and it'll tow the star and at least the inner planets of their system along with it -- allowing them to alter the orbit of their sun to run outside the dense part of their cluster where close stellar encounters are once in a billion years or so, instead of several times in a million.

And yes, this will also work with a Dyson Swarm -- just deploy a solar sail from each particle of the swarm and use those sails to keep all of the particles on one side of the star. It's slower than a solid reflector, but doesn't require dismantling the entire system to get the mass to build with.

Answer 6

I'm putting this in another answer, because it's very much different from my other one.

FRAME CHALLENGE:

In a dense cluster, planets that can develop life must be exceedingly rare -- perhaps nonexistent -- because, as I noted in my other answer, close encounters between stars must be relatively common. Current theory has most stars originating in gas clouds that amount to clusters similar to the Pleiades, and later being ejected from the resulting multi-star system. Globular clusters are far denser than these, and the only thing that could keep them from ejecting all their stars over time is if they formed in an organized state -- which would make them more like the disk of a galaxy than the globular shape we observe (meaning any globular cluster is continuously in process of ejecting its population).

The close encounters that eject stars from clusters need not be close enough to eject planets from their stellar orbits -- but some fraction of them will be, and the denser the cluster, the larger that fraction. The end result is that the habitable lifetime of a planet in a dense cluster will be much shorter than the lifetime of a star, and many of the planets in the cluster will be rogue -- wandering the cluster, waiting to be ejected even from the cluster's gravitational binding by another encounter (which will happen millions of years after the surface is frozen over).

Therefore, a Kardshev 1.8 civilization cannot be native to the cluster, never mind the planet they're on; they had to have colonized in cosmologically and geologically recent time (say, within the past million years), from much further away, and done so without the kind of careful forward checking that ought to be applied to such an endeavor.

That doesn't change the answers on what they need to do, but it might very much change the background of the fictional situation.

Answer 7

Propel the star using fusion rockets made of starstuff.

Your railgun projectiles are made of uranium and plutonium. They are designed to undergo fission when compressed in the outer layers of the target star. These fission reactions will produce heat exceeding that produced by the star as well as a shockwave through the starstuff, compressing the material and accelerating ongoing fusion within it. This accelerated fusion will release orders of magnitude more energy than the fission bomb itself. Many of these bombs arrive and detonate simultaneously and synergistically.

This reaction does not occur in the center of the star but off to one side. The result - an blast of energetic hot plasma from one relatively small area of star. It is a fusion rocket. The rocket you produce will push the rest of the star in the opposite direction.

Answer 8

Why bother trying to save a sinking ship, when you (should) already have lifeboats?

ɹɐqooɟ's civilization is a post-type-I Kardashev, which guarantees they have the ability to colonize other worlds, and indeed should already have done so as part of a rational strategy for minimizing the risk to their species as a whole from a catastrophic interstellar event like the one you're describing (averting "eggs in one basket"). Instead of trying to save their homeworld, they should instead focus entirely on vastly expanding their colonization project, with a view to relocating the entirety of their homeworld's population (including indigenous flora and fauna) to other, safer worlds. 6.7 millennia gives an excellent head-start on this, as well as terraforming efforts for their new worlds.

This may or may not be feasible, depending on how dependant they've become on the energy harvested from their Dyson sphere/swarm, but that will almost certainly be destroyed and/or displaced anyway by the rogue star, so they'll need to make a plan to deal with that loss either way.

I'm assuming of course that ɹɐqooɟ and his people have access to effectively faster-than-light transit. If not, colony/generation ships are probably going to be a safer bet (think Mass Effect's Quarians).

Answer 9

The star will arrive in 6700 years. In that time it needs to be moved perpendicularly by an unknown amount, comparable in size to 0.8 AU. So we need to change its velocity by about 1.1E-4 AU/year. When we take a moment to use factor label notation and check our units like we always should, that works out to:

(0.8 AU / 6700 y)(1 y / 3.15E+7 s)(150E+9 m / 1 AU) = 0.59 m/s

Our impactor has a speed of 70% c = (0.7 * 3.0E+8 m/s) = 2.1E+8 m/s.

Now momentum = relativistic mass * velocity, so we can say we need a mass 3.6E-8 smaller than that of the star. Assuming a red dwarf at, say, 100 Jupiter masses, that means (3.6E-8 kg impactor / kg star)(100 * 1.9E27 kg star) = 6.8E+21 kg. Oh but wait, we need to consider 1/gamma = (1-(0.7)²)^0.5 = 71% ... the actual rest mass of the object is 4.9E+21 kg. You need something smaller than Iapetus, larger than Haumea.

Answer 10

To deflect a star they would have to launch an object, comparable with the star in mass. 70% of the speed of light cannot change the required order of magnitude of mass.

So, it would be much, much easier to change the orbit of their planet the way they would want with such energy.

Answer 11

The civilization can poison the star with matter that squelches fusion.

They wouldn’t fire a single projectile, they’d fire a steady stream of refined metals — iron, titanium, just about anything from the lower half of the periodic table that is plentiful in their solar system will do.

The first projectile will impact the red dwarf thousands of years before it nears their solar system and with a steady firing rate, and in a few hundred years of such abuse the star will become unbalanced in terms of fusion and gravity and go nova — a safe distance from the civilization in question.

Answer 12

In terms of distance, 1 light year is about 65,000 AUs. The mass of the star is about 60% of our current star.

If you wanted to move its path slightly, you would probably want to smash something very large into it, since the momentum of the system (far away from the superplanet) will be the same regardless.

At 70% of the speed of light, there's about a 40% boost to momentum when compared to the classical calculations $\gamma=\frac{1}{\sqrt{1-\frac{v^2}{c^2}}}$.

The star is moving at about 46 km/s. According to this article, a star moving at 106 km per hour could pass at jupiter's radius without much trouble, or about 5 AUs.

You need to change its momentum by 1/13,000 of its current momentum (5/65,000), which corresponds to about 0.6 solar masses * 46 km/s, or 160 km/hour.

The momentum of an object colliding with the star, let's say perpendicular to its path, would have a momentum of about 3 * 10,000 km/s * M, where M is the mass of that object (in AU).

You need that momentum to equal about 1/13,000 times the momentum of the star, or

1/13,000 * 0.6 * 46 = 0.002123 AU * km/s

Now, since we currently have 30,000 km/s, we can solve:

30,000 km/s * M_0 = 0.002123 AU * km/s

M_0 = 7.08e-8 AU = 1.4e23 kg

That's about an object twice the mass of the moon.

The least amount of energy required to accelerate an object to 70% of the speed of light is 5 * 10^39 joules.

The sun generates 3.846 * 10^26 W. Per year, this is 1.212 * 10^34 joules, and over 6500 years, this would be 7.88 * 10^37 joules.

I'll assume the dyson swarm can capture roughly the same output of energy as our sun, with the F5V star being brighter and likely a bit bigger.

You are short the required energy to do this about 60 times, assuming perfect energy conversion.

Possible, more realistic assisting factor

Depending on what neighboring celestial bodies there are, it may be possible to deflect its path more. All you need to do is push is slightly closer to something that has a lot of mass. If, for example, the trajectory it was on relied on it being deflected by a black hole, then moving it slightly closer to that black hole could dramatically change its path, requiring far less energy than would otherwise be needed.