# How to effectively slow down a ship about to shoot through the Solar System at 0.6c?

Suppose that our random heroic brave interstellar expedition is returning to Earth from Alpha Centauri at 60% of the speed of light, but there was an accident in the nuclear reactor, forcing the captain of the ship to jettison the whole propulsion section in order to evade the nuclear explosion. The ship has lost all propulsion power whatsoever and is on a course that will pass straight through the solar system (the ship was supposed to brake during the last stage of the mission) and leave it again, to become eternally stranded in deep space, ultimately leaving our galaxy.

We still have got life support (that will keep functioning for 5 years) and that our current distance to Earth is 2 light years, which means that we are supposed to pass our home planet in 3 years and 4 months if our speed does not change before we will head into space again. The ship has two shuttles which are designed for reentry and landing on Earth.

The signal will take (you guessed it right) 2 years to reach earth, which means that we have 1 year and 4 months until the closest passage. At the time when the signal will reach Earth, the distance between the ship and our planet will be 0.84 ly.

If we take time dilation into account (+25% for the observer at 0.6c, that's why I chose this speed), the Earth has actually 1 year and 8 months until the closest passage.

As said before, if no change in velocity happens, we will shoot out of the solar system again at 0.6c with a minimal course correction induced by solar gravity.

My question is:

• How can the ship (or at least the crew) be saved within a reasonable time period (there are 3 years of life support left)?
• By "saved" I mean that the ship must be slowed down into a solar orbit where it can be accessed by rescue vessels.
• Bonus points for making it head towards earth or into an earth orbit so that the crew will only have to board the shuttles.
• No propulsion whatsoever may come from the ship itself unless you decide that sending a new nuclear reactor and propulsion module for rendezvous at relativistic speeds.
• You can use anything else as long as it is feasible in the year 2100 in a hard sci-fi setting.
• No redundant propulsion unit? The U.S.S. Lawsuit Waiting to Happen will likely not have enough time remaining to decelerate at a human-survivable rate - though I'll write up an answer when I have time to do the math. I think @Serban's answer is probably the most likely. Apr 25, 2016 at 15:16
• I'd hate to be on the planet where the discarded Mega-ton engine component eventually impacts at 0.6c. Apr 25, 2016 at 18:28
• @LorenPechtel: I’d wager that a ball of gas and dust travelling at 0.6c is still very much dangerous. Apr 26, 2016 at 6:16
• @Michael: entirely a question of how diffuse it is when it hits your planet: minuscule quantities of high-energy cosmic ray particles travelling far faster than that hit Earth all the time. One hopes that the enormous kaboom, combined with the 3+ years it will take to reach the nearest planet (and, if the jettison system is designed to make it miss Earth, the many, many years it takes for our growing dustcloud to hit anything else) means no single planet will be hit by very much of it. Surely this object can't be any worse than a supernova ;-) Apr 26, 2016 at 9:27
• Put the money you were going to spend on their rescue into cancer research and save a lot more lives... Those astronauts knew the risks! ;) Apr 26, 2016 at 10:02

# You Do Not

Instead, the optimum response is to accelerate an unmanned ship from Earth (or LEO) such that at some point it matches the speed of the oncoming returnees, offload, turn the rescue ship around.

• Why unmanned? Because g. You can accelerate faster without fleshy squishy things on board, so intersect maybe even inside Sol proper, which means our returnees get home faster, need less life support mass to live off, etc.
• Why not do something with the ship itself? Because Earth has GDP, ship not so much.

Most likely this is one of the hundreds of mission contingencies examined by the Planetary Space Agency before the ship was even designed, never mind sent out. Such a mission would likely cost hundreds of trillions of dollars by our current standards, so every possible outcome will have been carefully considered in advance given the expense. The core dump would have followed established protocol, and separation bolts would have had to be installed in advance. This makes it likely that an interceptor ship capable of matching the top cruising speed of the main mission is probably already in the docks, waiting for the contingency to be triggered.

• I completely agree with @Serban Tanasa. You don't even need to worry about the intercept deadline, either, because it would actually be EASIER to wait for the ship to pass by the Earth and launch a rescue ship after it going the same direction (though the launch would probably take place before hand to account for acceleration). Catch up with the ship, offload all the passengers/cargo/data/whatever, then turn around and come home in a ship with working systems. No one wants an old busted ship anyway! Apr 25, 2016 at 15:50
• Go on to a new mission; don't just turn around. Same cost either way. Apr 25, 2016 at 18:24
• GDP=Gross Domestic Product, i.e. industrial capability to build expensive new things quickly Apr 25, 2016 at 19:29
• @SerbanTanasa Good news - you're going to live. Bad news - we need you to work a little late on Saturday. You'll be working for the next 20 years. Apr 25, 2016 at 21:29
• I really love how "grounded" serban's answers are. +1'd Apr 26, 2016 at 13:46

Whatever method is used to slow the ship down, the energy source cannot come from within the ship, to any reasonable extent. The amount of energy involved in relativistic speeds is enormous.

Even assuming that the rest of the ship is abandoned, and the crew all enter a small chamber (which is then itself decelerated), the kinetic energy of a 1 tonne object (1/5 of the mass of the Apollo Command Module) at 0.6c is 2E19J.

This converts to roughly 5Gt of TNT, equivalent to 100 Tsar Bombas. Unless the crew have a backup energy source capable of providing the required 2E19J, hard physics prohibits the ship from slowing down unless that amount of energy is provided to allow the ship to gain sufficient delta-V of 0.6c.

Therefore, any salvation to the ship's crew must come from external sources. Good solutions already mentioned include providing fuel and a new reactor along the path of the ship, sending an unmanned rescue vessel, or otherwise providing the required energy from an external source.

Additionally, the propulsion sources must provide relatively large amounts of acceleration. The crew will die within 5 years without supplies, so the ship must return to Earth (or at least be resupplied) by then. In order to decelerate to 0 speed within 5 years, the ship must decelerate at a constant rate of 0.116g, and more if the ship overshoots Earth. This rules out any deceleration using low-impulse sources, such as light sails or clouds of gas, unless the ship is also resupplied.

• 100 Tsar Bombas doesn't sound like REALLY much. I will consider making the ship an Orion. Jul 25, 2016 at 19:24
• It theoretically could come from the sun, using wires + the heliosphere to slow the ship down. Sep 7, 2017 at 20:13

A few options, courtesy of Isaac Newton and his third law of motion:

1. Jettison the propulsion system in the direction of travel with really high speed (how high depends on the relative masses of the ship and the propulsion system; this might not be possible at all).
2. Have everyone get onboard a shuttle and launch it really fast going in the opposite direction of travel (and have it moonwalk into orbit, you smooth criminal).

Though come to think of it, deceleration from relativistic speeds to orbital speeds over the distance of the length of a spaceship would be more than fatal for the crew and likely the ship/escape pod.

• Option 2 requires an insane amount of energy not available on the ship. Apr 26, 2016 at 1:13
• @MarchHo Perhaps. Once everyone boards the escape shuttle, the entire main ship can be scrapped. Scrapping it, er, violently, could make use of any energy in the ship whatsoever to propel the escape shuttle - perhaps using components of the life support system to create an explosion that would otherwise be catastrophic, for example. Apr 26, 2016 at 1:44
• generating a delta-v of 0.6c is with a shuttle seems very very unlikely Apr 26, 2016 at 3:30
• @SteveIves isn't going from ~0 to 0.6c a delta-v? (relativistic effects aside) Apr 26, 2016 at 13:26
• @MarchHo : Option 1 is even less feasible.
– vsz
Apr 26, 2016 at 16:00

The timeframe makes this very improbable, but if the incoming track of the spacecraft is known with a high degree of certainty, a cloud of gas or even plasma could be laid in front of the craft. The spaceship will plow into this much like a contemporary spaceship reenter's Earth's atmosphere, and the friction created by the interaction of the spaceship with the medium will slow it down.

Now if we assume that a civilization which can create an interstellar spaceship capable of moving at .6 c won't have much difficulty in scrambling tanker spacecraft to fill up with gasses from the atmosphere of gas giant planets and getting in position to eject the gasses in the path of the oncoming spaceship to decelerate it.

While there are a lot of variables, two things stand out right away: you are coming in at very high velocity so the spaceship will suffer severe heating and erosion. We can assume that the front of the spaceship has shielding to protect it from erosion and radiation as part of the design (the ship will encounter gasses and dust during its flight as a matter of course), so there will be a level of protection built in. The crew will need to ensure the ship does not tumble during the deceleration (I will assume there is still a functioning RCS aboard the crew module).

While it is highly improbable that there can be enough deceleration achieved through the use of flying through clouds of gas to actually stop the incoming ship, there may be enough deceleration to allow a rescue mission to be launched and catch up with the crippled ship after it has slowed down enough.

## Send an engine and scatter fuel along their path.

As you may know, it takes a lot of energy to accelerate to or decelerate from relativistic speeds, and that the energy is proportional to the mass of whatever is being accelerated or decelerated. Also, don't forget that your fuel has mass, too, so any fuel that you are carrying contributes to your mass and makes it harder for you to accelerate or decelerate.

So the ideal would be to send up an engine and scatter fuel along their path such that they will be getting just enough fuel to keep their engine going at max power (the engine will only have minimal extra thrust, so as to minimize it's mass). Note that the fuel needs to accelerated to almost the same velocity as the ship will be going when the ship reaches the fuel, otherwise the collision with the fuel will be unpleasant.

The specifics of the engine and fuel will depend on what technology is available, but this basic idea can be adapted to a number of different types of engines and fuels. For example:

A solar sail and a railgun:

Using beam-powered propulsion, a solar sail (if the ship didn't already have one, which it easily could due to their usefulness) and a railgun would be sent on a path to perform a gravity slingshot around Jupiter (because the mission was timed such that Jupiter could be used in an emergency), then accelerated to match the ship's velocity. The solar sail would be deployed both to slow down the ship and to collect sunlight to generate electricity to power the railgun. The beam that helped accelerate the railgun would also be trained on the solar sail to provide additional power and deceleration.

Then, the railgun would be fired as fast as it possibly can. It will be designed to be as flexible as possible in what it can use as ammunition, so at the beginning it will be using everything possible from the ship itself. Unused section of the ship? It will have been broken up and prepared for the arrival of the railgun. Spare parts? They're going in too. Everything nonessential will be fed into the railgun to simultaneously reduce the mass of the ship and to decelerate the ship. There's likely to be a fair bit of mass they can jettison like this - the ship needed considerably more for interstellar travel than it needs for travel just in the solar system.

Thanks to careful calculations, the ship will reach the "fuel" path right as they run out of spare parts to jettison. For the railgun, this can just be chunks of whatever, likely an asteroid that has been broken up and scattered onto the path (avoiding the costly process of propelling that much mass out of the Earth's gravity well). There will be enough chunks for the railgun to continue firing at its maximum rate, but the ship will not collect more than what they need to sustain that rate.

All of this is simply to decelerate the ship as much as possible. Once the ship is no longer traveling at relativistic speeds, other options will be much more feasible - a replacement propulsion section, a resupply or rescue ship, etc.

• Actually, wouldn't you want the ship to collect railgun ammo as fast as possible? Each bit of mass it picks up slows it down slightly. Apr 26, 2016 at 0:45
• @immibis as fast as possible? Absolutely not! That's like being on the receiving end of the railgun. Apr 26, 2016 at 1:12
• Since this is hard sci-fi, you might want to estimate Power requirements of the rail gun, and the required solar collector area to provide that... I think the math will say, that anything "solar" will be totally inconsequential in slowing the ship... And same for any planetary slingshot. The total power and energy requirements are in a totally different order of magnitude for 0.6c velocities.
– hyde
Apr 26, 2016 at 20:09
• At 0.6c energy budgets, "slingshot around Jupiter" is like getting out and pushing to make a fighter jet go faster in the air (you know, by kicking your feet against the air and blowing with your lungs). Except ridiculously moreso. The energy budgets are on the wrong order of the wrong order of magnitude.
– Yakk
Apr 27, 2016 at 19:25
• @RobWatts Sure, but it won't work. Suppose your craft accelerates at 1 G out to Jupiter. By Jupiter it is going at about 0.01c. The time it is exposed to more than 1% of Jupiter's gravitational force is less than 5 seconds: detecting how much it would be deflected by Jupiter would take effort. There is no slingshot possible from Jupiter's gravity well at the speeds we are talking about.
– Yakk
Apr 27, 2016 at 22:56

I think the design of a ship would do everything it could to save fuel requirements, and find it worthwhile to have different systems to use for braking. It might deploy a solar sail, a magnetic sail, or various things to cause drag.

Even if it planned to use the main drive for some deceleration stage before using these other means, it might use the brake anyway with some useful effect. It might continue braking, gradually, even well past the sun and on into true interstellar space.

So, a follow-up or rescue mission is possible, with the next mission changed to rendezvous.

As a variation, they might cannibalize the ship and cut off everything that's not the brake mechanism and a minimal life pod, and parachute to a stop with the much reduced mass.

Or, if the drive mechanism is nonfunctional but they still have the fuel supply (e.g. anti matter and reaction mass) then, after somewhat slowing and continued braking on the way out again, a rescue resupply mission could be launched to just catch up to them with critical components, arriving (to them) empty.

Most of the answers here are totally out of it so I will address them in bulk.

First, lets consider the mechanics of intercepting the vessel. Assuming we want to keep the acceleration on the rocket to 1g (and we almost certainly do--a long term exposure to high g is going to be quite dangerous) we need to intercept them 7 months out (note: I'm using Newtonian math, Einstein would only make things even worse.) You have only 16 months for the mission so you now have 9 months to get that rocket into position. You need an absolute minimum of 4 months to get into position and since you only pushed your manned vessel to .6c I would think the interceptor would have similar limits, thus with a 7 month drift time. Oops, you're down to 2 months to build and burn (both the takeoff burn and the velocity match burn)--even if you can whip up a rocket with a replicator in nothing flat it's going to be burning at more than 10g. I doubt they have the technology. Even if somehow you have vastly more delta-v available you still have less than 5 months to build and burn. It's not going to happen.

Second, dust in it's path. This avoids the need to match velocities and thus makes it somewhat easier. It's also almost certainly going to destroy the vessel--the deflector system is going to overload and the vessel is either destroyed or fried. (Think of how any space vessel would fare if it's engine were pointed at it. The energy dissipation in the dust cloud is considerably higher than it's engine power as the cloud will be moving out at relativistic speed.)

This leaves only one approach that might possibly work: Launch your rescue vessel in the opposite direction. You have 13 months to get it on the way and there's no design time--it's an ordinary vessel. You might even have one around. The runaway rocket flies through the system on schedule, 4 months later the rescue vessel matches with it and takes off the crew.

Edit: Another problem comes to mind. Figuring an intercept and return to Earth assumes the rocket carries enough fuel to boost to .6c 4 times. This is the sort of fuel it would need if it had gone out and come back without refueling. However, this question has the "science-based" tag--and that is an incredible amount of delta-v. At .6c you're carrying 80% as much kinetic energy as rest mass. Assuming a theoretically perfect conversion of energy into kinetic energy (at a minimum this would require a reactionless drive of some kind) and for each boost you need almost half the rocket as fuel not counting the fuel needed to boost the fuel. (And that fuel will be considerable but my calculus is too rusty to tackle it right now.) After 4 boosts you're a bit past 90% of your rocket being fuel--and reality will be much worse than that indeed.

There's a reason most sci-fi authors handwave the power source of their stardrives!

If the rocket is refueled at it's destination the ratio is not so brutally high but that means you can't turn around in space. The intercept before Earth scenario isn't on the table at all, the intercept after scenario still works but the rocket is going to have to go to some other star rather than come home.

• they have ships that go at 0.6c all the way to other stars. They probably have more efficient rockets than ours. Apr 26, 2016 at 3:25
• @njzk2 Of course they do--we would be hard pressed to get any ship up to .6c, let alone anything manned. I'm simply assuming the rescue mission has pretty much the same tech as the rocket in trouble. Apr 26, 2016 at 5:10
• We'd be hard-pressed to get anything to 0.01c. Oh, anything that weighs more than a nucleus. Apr 26, 2016 at 11:04
• @LorenPechtel, you can send an unmanned interceptor at a 20g burn rate going out, if the ship can take it. Moreover, you can intercept in passing, as it goes by, and decelerate and turn around. No need to stop at Earth distance. Agreed that for < Kardashev1 building such a thing in months would be hard, hence having a rescue ship on standby. Apr 26, 2016 at 13:52
• @SerbanTanasa Sure, they could build a 20g rocket--but it's very unlikely they have already designed one. What's the mission for a 20g starship? And a 20g burn doesn't change the delta-v issues. Apr 27, 2016 at 2:50

Yes, there are many good answers, but either I missed it or nobody did came up with this, so another option: Laser.

Its plausible for the timeframe; they think about doing this right now. Okay, it will use way smaller vessels (unmanned), but its said you could sling a small probe to the next star within some years.

So lets assume they did think about a scenario where the ship does come back without any ability to break. Lets assume further they decided that using the laser-option to decelerate it was put into the ships design from the very beginning.

All you need to do is aiming in the correct direction (at both sides). When I think about it... hitting something that's incoming with 0.6c with a laser over half a light-year of the size of a small spaceship will be... well, you need to aim really good for this. To be honest, I'm not sure that's even possible at all.

But it offers you a fine plot point, if you are going to make a story out of this: That laser option wasn't planned in first instance, but somebody did remember how they did send probes to Alpha Centauri ages ago, so they install a makeshift-mirror at their vessel while the other side (earth) has the fine task to design a laser suitable for this task within a year or less.

But at the end... all you need to accomplish is getting that ship slow enough to have it half-orbiting the sun... How to explain... make an u-turn around sun, and the earth can send all that stuff some of the other answers did name (especially a new engine).

After all, no laser that wouldn't destroy that ship at the first hit would be able to eat away all the surplus speed. Just getting it slow enough so it would not fling out of the solar system at the other side. Sadly that will not yield any plus-points.

And to be honest: that whole situation does sound like a job for Jeb and the Kerbal Space Program :) But they would use a rope and attach parachutes, than try an air-break at Jupiter. Well... if everything else fails... still no.

Post Scriptum: Seriously, do not try to hit any atmosphere for air-brake at this speed - you could aim for concrete wall, that would not make any difference.

EDiT: Wait a minute, just some random idea that lured in the back of my head since I wrote down the Jupiter-parachute-idea:

Solar-Sail Its the same as the Laser: at its own it will never stop the ship in time, but you could try to use it like the space-shuttles did when landing to get some of your momentum eaten up before you reach the solar system. Well... you would need an incredible huge solar sail, and it would have a fun time passing through the Oorth' Cloud... at least you would make yourself more easy to spot for the laser-guys...

• Considering it's a spaceship, a concrete wall would probably be a lot better to hit :) Apr 26, 2016 at 9:06
• "ship does come back without any ability to break" in this scenario the one thing it's capable of is breaking. braking on the other hand... /sorry I couldn't resist Apr 26, 2016 at 15:18
• Ah... you see, that's why I recommend to shoot with a laser at it, if its unable to break by itself any more @JeffMeden ;) Apr 27, 2016 at 4:54

At .6c you aren't going to get much that will slow it down in only a year.

You can try lasers if you already have them in place, as it might knock a percent or two off, but considering for a laser to get a spacecraft up to .6c would take a huge amount of time, 1 year isn't going to do much.

You can try to get some huge thrusters and intersect with them, but just matching speeds is going to be pretty tough. you would essentially have to launch them out of the solar system the direction they are going and hope to get up to their speed before they go shooting by.

Honestly, their best bet at that speed would be to fly through the sun. At .6c they won't be in the sun long enough for the ship to heat up too much, though the turbulence will be pretty bad, so you'd only want to try this if you the ship is structurally sound, and the deceleration would be pretty rough too, so you may lose a few people to their organs rupturing, even with crash couches.

Edit:
Also, the EM field would likely be intense, so make sure your computer and other components are shielded, and maybe have backups stored in lead cases that can be swapped out for ones that overload.

Depending on the size and capabilities of the shuttles, you can mount them at the front of the ship and use their engines to slow down. It may not get you all the way down in speed, but even if it can cut your velocity in half, it both makes it easier for a rescue vehicle to reach you and doubles the amount of time Earth would have to mount a rescue. Keep in mind the ship has less mass with the engines jettisoned, so that works in your favor.

Also, depending on how your technology works, the ship's engineers might also be able to use contents of the primary ship as part of the propellant used by the shuttles to extend their fuel. For example, mixing in any gasses the primary ship has available (O2, argon, whatever is used for fire suppression, etc.) into the mix. Every little bit helps. Or even better, if the shuttles and main engines use the same type of fuel or propellant, while the shuttles are probably weaker, they have a huge supply to reload with.

• Your shuttles have a delta-v of >0.6c?
– TLW
Jul 21, 2016 at 19:47
• I don't have any shuttles. Jul 21, 2016 at 19:55
• Sorry: their shuttles have a delta-v of >0.6c?
– TLW
Jul 21, 2016 at 21:45

A few things to help with manoeuvring:

• RCS,
• the shuttles engines,
• the crew jetpacks,
• missiles just fire without releasing them,
• shooting whatever other weapons you have,
• or even venting air

You may even have solar sails, if you left for a long joney.

All that put together could help you plotting gravity assists on all the planets and planetoids you encounter, and then aero-braking.

You can even combine both by aero-braking inside Jupiter. (Just make sure you get out of it)

Or against Saturn's rings (assuming your ship can withstand the impacts, and then there is the asteroid belt before reaching Mars)

So to sum up:

• Various small resources are still on the ship.
• Hit comets, asteroids, rings, ...
• Aero-brake in any gas giant you cross, and in any atmosphere you come across
• Gravity assists

It's going to be a rough ride. Good luck!

• If you somehow managed perfect gravity assists with every planet (you can't) you'd still only lose ~0.001c. As for aero-braking (or worse, lithobraking), running into gas at 0.6c isn't exactly healthy. Remember: every kg of your ship has something like 1.4 MT of TNT worth of kinetic energy.
– TLW
Jul 21, 2016 at 19:55

# You can't

You have a ship, capable of transporting humans for years. That means it's HUGE, like Project Orion huge. 10 kilo tons of mass at the very least. Going at 0.6c. So you have 2.247×10^23 joules in that ship, currently.

Wolfram Alpha gives us some estimates how much that is. (e.G. it exceeds the amount of energy in fossil fuel we have on planet earth ~6 times)

If you want to stop that by throwing stuff at the ship, you will need to deposit that much energy into it, thus boiling it away. It doesn't matter if your stuff is photons, dust or concrete walls. Your ship will not tolerate that much energy (that would be 6.3118*10^20 Jules every day. Compare: the US uses 0.94*10^20 Jule per year). So you would need to radiate the heat of about 6 USA/year away every day. That's not going to work.

If you have a propulsion system that is half as efficient as the ideal mass:energy converter (which is not realistic in a hard science fiction scenario for the year 2100) you need 5 kt of material to reaction with. That was the amount of material you just jettisoned because it was your propulsion system (plus a little for the engine).

To send a rescue craft with a similar effective system, it will need 5 kt of fuel to stop you. It will also need 1,25 kt of extra fuel to accelerate that fuel to 0.6c to intercept you. But now you are stationary somewhere in mid space, outside of the solar system. To get back, you need another 5 kt to accelerate the ship back and 5 kt to break it again in the solar system. But that additional 10.000 t need to be brought to you as well, so you will need to start ~24 kt with the rescue ship (plus its engine). As you said, your fuel already started to go fusion bomb before, so its radioactive. You send uranium this time. That's 1.463 billion \$ for the fuel. (these are all back-on-the-envelope calculations, don't plan your spacetrip on them)

And that is just the basic. We did not have a backup rocket to save the Apollo Missions if they had gone wrong, so there is no reason to assume we will have a backup engine for our only spaceship ready when it breaks. So, not only will we now have to spend a year of the science fund only on the fuel, we will also have a multiple of that for building another engine and bringing it on course. This will not happen.

# What would actually happen

Your president will have a speech ready for this situation.

• Nice answer. It seems our explorers may need reactionless drives or loads of cheap antimatter to make it. Apr 27, 2016 at 18:02
• @SerbanTanasa the question basic premises (that a ship traveling at 0.6c had to jettison its reactor/propulsion) means that such propulsion already exists in the fictional world. So denying the possibility of accelerating and deaccelerating at such speeds is basically denying the scenario. Apr 27, 2016 at 20:26
• @Mindwin Not at all. If you have more time (like decades) you can harvest sufficient fissile material from the asteroid belt. Its just not going to work twice in such a short timeframe. Apr 28, 2016 at 6:57

Ok, lets get the bonus achievements:

• By "saved" I mean that the ship must be slowed down into a solar orbit where it can be accessed by rescue vessels.
• Bonus points for making it head towards earth or into an earth orbit so that the crew will only have to board the shuttles.
• No propulsion whatsoever may come from the ship itself unless you decide that sending a new nuclear reactor and propulsion module for rendezvous at relativistic speeds.
• You can use anything else as long as it is feasible in the year 2100 in a hard sci-fi setting.

XXII century Earth has the tech to get ships to 0.6c (else our shuttle wouldn't be in said speed). So getting a vessel to match the speed and vector of the incoming craft is not a daunting task. But it also means that Earth technology has means of reducing the effects of ultra-high G force on "squishy flesh things" like people. I'd assume such anti-G

And the answer is... SPACE NET

Send a ship composed of several propulsion units tied up to a folded net inside the ship. It would move itself outside solar system (beyond pluto orbit at least) into the path of the incoming shuttle, match vector and 99.99999~% of velocity(1) and deploy the net with the array of propulsion units. The timing of this operation would be such that i'd catch the shuttle a few weeks after deployment.
(1) enough so it won't damage or crush the shuttle and passengers

As soon as the net is securely latched to the shuttle, the propulsion units activate and begin deaccelerating. They also would steer so the gravity of the planets could be used to further slowdown and put the vessel in an intercept path towards Earth.

After reaching Earth orbit, the net would release the ship, and the crew could board the shuttles and dock with some facility easily.

• Reaching 0.6c with 1G of accelleration takes on the order of a year. No gravity shielding required. Hard part is energy, propellant and surviving interstellar particles.
– Yakk
Apr 27, 2016 at 19:29
• @Yakk It is assumed in the question body that humanity has the means to reach that speed. Or the runaway ship wouldn't be at such velocity. Apr 27, 2016 at 20:24
• Nothing in the question '[...] means that Earth technology has means of reducing the effects of ultra-high G force on "squishy flesh things" like people.' , as demonstrated by the fact that you can reach 0.6c at 1 G over less than a year. You claim the question implies gravity shielding. Gravity shielding is not very hard sci-fi at all.
– Yakk
Apr 27, 2016 at 22:47
• what advantage does a net have over just docking with the shuttle? Nets doesn't work that well in space to begin with.
– John
Jan 29, 2019 at 19:53
• @john if JJ can get magnetic falling bombs with smileys, I stand by my nets. They are better than boarding, everyone knows that a tiny net is a death sentence. Jan 30, 2019 at 11:44

Install some reactors, engines and fat electromagnets on a lot of very big boned asteroids you don't want, shove them out near to where the ship will pass. Turn them on, reverse railgun style. (The ship is the bullet in the metaphor).

One of two things happen, your ship flies through the magnetic field, and slows, leaving a slowly moving asteroid in its wake or if your magnetic field is BIG enough(not likely), you accelerate the asteroid to match the speed of the ship, increasing the mass and decreasing the speed.

I do not claim at all to have any idea of how magnetic fields work with a big relativistic differential(thanks Einstein), but it certainly seems like it would work. You could even possibly do something interesting with a variation of this idea(metal object moving through magnetic fields) and recapture some of the energy of the quickly moving projectile. This would be nice, as the energy that it would take to get the asteroids in place in time would be large as well. Might be more worth your time to slow them enough the physics works, then redirect them to your asteroid belt and then slowly spin them to a stop using your magnetic fields to guide them.

• Problem with this: the ship won't be near the magnets long enough to really be affected. And even if it was, you'd splat the passengers. 0.6c, remember? Your 100km long asteroid would whip by in half a millisecond.
– TLW
Jul 21, 2016 at 19:58
• More of a matter of how many asteroids/magnetic fields and how strong the magnetic fields are. There is an innate assumption here that you would keep the deceleration in at a survivable rate. As for how a magentic field works across a speed differential like that, I have no idea. I know this idea is currently used in particle accelerators, just on a different scale (and of course, to accelerate a particle to close to light speed instead of decelerator it). Jul 21, 2016 at 21:50
• Let me put it this way: there are ~1.5 million asteroids >1km in diameter in the asteroid belt. If you used every single one of them, each one would have to slow down the ship by ~120 m/s. A slowdown of 120m/s in less than 6 microseconds as the ship whips by is not exactly survivable, even assuming you could attain it.
– TLW
Jul 21, 2016 at 22:04
• I'll leave this one after this as there are undoubtedly a lot of holes that can be put through this suggestion, but using a circle, just like a particle accelerator does, then there woulnt be a limit as you reuse asteroids over and over. Considering the speed, using the Kuiper belt makes more sense, but as it has a diameter of .001 of a light year, you still would want to bring the speed down a lot (they would experience 720g if you didnt) before it gets there in order to not crush your astronauts as they turn, so move asteroids to slow them down as they approach or increase your radius. Jul 22, 2016 at 15:05
• A circle would not help. Or, to put it another way, halfway around the circle you are going in the opposite direction as when you started. Which means that you have to accelerate them more than if you simply stopped them.
– TLW
Jul 22, 2016 at 19:26

At this point in your story, it's time for the Star Trek answer. Choose 1 or more of:

• Reverse the polarity of the deflector dish
• Reverse the polarity of the tractor beam
• Invert the polarity of the shield generator
• Something with warp field stabilizers

It seems one of those 4 things can fix nearly anything that goes wrong on a star ship. So give it a try.

In other words, it's time to break out the near magical, but some how just vague enough tech to make story work.

• What about reversing the polarity of the neutron flow? Would that work? Sep 7, 2017 at 13:57
• You forgot to use a Graviton beam Sep 7, 2017 at 20:24

The difficult part is decelerating that much mass in that short amount of time. I think I'd suggest getting rid of all the mass of the original ship.

So you start with a very light unmanned rescue ship--as light as you can possibly manage. This ship will undergo almost constant maximum acceleration throughout it's entire life.

As the terribly light unmanned rescue ship reaches some distance between the out of control ship and the earth it must flip around and accelerate just as fast in the opposite direction so it can meet the big ship. It's going to have to undo all the acceleration it's accumulated AND then re-accelerate to .6 light speed to match up with the main ship. To do this it must be almost nothing but a giant pile of fuel on an engine.

It should match velocity and location with the original ship as soon as possible--Now for the really tricky part. In order for the deceleration to not take years, you freeze the squishy bits so they don't pop (Cryonics should be valid sci-fi for that time period), flip the rescue ship around again and fire engines on full for the last time. At this point the rescue ship has used up most of it's fuel and jettisoned any unnecessary hardware for the final deceleration so it's just an ice-cube sitting on an insulator sitting on a fuel tank sitting on an engine.

The insulation should keep the ice-cube from melting as long as it doesn't have line of sight to the sun. Assuming carbonite isn't an option, a thin wall of strong, light-weight material around the ice could keep it from fracturing under acceleration, so just keeping the ship pointed towards the sun and, again, fire engines on full. As the ship brakes for the last time it should meet up with a better equipped ship that can revive the occupants.

It would be bad if you overshot the sun while decelerating and exposed the payload to the sun's heat though...

• Or you could let it overshoot sol and intercept in its direction of movement... Apr 27, 2016 at 17:56

Forget the shuttles. Load the crew into their Slaver Stasis Field equipped deep sleep capsules, and then fire them at a large asteroid or small moon. When the high-speed indestructible capsules strike the much bigger and comparably stationary asteroid, the redistribution of energies should be quite spectacular. Once the show subsides, the severely decelerated, but still indestructible capsules should be moving much slower, making their retrieval by Earth forces significantly easier.

As for the Slaver Stasis Fields being available in a hard science set 73 years from now (2100 - (2016 + 7 years for the hero to get to Alpha-Centauri + 4 years for the partial return journey)), I think that our discovering how to suspend all atomic motion is about as likely as our reaching 0.6c in the next 7 decades. To accomplish either scientific miracle will involve our quickly learning many intense new secrets about how the universe functions. Given the scope of that learning, I don't think that accomplishing both goals is much more difficult than accomplishing either one separately.

• And they need computer to deactivate it a couple of nanoseconds after activation, because the stasis field is "several billion years outside to one second in". Jul 30, 2017 at 5:52

Just for fun, it may be possible.

Lets make a small assumption that you don't really need to stop the ship just slow it down enough to get some kind of solar orbit. Then you could interact with it normally.

With a year to work with you would need to apply a constant 20,818 m/s delta-v to pull this off. This probably isn't going to come from the ship, and it seems like a lot for gravity to make up.

Let's assume you have some kind of inertial dampers that means your squishy meat sacks won't explode as such a rapid speed change.

We also need to assume that the craft can withstand the mechanical forces that are about to come into play.

I would suggest a launch fro LEO to a solar orbit, that is going to intersect with the damaged craft. Again your going to have to pull some massive speeds to get to the correct orbit in the first place.

If done correctly, you can come up behind the damaged craft, accelerate to their speed, (that is going to need way more then 20,000 m/s) then transfer the crew to the new craft. The new craft can then decelerate back to a solar orbit, and finally transfer to an Earth orbit, and landing (or whatever)

Things to remember:

You don't have to capture the craft at the closest approach, in fact it may be better to capture after the craft exists the system, then decelerate the rescue craft till solar gravity takes over again, eventually hitting a more normalized orbit. Your going to burn a ton of energy catching up, but thankfully, once you decelerate, your need next to no energy (comperitivly) to land (crash) back to earth.

Your big engine is gonna have to put out way more power to reach the correct intercept, you can use gravity to help a little, but not much.

When you do actually capture the speed between the two craft will be low, but you will still be traveling at a high speed compared to other frames of reference. A stray dust particle is a big problem.

Go through the sun

It's the only way to decelerate. There is no way to get any rescue vehicles with propulsion systems onto a trajectory that can intercept at 0.6c given the time frame, not without ridiculous handwavium and warp engines.

The only possible chance is to fly through the sun, doing "aerobraking". Let's look at some quick numbers.

Entry speed would be 180,000km/s and the target velocity after braking will be anything under 600km/s (escape velocity of the sun). Achieving under this will guarantee the ship remains in the solar system, and opening up opportunities for further braking later on (could take hundreds of years, the ship would STILL be the fastest thing in the solar system).

To decelerate from 0.6c to 600km/s over the suns diameter requires your ship and crew to tolerate a sustained average of over 1 million g's, and the heat of the sun.

This is the most realistic scenario I can think of. Good luck!

The real question is how did your ship get up to .6c in the first place since the limits of regular propulsion is more like .2c or .3c, but assuming you could, and with nuclear fuel no less, then you'd probably have a lot of fuel and energy reserves. Dumping the nuclear core isn't a problem in space and you wouldn't lose all your fuel and such.

Another thing is that you obviously have another pretty big power supply otherwise you're already dead from all the micrometeors you're hitting.

So how do you slow, assuming you can't get another nuclear core and assuming you have all the fuel and energy to power your weapons to handle the stuff hitting you? Well... You'd eject the fuel in front of you. You'd also start taking apart your ship and reconstructing it into a larger reflective surface. And lastly keep firing your guns as much as possible, but not enough that you run out of ammo before you get to earth...

Do that should slow you a lot, inertia will slow you a lot, but the reflective surface is where you are going to get your help by people concentrating laser on you as you get closer to earth.

Once you get close to earth, blow the reflective surface and anything else just eject it which will slow you down more... If this is not enough, then there should be another ship that could catch up and attach to you and then slow you down, but assuming that it can't "attach" to you or catch up... You'd need to do the insane slow down maneuver of launching nukes set to detonate in front of the ship. Doing this can slow the ship down as long as it isn't too close to earth because then there is fall out and all that. Its risky but it's how you'd have to do it with modern tech... although you'd have to get down to .3c or .4c to get within what is possible with modern tech and when I heard of this strategy it was considered an insane thing to attempt to do, but hey it can work according to the math...

Robinson covered a very similar situation in Aurora, at least from a "large object moving too fast" standpoint. In short, cobble together as much propulsion as you can and use it to stop the least mass you can (shuttles).

Simply shoot against the ship. It's 22nd century and the ship should be able to withstand some relativistic shooting. There's a lot of space debris anyway on the way that you collide with occasionally.

If your weapon shoots at 0.1c, you need only about 50kg of ammo per 1kg of the ship weight, which may be feasible.

• And how will you hit it? You are aiming at an object the size of a house over the distance of a solar system. Whatever particles you send will be strayed out over a huge (like some planet-diameters) area. Apr 27, 2016 at 10:12
• @AngeloFuchs You can use powered ammo to correct the course, and as I say, you need to be very precise. However, if you travel between stars, you probably know all the bodies in the solar system and their trajectories, so you can compute the correct shooting direction.
– yo'
Apr 27, 2016 at 10:15
• At 0.6 + 0.1 ~=0.66c closing velocity, those shots are releasing over 7MT of TNT per kg of energy. Have fun with that, especially as you're requiring the debris shield to withstand >50x its own mass worth of shots.
– TLW
Jul 21, 2016 at 20:03
• Pretty good shields if you can land a few million nuke-sized hits on the ship without destroying it. Sep 7, 2017 at 13:46
• @Innovine Indeed, but c'mon, 22nd centurey is 22nd century :)
– yo'
Sep 7, 2017 at 13:54

You need an opposite motion to slow your ship down.

I have to assume we passed through the same area on the way out, and it sure would be handy to have the collected sensor data to know what kind of gas cloud and raw material are available to us.

If there are large hydrogen(or other burn/explode able) gases cloud in front of you, you could explode them to create reverse thrust.

Anything with exceptional gravity would also have useful gravitational effects. You would have to precisely plot a course that puts you close enough to the enter of gravity for a long enough period, so you have just enough escape velocity to escape.

1. Assuming we have strong metal cables or similar and pipes.

First you need to start sucking in as much space matter as you can safely without blowing up the ship, more mass less speed. Maybe like a window screen and then put a vacuum hose out to suck the mass in.

Then start harpooning (cables and pipes if necessary) asteroids moving slower than you, but not so slow or fast that you wreck your ship. It will be a game of create drag and release before a dangerous amount of force builds up. Obviously the head of the harpoon will have to expand to lock on and contract to release which should be well within the ability of science.

Meanwhile, the universe has tremendous amount of hydrogen and other burnable gasses. You can rig a rudimentary system that send the collected gas out the pipe and ignite it in front of the ship. This also would be an opposite force slowing the ship down further.

Maybe you could also exploit gravitational sling shooting in reverse to slow you down.

Also if you could magnetize the hull or a box you pull behind the ship, it to would lock onto passing masses and help slow you down. Gathering more and more mass. If it locks onto something too strongly and the forces created endanger the structural integrity of the ship, you can turn the magnet off for a second.

Finally, I would have some kind of net system to catch and slow down as the other person said.

• There are no black holes or other suns between us and Alpha Centauri. We would have noticed by now. Apr 27, 2016 at 13:25
• @AngeloFuchs removed the blackholes and suns. Apr 28, 2016 at 4:20