Refueling from stars

Question

My galaxy exploring ship is powered by a fusion reactor (back up with some solar panels but they spend a long time between stars). We know that stars are powered by fusion too, so they have all the ingredients we need and I've seen sci-fi use this...but the question is:

Could we actually get our fusion ingredients out of a star?

Is it too hot to approach close enough to pull any matter away? Do we need to only go to small stars where the gravity isn't so strong...or is the pull the same if you're going for a particular density of matter?

My main inspiration was the fuel scoop though the numbers there aren't important.

Answer 1

Depending on the density of fuel you are looking for, exactly what fuel you need, and how protected you are from heat and radiation, I would say your best bets for refueling a fusion-powered spacecraft are interstellar gas, gas giant planets, ocean/ice planets, and red giant stars.

Fuels

The easiest reaction for nuclear fusion (requiring the least temperature and pressure to fuse) is deuterium plus tritium. Both of these are isotopes of hydrogen, with one and two neutrons, respectively. Normal hydrogen, also known as "protium", has no neutrons, only a proton and an electron. Tritium is radioactive with a half-life of 12 years, so there are no long-lived stocks of it to collect. We will skip this possibility for now.

The next best reaction is deuterium + deuterium. Deuterium is stable, and was produced in the big bang along with protium and helium, and so it is a (very small) fraction of the interstellar gas, which is also what forms stars and gas giant planets; the primordial fraction is about 27 parts per million of the total hydrogen. This is similar to the fraction found in gas clouds, newborn stars, and gas giant planets, and you are unlikely to find it in higher concentrations on a truly large scale anywhere in the universe, although it can be expected to be somewhat enriched on terrestrial planets, because it is slightly more resistant to being blown away by stellar wind than normal hydrogen. For instance, on earth it is 156 ppm of hydrogen.

Moving down the list, the next fuel you might be interested in is ³He, the rarer isotope of helium. Its cosmological abundance is about 300 ppm of helium. Helium makes up about 23% of the mass of atoms in the universe, with hydrogen making up most of the other 77%. By particle number, however, it is only about 7%. Thus, ³He nuclei are about as common cosmologically as deuterium, and it can likewise be found in gas clouds, newborn stars, and gas giant planets.

The next useful fuel would be lithium. There are two relevant isotopes, ⁶Li and ⁷Li. ⁶Li, the rare isotope (about 8% on Earth) can be fused in potentially useful reactions with deuterium or ³He. Alternatively, both ⁶Li and ⁷Li can be reacted with neutrons (produced in other reactions) to form tritium and helium. This reaction has been used in hydrogen bombs; neutrons produced from the fission igniter and the initial fusion reaction react with a lithium plug to form more tritium, which will then create even more fusion with the deuterium. ⁷Li is primordial, although with a much lower concentration than ³He or deuterium. Thus, it can be found in dust clouds and newborn stars. It is also in gas giants, but since it is a solid at the cold temperatures that characterize the outer atmospheres of gas giants, it sinks into the middle. I'm not exactly sure what process produces ⁶Li, but I will assume it can be found wherever ⁷Li is found, just in smaller quantities.

Finally, if you have some very fancy tech that can compress gasses more effectively than the core of a star, then maybe you just want protium. This is the fuel used by the sun and other main sequence stars. However, even in the core of the sun, the reaction goes quite slowly, with the average power density lower than that of the human body. Protium is the most abundant atom in the universe, and you can find it in gas clouds, planets, and stars.

Deep space

The easiest place for a starship to get fuel, is probably to collect it slowly as you travel through interstellar space, especially within molecular clouds and nebulae. For one thing, you are already travelling there. The only disadvantage here is that the density is quite low. The densest part of molecular clouds may be up to one million molecules per cubic centimeter, and young planetary nebulae are similar. One million may sound like a lot, but keep in mind that a gas at standard pressure and temperature has on the order of 10^19 molecules per cubic centimeter. Remember we had about 25 ppm of deuterium and ³He, so that means at one million molecules per cubic centimeter, we can expect to find a couple dozen molecules of each. If you can scoop them up efficiently from a wide area around your ship as you travel long distances through the cloud, e.g. using a bussard ram scoop, this could get you a lot of material.

Gas giants

Alternatively, as mentioned in evil professeur's answer, gas giant planets are a much more concentrated source, and are much safer to be around than stars. Regarding the question about gravity, the gravity at the surface of a smallish gas giant, such as Saturn, is about the same as that of the Earth. Keep in mind, there is no actual "surface" on a gas giant, but the depth where the atmospheric pressure is 1 bar, about the same as that of the Earth, is considered the "surface". On Saturn, the temperature at this altitude is 134K, or -139°C. At this temperature, the gas will be about twice as dense (number of molecules per cm³) as Earth's atmosphere. Saturn is composed of about 96.3% hydrogen, with the balance mostly helium, so protium is easy, and deuterium and ³He are available as long as you can separate them from the bulk gas. As I said before, lithium will sink to the center of the planet, so it will not be accessible.

Heavier gas giants, like Jupiter, have greater gravity near the 1 bar level; for Jupiter, 2.5g.

Aside from the lack of lithium, gas giants do not shine, and so locating them from far away can be difficult. Still, with our observational capabilities, we have started to find them by the hundreds around nearby stars, and they seem to be quite common.

Ocean/icy planets and moons

Ocean planets, like Earth, can be a reasonable source of lithium and deuterium, and of course protium. All can be processed from seawater. However, they are generally quite poor in helium, especially ³He. Lithium and deuterium, as well as all nuclei heavier than helium, are relatively enriched on Earth, and presumably other terrestrial planets, compared to the cosmos as a whole. That's what makes them terrestrial planets!

Icy worlds are similar, with hydrogen (and therefore some deuterium) as a component of the water, ammonia, and methane which make up the surface ice, as well as any subsurface oceans.

Surface gravity of ocean and icy planets varies with size, but they have the advantage that you can land on them while you do your fueling, instead of needing to expend energy to "fly". All of the relevant bodies in the solar system have surface gravity equal or less than that of Earth.

Detecting ocean and icy bodies from interstellar distances is even harder than detecting gas giants. Ocean planets may (or may not) be quite rare, but if our solar system is any indication, icy moons are common around gas and ice giants.

Stars

Small, cool stars, including red and brown dwarfs, are the most common stars and at least red dwarfs shine brightly enough that they should be easy for a spacefaring civilization to find. Unfortunately they are depleted of deuterium and lithium. Because these are the easiest fuels for fusion, they get burned up first as the star is forming. This is actually the distinguishing feature between stars and planets: the smallest, coolest stars are brown dwarfs, which got hot enough to burn their deuterium, but not to start burning protium. Intermediate brown dwarfs have also gotten large enough to burn lithium. Both red and brown dwarfs are fully convective, meaning that the entire star is well mixed, and all the original deuterium and lithium pass through the core and are burned up relatively quickly.

Stars the size of the sun and larger are not fully convective, and so the outer layers contain deuterium which has never cycled through the core and burned up. However, as other answers have mentioned, the environment near the outer limits of a star is extremely hostile. Although it can be assumed that any starship has fairly extensive radiation shielding to protect it from ionizing radiation, there must be limits to that shielding. The outer atmosphere of stars, although diffuse enough that I don't think there would be too much conductive heating of a spacecraft, are hot enough to emit x-rays, which would be quickly deadly to any unprotected humans. Additionally, the strong, rapidly changing magnetic fields associated with solar flares can in extreme cases be enough to disrupt electronics even on Earth, even inside the protective magnetosphere.

Regarding gravity: The "surface" gravity of a star (or anything else) is proportional to its mass and inversely proportional to the square of its radius. For main sequence stars, which are burning hydrogen in the core, this actually means more surface gravity on smaller stars. For instance the surface gravity of the sun is about 28g, while the surface gravity of Proxima Centauri, a red dwarf with only 12% the mass of the sun, is about 170g, and Sirius A, the brightest main sequence star in our local neighborhood, is twice the mass of the sun but has a surface gravity of "only" 22g. The surface gravity on red giants is much lower, because they have the same mass that they had when they were main sequence stars, but a much larger radius. For instance Arcturus, with a mass only about 8% larger than the sun and a radius 25 times that of the sun, has a surface gravity of 0.05g. With this low gravity, in combination with the strong stellar wind, you might be able to use some very temperature-resistant solar sail to stay in place above the star while gathering fuel. Red giants are also cooler than a main sequence star of the same mass (4300K for Arcturus vs. 5700K for the sun), and so somewhat safer to approach. The red giant phase is a comparatively short period in the lifetime of the star, and so red giants are rarer than main sequence stars; however, their total luminosity is very high, so they are easy to locate. On the downside, near the beginning of the red giant phase, the outer part of the star becomes more fully convective, and so the concentration of easily fusable nuclei like deuterium and lithium will be somewhat decreased.

The outer limit of red giants is even more diffuse than for main sequence stars; depending on how dense you need your fuel to be, you can approach closer or farther. Indeed, at the end of their lives, red giants of moderate mass blow off their envelope completely, becoming planetary nebulae (see above).

Edit: Escape velocity

In my answers above, I talked about the "surface" gravity of different stars and planets. This is the gravity that the starship would have to hold itself up against while it is in the process of refueling from a star or planet that doesn't have a surface to land on. However, this is only part of the problem of gravity; before the ship can match velocity with the "surface" of a star or planet, it would have to lose the speed it gains by going into the gravity well, and after it was done fueling, it would have to escape the gravity well again. This in effect means it needs to pay the escape velocity twice. So, here are the escape velocities of the objects I mentioned. For stars, this is the escape velocity to interstellar space, starting from the "surface" of the star, and assuming that the ship is matching the rotation of the star. For planets and moons, I give both the escape velocity to interplanetary space starting from the surface, and also the escape velocity to interstellar space given the orbital speed of the planet. I don't know if your ship has a futuristic propulsion system where these kinds of velocity changes are trivial, but since you asked about gravity, here it is:

• The sun: 615 km/s to interstellar

• Proxima Centauri (red dwarf): 577 km/s to interstellar

• Sirius A (main sequence star bigger than sun): 655 km/s to interstellar

• Arcturus (red giant): 125 km/s to interstellar

• Saturn: 25.7 km/s to interplanetary, 29.5 km/s to interstellar

• Jupiter: 47 km/s to interplanetary, 52.6 km/s to interstellar

• Callisto (an icy moon of Jupiter): 5.9 km/s to interplanetary, 11.3 km/s interstellar

• Titan (an icy moon of Saturn): 4.7 km/s to interplanetary, 8.5 km/s interstellar

• Earth: 10.7 km/s interplanetary, 23 km/s interstellar

Molecular clouds and nebulae are already in interstellar space, so I won't calculate any escape velocity for them.

As you can see, stars have deep gravity wells, but red giants are again the best bet, because you don't need to come so close to the center of mass.

For gas giants, it would be most efficient to choose a light one far from its star (e.g. more Saturn than Jupiter), unless the ship is stopping in the inner stellar system anyway.

For icy moons, choose one far from its planet, and a planet far from the star. Icy minor planets and Kuiper belt objects would have even smaller escape velocity to interstellar space, but they are less likely to have a molten subsurface ocean, because they are not heated by tidal forces. So you would have to dig your fuel instead of pumping it. They would also be substantially harder to detect, even from within the system.

Edit: Ionization

If your ship uses magnetic fields to pull in fuel, then you need your fuel to be ionized. This rules out gas giants, molecular clouds, and icy/ocean planets/moons. The photosphere (aka "surface") of cooler stars is also largely unionized. However, the stellar wind is ionized, and will be densest a short distance from the star. Planetary nebulae also tend to be ionized, at least when young.

Answer 2

is the pull the same if you're going for a particular density of matter?

Yes, gravity works on mass not volume

The only materials you would be able to reach would be those emitted from the star. Both gravity and heat would prevent you from scooping up anything from the star itself. A picture of the sun should dissuade you from trying. The forces are just too titanic to be messing with.

The only thing known to be able to escape the suns gravity is the solar wind, you may be able to do something with that.

But if it's just the ingredients you need then you'd be better off taking it from any other source. The Sun generates its energy by nuclear fusion of hydrogen nuclei into helium. Hydrogen is the most common element out there. You don't need to go to the sun to get it.

Answer 3

That obviously depends on the tech level you have available in your story. An extremely advanced civilization would probably find more efficient ways of producing energy long before figuring out how to do something like that safely. On the other hand for a deep space exploration it just might be a prerequisite.

Check out Stargate Universe. Destiny, the ship that is the focus of the show, did exactly that to carry its mission out over millions of years.

EDIT:
If you were to go a similar way Destiny did, there are several problems to consider. Heat shielding is the obvious one. To survive in our Sun's photosphere for instance your ship would need to withstand temperatures of up to around 6000K. Another one is the method of acquiring the material itself - are you going to use ram scoops while traveling at speed or is the ship going to be stationary? If it's traveling, it's going to have to withstand the resistance of the matter (again, in case of our sun the density is $2*10^{−4} \frac{kg}{m^3}$, so depending on your velocity, this might just become a brick wall). If it's stationary, you will have to have a way of defeating the star's gravity.

In short, it's theoretically possible, but a civilization that could do it might already have a better alternative.

EDIT2: Come to think of it, if you're only after hydrogen or helium, why not simplify the problem and look for jovian planets instead of stars themselves? They would generally have a very similar composition, but the temperature and gravity would be far more manageable. Because of the lower temperature, there will be more complex molecules present, however. Planets will also be somewhat rarer (debatable, evidence suggests there might be more planets than stars) and more difficult to find. Also, it's not nearly as flashy.

Answer 4

Stars are very hot. You don't want to get too close to one. Certainly not close enough to pull matter away from one.

That is, most stars. Red giant stars might be manageable, but you would need some pretty aggressive cooling.

Also, "pulling matter" is not something you do in a scientifically accurate story. There are no tractor beams. Instead you travel through the outskirts of the star and simply collect the matter as you pass by.

You want to go fast to avoid getting too hot. But the faster you go, the harder you hit the matter you are trying to collect.

Both problems get better as you pass further away from the star, but there is less matter to collect. You might need to do many passes.

I am sorry that I have any clear numbers to give you, but this should at least give you somewhere to start researching.

This is not what you ask, but I think your best bet is to go find some nice cold ice asteroid to mine. You will more than enough hydrogen to fuel any reasonable sized ship. You will also find other useful stuff, like oxygen.

Answer 5

You could "pull" your fuel using a massive enough fishing hook, the same way white dwarfs in binary stars eat their larger sisters. A small neutron star could do. To be able to take your hook back you need a pretty massive ship (probably around the mass of the sun itself, so please don't come near this solar system). And a long unobtanium wire that doesn't melt (superconduces phonons, quickly radiating per surface unit) or breaks (almost infinite tensile strength). This scheme would allow you to keep some distance from the sun, but if you can manage neutron stars you may not care about the fission reactor or the sun's gravity.

Alternatively, just open a wormhole and drop one end to the sun's surface so that some of your fuel is carried to the other end (your very well insulated and reinforced fuel depot). Probably you need to bury the intake end under the surface of the sun in order to have some at the outtake end, your wormhole acting as a communicating vessel. In both cases, you should be careful of tidal waves, since gravity would as well cross your wormhole and try to rip your ship apart. Also, wormhole technology is a really powerful one, particularly recognized by its plot destroying capabilities. Consider you would need to find excuses for it not to work at domestic scales, apart of explaining why you have to get close to the sun to use it.

Maybe more approachable is being able to predict and intercept coronal mass ejections, at a distance of the sun that is safe for your ship but efficient for your mass collectors to work.

Answer 6

Assuming you don't want to get your nice spaceship (that you're riding in) too close to the sun, you could always throw something at the sun and let it skim the corona or even the surface, or skim a coronal mass ejection, collecting matter/heat/energy as it passes through, then you catch it on the other side.

If your ship's engines are strong enough to take you from star to star before you grow old and die, they should be strong enough to push a few rocks at a sun before you grow old and die too.

The sun's corona during a solar eclipse, it's big & relatively easy to throw things at. [Image courtesy Wikipedia By I, Luc Viatour, CC BY-SA 3.0]

A basic empty box or tube could be enough to collect plasma or solar matter or whatever you're interested in.

• "But I'm on a long range, long term mission, I'm not made of tubes to throw at the sun! They'll get all melty and ruined!"

Ok, how about you push an asteroid at the sun? It could pick up & carry enough solar matter with it for you to catch it on the other side & collect whatever you'd like. A hollowed out stone or metal asteroid might collect a lot of "sun stuff" on it's trip too, not just whatever gets knocked loose and comes along for the ride as with a solid asteroid. Or even push a comet.

• "A comet? Come on, it'll melt worse than my tube!"

Actually, comets have survived passing through the sun's Corona, like Comet Lovejoy here re-emerging from the sun's corona in 2011 or ISON in 2014,but since you get to pick your own more solid rock or metal ball (or whatever) to throw, your odds of survival should be better.

• "I just want to be close to a big explosion to collect released energy and stuff (more than a normal sun, that's too slow!)"

The above article also says a team led by John Brown, Astronomer Royal for Scotland, speculates:

If a comet is big enough and passes close enough, the steep fall into the sun’s gravity would accelerate it to more than 600 kilometres per second. At that speed, drag from the sun’s lower atmosphere would flatten the comet into a pancake right before it exploded in an airburst, releasing ultraviolet radiation and X-rays that we could see with modern instruments.

The crash would unleash as much energy as a magnetic flare or coronal mass ejection, but over a much smaller area. “It’s like a bomb being released in the sun’s atmosphere,” Brown says. The momentum propelled by the comet could even make the sun ring like a bell with subsequent sun-quakes echoing through the solar atmosphere.

If you could pinpoint where one of those will explode by throwing a comet there, it could provide lots of energy & matter to collect.

• "I give up, I'm just going to go find a nice cool gas giant"

Ok, fine, good idea I guess. But you can still throw stuff at gas giants, and it's a lot safer to get closer and catch the "splashback"

Answer 7

The pull will be dependant on the star's mass, not density. As for gathering fuel, you'd need to work out heat protection first, and radiological protection second - any star is, after all, a thermonuclear reactor, technically speaking. Thus you get a large amount of ionizing radiation output, especially up close (as radiation exposure scales up/down with distance. Our sun (and any other star too, for that matter...) could very well kill us with radiation if we got too close without shielding)

Answer 8

How about getting the hydrogen from molecular clouds? There should be some, and they are pretty large (up to hundreds of light years diameter). They aren't extremely dense (up to 10000 particles per cubic centimeters) but there might be lumps that could be collected. Their detection is a bit problematic, though.

Answer 9

An option I haven't seen presented is to release a tethered mass into the star's gravity well and harvest the resultant momentum.

For example, drop your accumulated trash, tied by a very long, thin, and very strong line (a braid of carbon tubes, for instance). Allow it to reach blistering speeds, then at some point transfer the energy to something. It could be a flywheel, or a chemical storage medium. For instance, it could spin a dynamo which results in electrolysis of a solution which releases energy as it winds down.

A more sophisticated society may have means to reverse (so to speak) nuclear reactions this way. Alternatively, you might have some captive microsingularities (baby black holes) which are strongly affected by a resulting electromagnetic field (electromagnetism being deemed a "stronger" force than gravity), reseating the singularities into position within a mechanism which releases their potential energy over time (maybe allowing one to fall toward another, turning a crank or generating heat).

Answer 10

As others have stated, the physics of such an attempt don't stack up too well. However, since you're in sci-fi, you can mess with what's possible a certain amount.

How about doing something similar to the Death Star Mk2 in The Force Awakens? In that film, the Death Star is seen to be 'sucking' material out of a star. You could conceivably do something similar...

For example, your super-ship could place into solar orbit a micro black-hole. That has such enormous gravity that it causes an amount of solar matter to be 'sucked' out of the star and into space, where it rushes towards the black hole, and a proportion of it will be 'sucked in' to the black hole. However, some around the edges will be moving so fast that it escapes the black hole's gravity and into the loading bay of your space ship (sort of like a gravity lens does with light).

When your tank is full, you somehow either dismantle the black hole (which is now whatever you put there + an awful lot of the star), or maybe better still is you 'push' it into the star itself. Therefore, the star gets its material back and presumably can consume or otherwise integrate the black hole you created. I guess you could initially put the black hole into a deliberately decaying orbit so that it will automatically fall into the star when you're full and out of the way.

Of course, how any of this gets done is up to you. Maybe your ship works by making its fuel super-dense, and so the micro black hole is just a blob of super-dense (spent?) fuel? You get a few 'green' points for that one because your spent fuel returns to the stars, or maybe you can redefine spacetime a bit and sort of 'punch' a dip into it to make the gravity (although it's theorised that 'dips' are created by gravity, not the other way around, so you may get into trouble there). You could go a bit 'Stargate' and open a wormhole to an actual black hole, and therefore have a point-source of gravity with which to create the 'lens'.

Ooh... this is fun :-)