Ion Drives: The Slow Rockets That Win Space

Introduction to ion drives

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0:30In space, patience is not merely a virtue. Sometimes it's a propulsion system. Sometimes the tortoise wins the race.

0:44We tend to imagine rockets as thunderous things. Pillars of fire, clouds of steam, engines roaring hard enough to shake buildings. That is what rockets have to be when they are lifting off from Earth, because Earth is a large planet with strong gravity and an enormous atmosphere that reacts poorly to anyone trying to leave. The sky is the limit, or at least the rule we seek to defy. But once you are already in space, the rules change.

1:15Out there, you don't need to keep fighting the ground. There is no runway, no sea, no hill to climb, and no air resistance constantly stealing your speed. A very small push, applied for a very long time, can eventually become a very large change. This is the secret of ion drives. An ion engine may push with less force than a sheet of paper, resting on your hand. It may accelerate a spacecraft so gently that, if you are standing beside it, you might not notice it moving at all.

1:50Yet that tiny push can continue for weeks, months, or even years. And after enough time, the slow rocket has quietly done what the mighty chemical rocket could not. Change its velocity by kilometers per second while using only a small amount of propellant. Today, we are looking at ion drives. How they work, why they are so efficient, why they already matter, where they fail, why it is so hard to increase their thrust,

2:22and why these slow rockets may one day be the engines that help turn space from a place we visit into a place we build. Because the future of space travel will not be all fire and fury. A lot of it may be a faint blue glow, pushing patiently through the dark.

The rocket equation loophole

2:42The Rocket Equation's Patient Loophole The basic problem of rocketry is simple and brutal. A rocket moves by throwing mass in one direction, so the spacecraft moves in the other. That is true whether the exhaust is hot gas, plasma, nuclear heated hydrogen, antimatter, or a stream of charged particles. Rockets do not push against air, they push against their own exhaust. The trouble is that a spacecraft must carry its propellant with it.

3:13If you want to go faster, you need to throw more mass behind you, or throw that mass faster. But carrying more propellant makes the spacecraft heavier, which means you need more propellant to move the propellant, and the rocket equation starts grinning at you like a tax collector. Chemical rockets are very good at producing thrust. They can lift payloads off planets, perform rapid burns, and land where gravity is trying to kill you. But their exhaust velocity is limited by chemistry.

3:43Chemical bonds only contain so much energy, and that sets a ceiling on how fast you can throw the exhaust. Ion drives attack the problem from the other end. They do not try to throw a lot of propellant. They try to throw a small amount of propellant extremely fast. Instead of burning fuel, an ion drive uses electricity. It takes a propellant, often xenon, strips some electrons from its atoms to create ions, accelerates those ions with electric or magnetic fields, and ejects them from the engine at very high speed.

4:17The spacecraft moves forward because the ions are being thrown backward. That is all an ion drive is, in the most basic sense. An electrically powered device for throwing charge particles very fast. But that simple idea changes mission design. Consider a specific impulse, or ISP. You can think of it as a measure of how efficiently a rocket uses propellant. It is closely related to your exhaust velocity. The faster you throw propellant out the back, the more impulse you get from each kilogram.

4:49It is measured in seconds because, in old rocket engine terms, it describes how many seconds, one pound of propellant could produce one pound of thrust. Conceptually akin to how long you could hover a rocket on Earth. That is a loose approximation, and hovering is usually a terrible way to get to space anyway, because every second spent hanging in the air is another second gravity is stealing energy from you. Chemical rockets burn propellant quickly because they need high thrust. You have to climb out of the atmosphere, fight drag and gravity losses,

5:22and build the sideways speed needed for orbit. Chemical rockets usually have specific impulse in hundreds of seconds. Ion engines can reach thousands. But they spread that efficiency over such low thrust, and such long firing times, they could never launch from Earth, the Moon, Mars, or any sizable world. At most, an ion drive might lift off from a tiny asteroid, where a person in a spacesuit could jump into space by the sheer power of their own legs. That does not mean ion drives violate physics.

5:53It means they trade thrust for efficiency, getting far more velocity change from each kilogram of propellant, if you can afford to wait. This is why ion drives are often described as high efficiency, low thrust propulsion, and both halves of that phrase matter. High efficiency means they can eventually produce a large delta V, the total change of velocity a spacecraft can make. Low thrust means they cannot produce it quickly. An ion drive is not the engine you use when you need to leap away from the launch pad.

6:23It is not what you use when you need to land on Mars, dodge debris at the last second, or throw the crew into their seats with heroically crushing acceleration. It is what you use when you can afford to be patient. A chemical rocket is a sprinter. An ion drive is a marathon runner.

How ion drives work

6:43How ion drives work. The classic ion engine is the gridded ion thruster. In one of these engines, neutral propellant atoms are fed into a chamber. Electrons collide with them, knocking loose more electrons, and creating positively charged ions. Then, a set of charged grids accelerate those ions out the back of the thruster. A neutralizer releases electrons into the exhaust, so the spacecraft does not build up a dangerous electric charge. Without that neutralizer, the spacecraft would be ejecting positive ions,

7:16and gradually charging itself negative, which would eventually pull the ions back, not to mention charged particles drifting in the void. This gridded design can achieve very high efficiency, and high exhaust velocity. But the thrust is low, and the grid suffers erosion over time. You're accelerating charged particles through delicate hardware for thousands of hours. It is a slow, polite cousin of the plasma rifle loved in science fiction, and generally speaking, weapons take a lot of beating when they fire.

7:48That hardware, ion drive or plasma gun, has to survive a slow sandblasting by plasma. Another major family is the hull effect thruster. Hull thrusters use electric and magnetic fields to ionize and accelerate propellants in a channel. They usually have lower specific impulse than the best gridded ion engines, but they can produce more thrust for their size and power, and they are extremely useful for satellites and deep space missions. They have become one of the workhorses of modern electric propulsion.

8:19There are also tiny electric thrusters, such as electrospray and fuel-emission systems, used for small spacecraft, precision pointing, and formation flying. At the other end are high-power plasma concepts, magnetoplasma dynamic thrusters, vasemir-like engines, and other systems where the equation shifts to where do we get all the power? Because that is the other side of ion propulsion. Ion engines are not hungry for propellant, they are hungry for electricity.

8:50A chemical rocket carries its energy in its propellant, burn the propellant, release the energy, throw the exhaust. An ion drive separates the energy source from the reaction mass. The propellant is mostly there to be thrown. The energy comes from solar panels, batteries, nuclear reactors, power beams, or some future power system. That is both a blessing and a limitation. It is a blessing because you are not restricted to chemical bond energy. If you have enough electricity, you can sell the exhaust to much higher speeds.

9:23In the extreme case, you could view the ship as having something like a particle accelerator on the back, throwing ions out at relativistic speeds. It is a limitation because electricity is not truly weightless. Solar panels have mass. Reactors have mass. Power converters have mass. Radiars have mass. If you want more thrust, you need more power. This is why ion drives are not magic. They save propellant by spending power and time.

Xenon and other propellants

9:54As mentioned, the most common propellant for ion drives has been xenon. And xenon is close to the gold standard for this job. It is a noble gas, so it doesn't bind other atoms to form molecules. It's chemically inert, and does not want to corrode your engines either. That is a very desirable trait in a propellant. You plan to run through an engine for thousands of hours. Xenon is also heavy. A xenon atom has an atomic mass of about 131, compared to about 40 for argon,

10:25and only about 4 for helium. That matters because thrust depends on momentum, which is mass times velocity. Double 1, and you double the particle's momentum. At the same accelerating voltage, lighter ions leave faster, but heavier ions carry more momentum per ion. So at 1 kilovolt, xenon ions leave at roughly 38 kilometers per second, argon about 70 kilometers per second, and helium at about 220 kilometers per second.

10:56That makes helium sound amazing. Do you remember that for the same beam power, higher exhaust velocity gives you less thrust? Xenon gives a slower thrust, but gives more push per watt, which is a particularly big deal if you are generating your power on board. As I said, momentum is mass times velocity, but kinetic energy of motion is one half mass times velocity squared. So to double the speed takes four times the energy, but only doubles the momentum.

11:28If you have propellant mass to spare, heavier particles are handy because they give more shove per watt. If you have energy to spare and need extreme speed, lighter particles become more attractive. But xenon is also relatively easy to ionize. Distributing the first electron from xenon takes about 12 electron volts, compared to about 16 for argon, or around 30% more, and about 25 for helium, more than double. Per kilogram, the difference is even more dramatic, because helium atoms are so light

11:59that you need vastly more of them. Ionizing a kilogram of xenon takes about 9 megajoules, argon about 38 megajoules, and helium about 590 megajoules. For context, a gallon of gasoline stores about 120 megajoules, while gasoline and kerosene are worth around 40 to 45 megajoules per kilogram. Kerosene, or refined RP-1, is one of our classic rocket fuels. Now, that ionization costs becomes less important

12:30and very high-voltage engines, because accelerating each ion through 10 kilovolts, or even a megavolt, dwarfs the few dozen electron volts needed to ionize it, but for practical modern spacecraft engines, especially lower-power ones, xenon is a much nicer compromise. There is also storage. Xenon is dense and compact compared to lighter gases, 33 times denser than helium at the same pressure and temperature, so a spacecraft can cure useful amounts that have certainly large tanks.

13:01You might cover these tanks with solar panels or microwave power receivers, but that's more of a silver lining to the issue that bigger tanks cost more mass. Helium is especially annoying here. It is hard to store, leaks enthusiastically, and gives poor thrust unless you're willing to spend enormous power. But xenon has a problem. It is rare and expensive, around a thousand bucks a kilogram, which isn't a big deal for a billion-dollar space probe, but starts being a big deal

13:32for constellations of satellites with ion drives for station-keeping, whereas argon is under a dollar a kilogram. Argon makes up almost 1% of Earth's atmosphere, while helium is only a few parts per million, and xenon is less than a tenth of a part per million. So argon is more than a hundred thousand times more common in the air than xenon. Helium is cosmically abundant, but not conveniently concentrated near Earth, and is still awkward to store and handle.

14:02That matters if we imagine a true space economy with thousands of tugs-in freighters, or a megaton colony ship that cannot casually fill its tanks with one of Earth's rarest atmospheric gases. So xenon is probably the premium propellant for today's ion drives. Heavy, inert, compact, and easy to ionize, argon is the cheaper industrial workhorse candidate. Helium is usually a dreadful propellant for ordinary ion drives, but in a far future ship with megavolt accelerators,

14:33enormous power supplies, and a need for extreme exhaust velocity, helium, or even lighter propellants, might make more sense. For example, if we assume single ionized atoms accelerated through a megavolt, xenon comes out at about 1210 kilometers per second, or at about 0.4% of light speed. Argon comes out at about 2200 kilometers per second, or about 0.7% of light speed. Helium comes out at about 6940 kilometers per second,

15:04or about 2.3% of light speed. That difference matters when the ship is trying to reach speeds near or above its own exhaust velocity. To reach 1% of light speed, and then slow down again at the destination, helium needs a mass ratio of only about 2.4 to 1. Argon needs about 15 to 1. Xenon needs about 140 to 1. Push the target to 10,000 kilometers per second, or 3.3% of light speed, with braking at the end, and the mass ratio jumps to about 18 to 1 for helium,

15:368,800 to 1 for argon, and roughly 15 million to 1 for xenon. So xenon is better for practical thrust. Helium is better when the alternative is bringing a second colony ship made entirely of propellants. But for fast interplanetary travel, rather than interstellar precursor speeds, xenon and argon can still win big. If your goal is around 300 kilometers per second, or a tenth of a percent of light speed, xenon provides that exhaust speed at about 60 kilovolts.

16:07Argon can do it at about 20 kilovolts. Helium can do it under 2 kilovolts, but at that point, helium's extreme exhaust speed is mostly overkill, while its poor thrust and miserable storage remain very real problems. Megavolt electric propulsion is not forbidden by physics, and we can generate those potentials today in large accelerators. Gigavolt ion energies are also routine in particle physics, but not as compact spacecraft thrusters. For a starship-scale electric rocket,

16:38the hard part is not inventing a big voltage. It is handling the power, current, insulation, beam neutralization, waste heat, and erosion, while throwing tons or millions of tons of propellant instead of a laboratory trickle. For my part, I expect the truly fast versions of this would eventually blur into beam power portion. You might push a ship with high energy ion exhaust whilst velocity is still below what the engine can efficiently provide, gaining power from a laser or energy beam,

17:09then switch to laser sails or other beam-driven systems once the ship is moving faster than its own practical exhaust. We have discussed that sort of thing in other episodes, like interstellar relays. And that is a recurring theme in space development. The best technology is after not the best in isolation. It is the best fit for the mission, the power supply, and the supply chain.

17:34From exotic engine to space workhorse.

17:39Iron proportion has already flown successfully many times. Deep Space 1 was one of the early proof-of-concept missions for Deep Space Ion Proportion. Don was an especially famous demonstration, using Ion Proportion to visit Vesta and Ceres, orbiting one body, leaving it, and then orbiting another. That is the kind of mission profile Ion Drives make possible. A Capcom spacecraft could do it, but would need much more propellant or a much more constrained mission. Electric proportion is also common in satellites.

18:11Many satellites use electric thrusters for station keeping, maintaining their orbital slots, or gradually raising orbits. It is often slower than chemical propulsion, but propellant mass is money. More recently, solar electric propulsion has become a standard part of serious mission planning. It is attractive for missions to asteroids, Mercury, and other destinations where patience and efficiency matter more than raw thrust. And that brings us to where Ion Drives shine. They are excellent for missions

18:42where spacecraft is already in space and it is time to spiral outward, spiral inward, rendezvous with small bodies, or slowly adjust its orbits. Asteroids are a natural target. They have weak gravity, complicated orbits, and enormous scientific and industrial value. An Ion-powered spacecraft can take its time matching velocity with an asteroid, surveying and sampling it, or perhaps one day moving my material around the solar system. The last point matters because Ion Drives are not just

19:12engines for probes. They are engines for logistics.

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The trucking industry of space

19:48The trucking industry of space. Humans are in patient cargo. We complain, eat food, require shooting, and generate paperwork. But water, metal, spare parts, propellant tanks, food packets, reactor components, and habitat modules generally do not mind taking the slope out. So a future Mars program, lunar base, or asteroid settlement might use chemical or nuclear thermal portion for crews,

20:18while ion tanks slowly move cargo ahead of time. The crews arrive faster, the supplies arrive cheaper, the machines do the boring work as machines are meant to do. This cargo role also matters for cis-lunar space. Imagine reusable electric tugs moving supplies between low-Earth orbits, high-Earth orbits, lunar orbits, and depots. They do not need to be glamorous, they just need to be reliable. The history of civilization is not written only by explorers,

20:49is written by carts, ships, canals, railroads, trucks, and containers. Ion drives are a candidate for the trucking industry of near space. Now if we build large space habitats, solar power satellites, asteroid mines, or industrial platforms, we will need a lot of trucking. This is why ion drives should not be judged only as engines for probes. They are engines for logistics. Exploration gets headlines. Logistics builds civilizations.

Solar and nuclear power sources

21:19Solar power, nuclear power, and the tyranny of electricity. Now the power source determines a lot. You can build a higher thrust ion drive with a higher power generator. There is more to the problem than just throwing more power at it. Solar electric propulsion is the obvious option for the inner solar system. Solar panels are proven, scalable, and do not require carrying fuel for the power plant. Near Earth, solar power is abundant. Farther from the sun, solar power fades with the square distance.

21:50At Jupiter, sunlight is only about 1 25th as intense as near Earth. At Saturn, it's about 1%. You can build bigger arrays, but there are limits. Large arrays are massive, fragile, and awkward. They complicate maneuvers and create engineering headaches. Still, solar electric propulsion is a powerful tool from Earth's neighborhood through the inner solar system and into the asteroid belt. For the outer solar system, nuclear electric propulsion becomes much more attractive.

22:20A nuclear electric spacecraft uses a reactor to generate electricity, then uses electric thrusters to accelerate propellant. This is different from the nuclear thermal propulsion where the reactor directly heats propellant and throws it out. Nuclear thermal gives higher thrust, nuclear electric gives higher propellant efficiency, at the cost of lower thrust and a more complex power conversion. Nuclear electric propulsion could be very useful for outer planet missions, heavy cargo, asteroid belt logistics, and crewed mission support, but it is not a magic torch drive.

22:53The reactor has mass, the shielding has mass, the power conversion system has mass, the radiators may be enormous because in space, getting rid of race heat is not optional. Thermodynamics always sends the invoice. Still, nuclear electric propulsion is one of the plausible bridges between today's robotic ion missions and tomorrow's serious deep space infrastructure. If you put megawatts of electric power on a spacecraft, electric propulsion changes character. With enough power,

23:24you can move substantial payloads on useful schedules. With gigawatts, the skill of serious rocket launch power, at least for a few minutes, you can get real thrust, assuming your ship is not gaining mass faster than gaining power generation, is still not viable in a thick atmosphere. You probably still have a terrible thrust-to-weight ratio and your ionized plasma beam is slamming to air molecules almost immediately, adding high voltage, arcing, sparks, and the fact that your spacecraft is starting to resemble

23:54a lightning machine with a guidance computer is not getting you off of Earth. It might have niche uses in very thin upper atmospheres, or around worlds with much thinner air, like Mars, but is not an airplane engine. That leads to one of the most common questions. Can ion drives take humans to Mars? The answer is, perhaps, but not in the way people often imagine, and definitely not for the landing. A small ion engine like those used on robotic probes

24:24is not going to push a crewed Mars ship quickly. The acceleration would be too low, and the trip could take too long. Human missions care about time, because time means life support, radiation exposure, medical risk, psychological strain, and fewer abort options. But electric propulsion could still be vital for human Mars missions. It could pre-position cargo, it could new fuel, habitats, water, shielding, and equipment before the crew departs.

24:55It could help assemble mission architecture in Earth orbit or lunar orbit. A high-powered nuclear electric ship might transport cargo or even crews if the system is powerful enough, reliably, enough, and mission planners are comfortable with the trajectory. High-powered electric propulsion could also be useful for maintaining larger transportation systems like Aldrin cyclers, skyhooks, and votivator tethers. Or an Aldrin cycler with a votivator tether extending from it, which I don't recall anyone suggesting before,

25:26but strikes me as an awesome combo. Aldrin cyclers are big spacecraft, even converted asteroids, on highly elliptical orbits that pass two celestial bodies regularly, like an Earth-Mars cycler. They can carry tons of shielding and life support supplies like hydroponic bays, and shuttles go to and from them as they pass a planet. Themselves lighter for not needing all that travel gear and shielding. They could be rotated for spin and gravity, and if you're tether coming off that, its end is moving a lot faster.

25:57In this case, any tether lets a shuttle or cargo pod be snatched by the cycler with a modest velocity difference, and reeled in or flung away, and then the cycler regenerates that momentum over several months from its solar panels or nuclear reactor. And while that shuttle could never use an ion drive itself, it could be carrying ion drive fuel for the Aldrin cycler. Anyway, those tether systems can give spacecraft quick momentum exchanges, while electric proportions slowly restores

26:27that momentum over time. That is one clever way to turn low, steady thrust into fast, useful transportation. So ion drives probably do not replace chemical rockets for launch, landing, or fast crew transfers. They complement them. A mature space economy will use many propulsion systems. Chemical rockets, nuclear thermal engines, electric thrusters, mass drivers, tethers, sails, aerobraking, and eventually more exotic systems. The right question is not which engine is best,

26:58but best for what? For leaving Earth, use high thrust. For landing on a planet, use high thrust. For emergency maneuvers, use high thrust. For slowly moving cargo across millions of kilometers, use patience. Ion drives are patience made mechanical. What ion drives cannot do. Of course, patience does not remove engineering problems. Ion thrusters must operate for thousands or tens of thousands

27:29of hours. Their grids erode, their channels wear down. Cathodes fail. Power processing units must survive years of operation. Solar arrays degrade. Thermal systems have to keep everything in the right temperature range. The spacecraft interacts with its own plasma environment. Tiny effects accumulate because the whole point of the engine is to run for a very long time. Longevity is not a bonus feature for ion drives. It's a core requirement. A chemical rocket engine may only

28:00need to burn for minutes. An ion thruster may need to work for years. It is not easier. It is difficult in a different direction. There is also a trajectory complexity. And they have trouble exploiting the Oberth effect, where a rocket gains more useful orbital energy by burning hard while it is already moving fast, usually deep by gravity well. Chemical rockets can do that because they can dump a lot of thrust into a short periapsis burn. Ion drives usually cannot. Their thrust is spread out

28:30over hours, days, or months. So they tend to smear out the maneuver and lose much of that advantage. A high-power electric ship might still plan its thrusting to favor periapsis, but it's still a real downside, because the Oberth effect is one of the classic ways mission designers turn a short burn into a large gain in orbital energy. A chemical rocket often performs impulsive burns, fire the engine hard for a short time, then coast. Mission planners can approximate

29:01that as an instant velocity change. Ion drives our continuous or semi-continuous low thrust systems. The spacecraft is changing velocity all the time, which makes trajectory design more complex. That was a bigger concern in the past. In an era of cheap and rapid calculation, it is not a trivial problem, but no great limiter either. But the result can look strange compared to classic spaceflight. Ion-powered spacecraft may spiral outward from Earth rather than depart in one

29:31dramatic burn. They may slowly reshape their orbit over months. They may approach a target, not with a single braking maneuver, but by gradually matching its path. That is less cinematic, but very practical. And in real spaceflight, practical tends to beat cinematic, though with worse soundtrack options. My guess though is that this is where skyhooks and rotavators would come in, as they can kick a spacecraft to high speed quickly, and while folks tend to think of them as a way to get off Earth, they actually work

30:02better and easier in high orbits. Ion drives also invite misconceptions. They are not reactionless drives. They still throw mass out the back. They are not warp drives. They do not cancel inertia. They cannot lift off from Earth under their own thrust. They cannot make propellant irrelevant. They are not free energy machines. And they are not useless just because their thrust is tiny. They are real engines with real strengths and real weaknesses. That may be the most important

30:32lesson. In science fiction, propulsion systems are often ranked like weapons in a game. Chemical rockets are primitive, high end drives are better, fusion drives are better still, anti-matter drives are the end game, and someone eventually discovers hyperspace by reversing the polarity of some made-up field. But real propulsion is not a ladder, it's a toolbox. A hammer is not obsolete because a screwdriver exists. A cargo ship is not useless because jet aircraft exists.

31:02The slow vehicle may be the one that moves most of a civilization's mass. Chemical rockets will still matter. Nuclear thermal rockets may matter. Tethers, sails, mass drivers, air braking, and beam propulsion may all have their roles. Ion drives fit into that toolbox as engines for efficient, patient motion. They let us trade time and electricity for propellant savings, and in space, that trade can be extraordinarily valuable. This matters because propellant is not just fuel.

31:33Propellant is mass that had to be launched, mined, processed, stored, insulated, transferred, and protected. Every kilogram saved can become more instruments, more shielding, more cargo, more lifetime, or less cost. In the early space age, the big challenge was reaching space at all. In the next age, the challenge was moving around space affordably. Ion propulsion is one of the tools that helps us do that. A lot of what we

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The engines that build starships

33:06The Engines That Build Starships Now, can ion drives take us to the stars? Not the ion drives we had today, at least not on human timescales. A Don-style engine is not an interstellar drive simply because it is efficient. Even very efficient engines need power, propellant, and time. And getting to a significant fraction of light speed is a very different problem from reaching an asteroid or Mars. Far-future electric rockets are another question. If you have megavolt or

33:36gigavolt accelerators, huge power supplies, beamed energy, and propellant streams measured in tons rather than grams, the electric portion begins to shade into the territory of interstellar precursor craft. But by then, we are no longer talking about today's ion drives. We are talking about spacecraft that look partly like ships, partly like power stations, and partly like particle accelerators. But even if ordinary ion drives never become starship engines, they might still help build an interstellar future.

34:06Before you launch starships, you need industry. You need power collectors, mining operations, fuel depots, telescope arrays, habitats, shipyards, and perhaps beam stations. They must be moved, assembled, maintained, and supplied. Ion drives may help build the infrastructure that later enables faster propulsion. That is their proper place in the grand hierarchy. They are not the starship engine of our wildest dreams. They are one of the engines that builds the civilization that builds the starship.

34:38And that may be more important. Ion drives are not dramatic. They are not loud. They do not shake the ground. They do not pin astronauts to their couches or carve fiery paths to the sky. Whisper. But in space, a whisper can last for years. And given enough years, that whisper becomes velocity. Given enough velocity, it becomes access. Given enough access, it becomes industry. Slowly and steadily, these fake blue

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