Five years ago, I made the (questionable) decision to launch my Substack over Thanksgiving weekend. So every Thanksgiving or thereabouts, I do a roundup of seven important themes from the past year, along with a few themes to watch for in the upcoming year. Here’s last year’s edition. The links below are all links to other posts I wrote over the past 12 months, so you can use this post as a reference for what I wrote about in 2025.

I’d also like to thank everyone for reading and supporting Noahpinion. A year ago this blog had 280,000 readers; now it has 414,000. I never expected my blog to get that big, and I’m incredibly grateful to all of you for helping to make that happen. Please remember to recommend Noahpinion to your friends, family, and coworkers! I also published a book this past year, though so far it’s only in Japanese; this upcoming year I’m going to write an English-language book about macroeconomics, so be on the lookout for that.

Anyway, here are the seven themes for 2025.

Tariff madness

This was the year that Donald Trump, true to his campaign promises, upended 70 years of American economic policy. On April 2, which he dubbed “Liberation Day”, Trump announced truly enormous tariffs on almost all of the countries in the world. Many of these tariffs were eventually walked back, sometimes after “deals” in which other countries made various promises to the U.S. and/or to Trump and his family. Thankfully, none of the worst-case scenarios have yet reared their heads.

But some tariffs remained in place, and these tended to be tariffs on America’s allies rather than on China. And general uncertainty about future tariffs has exploded. This, along with worries about U.S. political unrest and national debt, has led to a depreciation of the dollar as some investors hedged their bets by moving money out of the country.

The tariffs haven’t yet tanked the economy or raised inflation, but they’re exerting a corrosive influence on the economy, pushing up prices, weighing on employment, and hurting the manufacturing sector. This was entirely predictable; economists have long understood that tariffs on intermediate goods hurt manufacturing by disrupting and shrinking supply chains. Trump’s team, unfortunately, makes it a point of pride not to listen to economists, instead choosing to invent a blizzard of dubious ad-hoc justifications for the President’s whims.

In fact, one reason Trump was able to get away with his tariff policies was that a great deal of B.S. and myth has grown up around trade and trade economics. Lots of people persist in thinking that trade deficits make countries poorer, because of the way GDP is broken down into components. But they do not. It is now widely accepted that globalization hollowed out the American middle class. It did not; in fact, nothing did. The whole case for Trump’s tariffs was based on misconceptions.

Which is a shame, because pure free trade is not the ideal policy. If America were smart about strategic trade, we could craft approaches that would enhance national security, protect infant industries, help American companies scale up, and discourage Chinese mercantilism. This would require America to trade freely with allies while putting tariffs on China, and to implement the kind of Biden-esque industrial policies that Trump had disdained. But at this point, no one in the halls of power seems to be thinking strategically, listening to experts, or worrying about details. And so the madness continues.

The AI boom (and possibly, the AI bust)

The biggest reason that the U.S. economy is still doing OK, despite the pressure from tariffs, is the AI boom. Data centers are being built out at a stupendous rate, exceeding the 1990s telecom boom and drawing comparisons to the railroad boom of the 1800s. A lot of people are worried that this construction bonanza is being financed by shady private credit deals that could hurt the macroeconomy if the AI sector goes bust.

That possibility has fueled a lot of debate over whether AI is as useful as its proponents believe. But this debate misses the key point that railroads and telecoms were ultimately even more useful than their proponents believed, and yet both still experienced busts along the way. If AI fulfills everyone’s wildest dreams, but slightly too slowly to pay back the data center loans, there could still be carnage in the financial markets.

An AI-driven financial bust could also happen if the AI industry turns out to be much more competitive than the traditional software industry. There are plenty of essential industries that make low profits — airlines, solar panels, and traditional agriculture come to mind. Traditional software depends a lot on human capital (engineers), but AI depends a lot on physical capital (compute), so it could end up being a lot more competitive of an industry.

In any case, it seems inevitable that our economy is going to make a giant macro bet on AI.

Meanwhile, a lot of people continue to worry that AI is going to take lots of people’s jobs. But nobody really knows whether that will happen, and the people who make strong claims about it are just being overconfident. So far, it looks as if industries more exposed to AI have been hiring fewer entry-level workers, but hiring more experienced workers. So it’s possible that AI is biasing the playing field toward people with more experience…or it’s possible companies just over-hired young workers back in 2021 and are now correcting. Only time will tell.

The rise of the Electric Tech Stack

AI is only one of the big inventions remaking the world right now. The other big one is what Sam D’Amico and I call the Electric Tech Stack — batteries, electric motors, and power electronics. Together, these technologies have made it economical to use electric power instead of combustion in a large variety of applications — cars, appliances, certain industrial processes, and power generation itself — in addition to enabling lots of new robots, drones, and so on.

The problem is that America is falling way behind China in terms of mastering the Electric Tech Stack. Americans seem to have collectively decided that the Electric Tech Stack is all about climate change, and so it has become a culture-war football, with Republicans trying to cancel battery manufacturing. Very few Americans seem to understand that as battery-powered drones master the battlefield, whoever can build more batteries and motors will rule the skies. The Electric Tech Stack is about power, first and foremost.

(Of course, saving the world from climate change isn’t nothing. Right now it’s China that’s doing that, by flooding the developing world with cheap solar panels and batteries.)

Anyway, I’m pretty worried that Americans’ rejection of the Electric Tech Stack is a sign that they’re starting to fear the future itself. It’s possible to see anti-electric sentiment as of a piece with anti-AI fears, antivax craziness, and various other anti-tech outbursts. If so, it’s a very bad sign.

The Chinese Century and its discontents

This year was really when China’s ascendance over the developed democracies became apparent. Trump’s battles with domestic opponents, isolationist instincts, and eagerness to start fights with allies have accelerated the rise of Chinese power, and helped to undo much of the damage China did to its own image through “wolf warrior diplomacy” during the Biden years. Meanwhile, China’s economy is now bigger than America’s by most reasonable measures, and it dominates the manufacturing technologies that would prove decisive in a protracted military confrontation.

It’s therefore safe to say that we’re now living in the Chinese Century. Demographic and macroeconomic factors will present headwinds for China, but won’t be able to knock the country off its perch.

But even at its glorious peak, China’s civilization may prove underwhelming in certain ways. It’s a scientific and technological leader, but it doesn’t yet seem to be driving breakthrough progress the way the U.S., Britain, Japan, and Germany did in their heydays (and which the U.S. still does). It’s cities are visually impressive, but sprawling messes on the ground.

Meanwhile, China is making some mistakes. In recent years it has unleashed the biggest industrial policy push in the history of the world, intent on replacing the real estate industry and filling the hole left by that industry’s collapse. But by paying a bunch of Chinese companies to compete each other’s profits to zero, China unleashed “involution” that is hurting corporate balance sheets and causing deflation. And thanks to that involution, China’s people are working hard but seeing relatively few benefits. As Dan Wang wrote in his popular book Breakneck, China’s leaders are a bit too focused on feats of technical and social engineering. and not focused enough on making their people happy.

And in the distance, Xi Jinping’s succession looms. The most powerful Chinese leader since Mao is 72, but he has not yet picked an heir apparent, and appears intent on ruling well into his dotage. Either a superannuated leader or a vicious succession battle could present major problems for China.

The ongoing collapse of progressivism, and the rise of new ideas on the left

Trump has generally been a terrible President in his second term. The only thing still keeping him afloat — indeed, the only thing that allowed him to win in 2024 despite all he’s done and said — is the American public’s deep disdain for the Democratic Party. Some of that disdain is due to Democrats’ seeming weakness in confronting Trump. But a lot is due to general exasperation with the progressive movement, which has lost credibility on a wide number of fronts in recent years.

On one issue after another, progressive approaches have proven inadequate to America’s needs. Many progressive state and local governments have gone soft on crime, allowing a breakdown in public order that victimizes the most vulnerable and also makes it politically impossible to build dense transit-rich cities. Progressive procedural requirements have made building infrastructure, transit, housing, and green energy very difficult in America — thus hampering a lot of progressive causes. The progressive approach to education has emphasized “equity” instead of teaching kids how to do math.

Ideologically, progressives remain enamored of the disastrous idea of degrowth. They’ve ignored or dismissed the recent successes of free-market economics, instead blaming “corporate feudalism” for America’s problems without much evidence. Progressive online culture, once so dominant, has become downright ghoulish, cheering the murders of business executives and spending untold hours trying to “cancel” each other on Bluesky and in other deeply progressive spaces. Progressives have embraced “land acknowledgements” that might seem harmless, but which ultimately delegitimize the U.S. as a country. And so on.

Some progressives have begun to think of useful alternatives to the canon inherited from the 2010s. Derek Thompson and Ezra Klein published a blockbuster book called Abundance that proposed a new progressive agenda based on getting Americans more material stuff. Some progressives leapt to attack the new book, but their attacks fell short and looked petty. Hopefully the abundance movement will be able to steer progressivism out of its current dead end.

Besides embracing abundance, progressives also need to moderate on all the issues where their approaches have demonstrably failed — and to be honest with themselves and with their activist base about why they failed in the first place. The liberalism of Bill Clinton and Barack Obama, and the idea of liberal nationalism itself, represent a much better way forward for the Dems than an accelerating slide into radicalism. Fortunately, the recent No Kings protests displayed plenty of patriotism, suggesting that the tide may be turning on the left more generally.

Trump’s gangster regime

This whole year was lived in the shadow of Donald Trump’s 2024 election victory. Trump’s first term turned out to be relatively benign, and even featured some important successes. There was always the possibility that his second term would be similar. Sadly, this was not what ended up happening.

As I predicted, Trump has spent much of the early days of his administration feuding with American institutions — the media, the Fed, the courts, the electoral system, and so on — and threatening his domestic enemies. As it became apparent that institutions weren’t resisting him as they had in his first term, Trump began to overreach, issuing a blizzard of executive orders that assumed far more executive power than U.S. Presidents have exercised in the past except in the middle of total war.

Eventually the Supreme Court emerged as the one institution in the country that Trump wasn’t willing to openly defy, producing an uneasy stalemate. But the U.S. now feels like a much more authoritarian country than it did a year ago. This sadly fits with the global trend toward strongman rule.

The assassination of Charlie Kirk by a leftist radical prompted Trump and his allies to issue dire threats against Democrats, and to threaten restrictions on freedom of speech. Some Trump officials even used the rhetoric of civil war for a few weeks after the assassination.

In foreign policy, Trump has abandoned the liberal nationalism that marked both Republican and Democratic administrations since World War 2, acting instead like a bully trying to extract tribute from weaker nations. Although he did manage to broker a successful cease-fire in Gaza, Trump’s abdication of America’s traditional stabilizing role in world affairs has pushed the world a bit closer to war.

But despite the ferocity of its rhetoric and its reckless assumption of power, the MAGA movement feels strangely weak. Contrary to the hopes of its adherents, Trumpism is not building any new communities, institutions, or organizations in America; instead it’s just a blast of mostly online rage. And Trump’s policies seem startlingly incompetent, from his rejection of vaccines to his infinite tax cuts that explode the national debt. The Tech Right, which was supposed to provide an injection of elite human capital into the MAGA movement, has instead withdrawn after the failure of DOGE. International actors are quietly laughing at America’s fumbling lack of competence.

And the biggest question — what happens to the right after Trump and his personalistic rule are gone — has yet to be resolved. The most likely scenario is that without Trump’s personal charisma to hold it together, MAGA will become more ideological, conceiving of itself as a crusade to save Western civilization from immigration and liberalism — in other words, a typical right-wing movement. Whether that sort of movement can succeed in America has yet to be seen.

America’s identity crisis

Why is America being forced to choose between Trumpian gangsterism and dysfunctional progressivism? The fundamental reason is the age of sociopolitical unrest that began in the U.S. around 2014. That era of unrest is slowly fading at the grassroots level, as Americans tune politics out, but we’re still dealing with the institutional and political consequences.

Fundamentally, unrest was touched off by social media, which threw Americans all into the same room as each other, destroying our ability to spread out and tolerate our differences from afar. Curbing social media use among the youth and fragmenting the internet into more private, curated conversation spaces will help undo some of the damage. But the more fundamental crisis — the thing social media revealed — is a crisis of identity. As America diversifies, ethnic notions of nationhood are being strained, even as the internet fragments our cultural unity.

This identity conflict was most intense in 2014-2021, but it’s still roiling, especially online. Anti-Indian sentiment has risen on the right, and antisemitism has made a comeback on both the right and the left. Immigration, once cast as an economic issue, is now the main culture-war flashpoint, with both rightists and some progressives seeing it as a tool for reengineering the American populace. The Trump administration’s xenophobic policies are a reflection of that conflict.

And in the background of all of this is the end of the aftermath of World War 2. The liberal, tolerant values that emerged from the resolution of that conflict have weakened as the generation that fought for them has passed away.

Still, I am optimistic that America will eventually resolve its identity crisis. These things have happened in the past, and the nation always emerged stronger after a period of unrest and division. We just might pull it off again.

Looking ahead to 2026

If 2025 was Trump’s blitzkrieg, 2026 will be a year of retrenchment. The administration’s falling popularity, as well as emerging divisions between various factions on the right, will act as a partial check on Trump’s program. So I predict a more static, less terrifying year in the world of politics and policy.

The Democrats, meanwhile, will be energized by Trump’s low poll numbers and by the likelihood of retaking the House of Representatives in next year’s election. That feeling of confidence will paper over some (but not all) of the bigger divisions and recriminations that followed Trump’s victory in 2024. But there will still be a constant ongoing debate between those who want to take the party in a more leftist direction — led by the charismatic Zohran Mamdani — and those who want to moderate in order to win.

Economically, the big looming question is whether there will be an AI crash. I personally doubt that 2026 is the year — the big AI “hyperscalers” are still funding too much of the data center build-out with their own considerable cash reserves, and borrowing still hasn’t hit the absurd levels that we usually associate with a major crash. The continuing boom will keep the economy mostly afloat, though we’ll keep seeing some signs of tariff-related deterioration as more tariffs go into effect.

If, as I expect, the AI boom continues for another year without a crash, it’ll convince a lot of people that a crash is never coming — which of course will induce more reckless borrowing, and set things up for an actual crash in the years to come.

The international situation is the biggest question mark by far. Will this be the year that China’s leaders finally decide to pull the trigger and invade Taiwan? If so, will Trump try to put up a fight or just let them have it? Will this be the year that Trump finally pulls the trigger on aid to Ukraine, and washes his hands of the conflict? What effect will that have on the war? Will Europe step into the gap? I don’t know the answer to any of these questions, but the danger of a further expansion of great-power war is certainly there.

All in all, Trump’s loss of momentum, the looming uncertainties of an AI crash, AI itself, and the volatile international situation mean that 2026 is likely to be a year where America finds itself at the whim of events beyond our control.

But whatever happens, I’ll be there to write about it and hopefully to explain what’s going on. Hang in there, and enjoy another year of Noahpinion.

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“Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.” Douglas Adams, The Hitchhiker’s Guide to the Galaxy

Our vision for the future has humans traveling between planets much faster than our ancestors sailed across oceans, but no existing rocket technology can achieve that. We’re going to need something significantly more energetic, and antimatter is the key.

In August 2024, I wrote a primer examining the merits of antimatter propulsion from first principles. In this post, I will lay out in some detail a specific plan for an antimatter “Manhattan Project”. If we don’t do it, someone will.

Credit: Avatar: The Way of Water. Canonically, these spaceships use antimatter fusion drives.

Why?

Chemical propulsion is the standard for launch today, while some satellites use electric propulsion for station keeping.

Credit: Trevor Mahlmann for SpaceNews

Starship is terrific but it’s not capable of flying to a nearby star at 30% of the speed of light and then landing. For that we’re going to need something far more energetic, and we are fortunate to have it.

Antimatter is powerful because its embodied energy gets to use the equation.

E = mc².

c² is a very large number, approximately 10¹⁷ = 100000000000000000. When antimatter encounters ordinary matter, it annihilates completely converting a tiny amount of mass into a huge amount of pure energy. This is 100-1000x more energy than even the most energetic nuclear fission reactions.

With rocket propulsion, the distance you can go is determined by the change in velocity, Δv (“delta vee”), you can achieve with all the fuel you brought. The Tsiokolsky rocket equation boils down to Δv ~ 2 v_e, the exhaust velocity, while thrust is given by the mass flow times the exhaust velocity, T = ṁ v_e. Exhaust velocity is usually expressed as specific impulse (Isp = v_e/g) and measured in seconds. It is the amount of time a given propellant can generate 1g of thrust. A more powerful propellant can provide the thrust for longer.

This graph shows the rough domains of existing and hypothetical propulsion systems. Chemical can generate terrific thrust, but is limited by low specific impulse. Electric propulsion can achieve much higher specific impulse, but is plagued by low thrust. This applies also to nuclear electric propulsion systems, which enjoy all of the hassles of nuclear reactors and still don’t achieve the desired high thrust, high Isp, high power operating mode.

Credit: Adapted from graph by Frans Ebersohn.

An antimatter rocket cycle can bridge this gap. Like chemical propulsion, there’s no upper limit to thrust. And given that the default antimatter reaction product is hard gamma rays, there’s no real upper limit to Isp either. If humans ever find a way to cross the gulf between stars, it will be with antimatter powered propulsion.

How

I could spend another 10,000 words singing the praises of antimatter propulsion, but if you’re not bought in at this point, why bother? Let’s focus on the how.

Usually, when talk begins of exotic propulsion methods, discussion immediately centers on particle accelerators and superconduction magnets. Hold it right there! We’re trying to launch this on a rocket. Let’s conceptualize around a Starship upper stage, so we’re talking 1000 T of propellant, 100 T of structure, and 10 T of engine. Launch is a dynamic environment, which means everything needs to be able to withstand shock and vibration. I like particle accelerators as much as the next guy, but let’s begin by deleting as much complexity from the critical path as possible.

Let’s discuss the various parts of the antimatter problem that need to be solved: Production, storage, and use.

Production

The model for antimatter is that it is produced on Earth using the power and skill of our entire industrial base. Like aluminum, but to a far greater extent, it exists as an extremely condensed form of stored energy that can then be readily transported into space. We cannot easily lift the entire grid of the US into space, but its 1.3 TW capacity, run for an entire year, condensed into antimatter, would weigh just 227 kg (less than 500 lbs), which is well within our launch capacities. 227 kg of antimatter is also easily enough to launch hundreds of enormous spacecraft to nearby stars, so we will begin with a somewhat more modest quantity.

As of late 2025, humanity is able to produce antiprotons and antihydrogen in the thousands of atoms per day and millions in total. This is incredibly impressive even by the standards of a decade ago, but it’s roughly analogous to our plutonium production capacity in late 1940. We have a ways to go here.

Antimatter production is something like 0.000001% efficient. It requires quite large particle accelerators and vacuum storage rings.

The good news is that even at this efficiency, I think it’s worthwhile to scale up production for deep space propulsion, which is astoundingly expensive and profoundly limited by default. Remember, getting some marginal Δv when you’re a long way from home is essentially completely inelastic. There are no other options, and the solar system is the size it is.

Obviously the utility is enormously increased if the production cost can be brought down, so the even better news is that it’s hard to imagine ways of making it less efficiently than we already do. We’re very early. We’ve barely even begun to think of ways to do this better. For example, CERN recently demonstrated 8x higher production efficiency, with a fairly obvious hack. That’s almost an entire order of magnitude. Three or four more advances like this and we’ll really be getting somewhere.

Currently, antimatter is made by bombarding a tungsten target with a high energy particle beam, which produces a few antiprotons. Then, if they happen to be at the right energy and going in the right direction, we can capture and store them for a while, gradually slow them down, and combine them with antielectrons (positrons) to form neutral antihydrogen. When I was a child, no-one was even sure if this was possible. Even today, there is serious work underway to investigate if gravity works on antimatter the same way as on normal matter. Perhaps it doesn’t!

As told in “The Making of the Atomic Bomb”, Leo Szilard had been obsessed with nuclear power and weapons for many years. In 1938, he realized that uranium, uniquely of the naturally occurring elements, could support a chain reaction. This led to a letter signed by Einstein and delivered by Alexander Sachs to FDR on October 11, 1939. Even then, the Manhattan Project wasn’t officially begun until August 1942, nearly three years later.

I think it should be possible to make antimatter with better than 0.01% efficiency, which would make high performance flight to Mars, Jupiter, and Saturn possible at scale within existing spaceflight budgets.

Humanity is on the cusp of being able to make useful quantities of antimatter, and obviously controlling the technology is strategically vital – and not just for high performance rocket engines.

Storage

Conventionally, antimatter is stored as a charged plasma in electromagnetic “storage rings”. It is non-trivial to design containment for a form of matter that instantly annihilates on contact with any ordinary matter. Storage rings are large, heavy, and persnickety. It would be ideal to find a more robust method for storing up to milligram quantities of antimatter.

In the interests of simplicity, I think the path forward might be electrostatic containment. A small, cryogenically cold vacuum chamber (similar to devices used for quantum computers today) stores antihydrogen as a liquid droplet or ice crystal. A small net charge allows active containment in a 3D electrostatic trap. Surface charge also modulates surface tension and partial pressure, by which atomic quantities can be emitted from the surface for use, similar to a bubble jet printer head.

Diagram showing conceptual antimatter droplet containment system. A charged drop of liquid antihydrogen is held electrostatically between actively controlled electrodes. Surface charge is modulated with an electron gun. The system is held in vacuum and kept well below 20 K. Boiloff is directed out to the engine.

Here’s Gemini 3’s image model version. Not bad!

In concept, this is a relatively small, easy to build piece of laboratory equipment. It can be tested on regular hydrogen with no special hazards, simply by inverting the charge on the containment system. A hydrogen droplet storage and manipulation system could be built from scratch for less than a million dollars. Of the three parts, it is the cheapest to test and retire risk early.

Engine cycles

Here we come to the fun part. How to actually use this incredible form of stored energy?

The fundamental problem is one of transduction – the same problem with any rocket engine. The propellants are enormously energetic, but they really want to just make heat and noise and light. How do you get them shoving mass out the back at high speed, safely and reliably?

This particular problem underscores the difficulty of making nuclear fission propulsion work. Nuclear fuel is about a million times more energetic than chemical fuel. Given that E = ½mv², we should expect a nuclear rocket to be able to deliver about 500,000 s of Isp at high thrust.

The only known way of doing this is via Freeman Dyson’s infernal contraption Project Orion. Yes, it achieves high Isp and high thrust, by detonating thousands of nuclear bombs behind it on its way.

Credit: PanzerSoldat_46

Less concussive methods of production instead rely on nuclear reactors. Nuclear thermal propulsion uses a nuclear core to heat passing hydrogen, achieving an Isp of around 900 s with a steep complexity and mass penalty and lower exhaust temperature than high performance hydrolox engines such as the BE-3. Why? Nuclear materials such as zirconium and hafnium aren’t stable enough at higher temperatures. In a world of Starships and orbital refilling, it’s hardly worth the effort.

Nuclear electric propulsion instead takes a nuclear power reactor and runs electric propulsion, which can achieve Isps well into the thousands of seconds, albeit at much lower thrust. In this case, we take the incredible power of nuclear fission and tie it down with a boring Brayton cycle nuclear thermal reactor, spin a turbine, make electricity, then run that through a Hall effect thruster or similar. Each step takes a 90% cut and pretty soon, we’re once again left with some watered down weak sauce propulsion system that, at best, can achieve lower acceleration than an infinitely cheaper and easier solar sail, at least anywhere inside the orbit of Saturn. It’s not even more compact, since any space nuclear reactor needs a giant radiator to keep the cold side of the heat engine cold – a radiator that is bigger, heavier, more expensive, and more complex than a space solar array that would generate equivalent power without any nuclear reactor at all. Do not optimize something that should not exist. Delete!

Antimatter thermal propulsion

Antimatter suffers from the same problems. But maybe we can find some way to use it without particle accelerators and superconducting magnetic fields?

We can afford to “waste” almost all the inherent energy, provided at least some makes it through to the business end of the rocket and produces a high thrust, high Isp result.

The simplest method is thermal propulsion, similar to nuclear thermal, but better. Emit a stream of antiprotons into a block of high temperature refractory, such as hafnium carbide, with a melting point of about 4000 C. The antiprotons annihilate against the block, producing a stream of hard gammas that are absorbed by the block, heating it. Then flow a propellant through. Hydrogen has low molecular mass, which creates a higher exhaust velocity. But its storage density as a liquid is about 12x lower than water, which also doesn’t require cryogenic temperatures. Personally, I’m in favor of the higher thrust and higher mass fraction of a denser propellant, and I’m prepared to sacrifice some Isp. But if you’re determined to fly a brachistochrone trajectory to Pluto and back with a human crew, other trades may apply.

For reference, an antimatter thermal cycle running steam can produce 900-1000 s of Isp, slightly better than a nuclear thermal rocket and without needing a local nuclear reactor. Hydrogen propellant can produce around 1700 s. Total achievable Δv is about the same, at about 24 km/s. This is easily enough to fly to and from anywhere in the solar system on a Hohmann cycle (slow, efficient) orbit. Very respectable!

The principle virtue of the antimatter thermal cycle is that it’s simple. No moving parts in the hot zone, and only a simple pump to flow water through a refractory block and into space. The main downside is that it leaves a lot of performance on the table. The antimatter thermal cycle can be thought of as a uranium gun type bomb. Simple, crude, underpowered. Today, we have dial-a-yield shelf-shable thermonuclear bombs with 100x the yield that are lighter and smaller. What might be the antimatter rocket equivalent of the plutonium implosion device?

Antimatter-catalyzed fission fragment propulsion

An antimatter thermal engine is a good start but to unlock the solar system we’re going to need a method to get to higher thrust at much higher exhaust velocity. But hafnium carbide is about as hot as solid materials can get. We need a way to get much much more energy into the exhaust gas stream without relying on heat transfer from a convenient solid. We need some kind of antimatter-fueled afterburner.

Unfortunately, simply shooting antimatter into the exhaust stream won’t accomplish much. The antimatter will annihilate, producing a bunch of gamma rays that will zoom off into the universe. They can penetrate about 10 cm through a dense solid like hafnium carbide, and about 1 km through relatively hot, sparse exhaust gasses, even at the throat before expansion. Since engines are much smaller than this, we’re going to need a mechanism to transduct the extreme energy of gamma rays into a form that can further heat exhaust gasses.

Credit: Future of antimatter production, storage, control, and annihilation applications in propulsion technologies

We need a “kinetic cascade”. I previously wrote about this quirk of mechanics in my post on orbital debris. In that case, dense, fast-moving satellites and debris in Earth orbit are not slowed down enough by the sparse atoms of Earth’s upper atmosphere. Instead, we can launch tons of 30 micron powder into the desired orbital regions. This powder greatly increases the flux of non-destructive momentum-sapping drag-inducing collisions, filtering debris below some density threshold out of orbit and pushing it lower, where it burns up in the atmosphere. Meanwhile, 30 micron powder is sparse enough that it is materially affected by residual atmospheric drag, and also gets de-orbited in a few months. The powder creates a bridge that enables momentum transfer from big things (debris) to medium-sized things (powder) to small things (air molecules). This is a kinetic cascade.

The same principle can be applied to antimatter propulsion.

Every middle schooler in America knows that neutrons induce fission in U-235, the lighter isotope of uranium. Both U-235 and neutrons can be hard to come by, mostly for the better. What only a few middle schoolers really understand is that antiprotons can induce fission in U-238, the inert naturally-occurring form of uranium used routinely in ceramic glazes. In both cases, the result are two large fission fragments, one typically larger than the other.

The actinides, including uranium, can all be fissioned by antiproton collision.

A chart showing daughter nuclide distribution. The daughters are usually also radioactive and decay over a period of seconds-to-days into much less radioactive products – none of which are relevant over the timescales for engine propulsion, which is far less than a second.

The important point is that one antiproton can collide with a uranium atom, producing two highly charged daughters moving at 0.05 c, instead of two gammas. Yes, the fission consumes about 40% of the antiproton’s embodied energy, but two highly charged massive and highly energetic atomic nuclei are much much easier to work with than two highly introverted gamma rays. Instead of passing through kilometers of exhaust gas, fusion products stall in centimeters of air, enabling the remaining 60% of the energy to be dumped directly into a moving gas stream. This is the afterburner!

Cherenkov radiation is actually caused by much faster particles moving through a medium faster than the speed of light in that medium, but it gives a good intuition for the scales and ranges involved.

At these temperatures, the exhaust gas becomes a dissociated plasma. Plasmas can be controlled and directed with magnetic fields, but this is extremely challenging for high thrust engines with lots of gas flow. I think that a “film cooling” approach is probably best. Dump the heat into the core of the engine exhaust, and allow cooler steam to contact the metallic parts of the engine directly. The engine bell would naturally be regeneratively cooled anyway, using the inflowing water to absorb heat prior to flashing to steam in the injector.

The way this actually works is that there’s a water injection plate at the upstream side of the engine “combustion” chamber, and in the middle is a pintle injector able to dispense almost microscopic quantities of liquid U-238 (it’s molten at these temperatures) and very microscopic quantities of antihydrogen. They interact instantly in a tiny volume, throwing out particles around 10 cm into the surrounding steam volume, then expand out into the exhaust. No magnetic fields. No particle accelerators. No radiation shielding needed.

Diagram showing how antimatter-uranium reaction at the tip of an injector produces energetic fragments that heat an advecting layer of dense steam, superheating exhaust gasses.

The neat thing about this approach is that the relative flow of antimatter and water can be modulated. The antimatter essentially sets the engine power, the water flow cools that, setting the temperature. Higher water flow = higher thrust at lower Isp. Lower water flow = lower thrust, higher Isp. Essentially any Isp between chemical ranges (400 s) and electric propulsion (5000-20,000 s) and even beyond is possible.

It’s even possible to reduce the amount of antimatter required for a given engine and mission profile. Enriching the uranium supply with around 20% U-235 can create localized chain reactions, with hundreds of fissions and fragments per antiproton. This actually creates an amplification effect. Instead of fission consuming 40% of the inherent energy of the antiproton, it produces a 100x the output. Either way, the exhaust will always be mildly radioactive and thus probably undesirable as the engine of a first stage Earth-launched rocket, at least until social norms around radiation exposure (other than sunbathing) changes or better, we figure out how to upregulate our DNA repair machinery and achieve something like immortality.

The fission fragments themselves, reflected with an electrostatic nozzle, would achieve an Isp of 1,500,000 s, enough to propel a spacecraft to perhaps 10% of the speed of light. But if you are attempting to optimize for travelling to the stars, there are other cycles that may work better.

To summarize the complete cycle, a small cathode ray electron gun modulates the surface charge of a stored droplet of cryogenically cooled antihydrogen, spalling off antiprotons. They travel through an injector, meeting tiny droplets of molten U-238, potentially with some U-235 alloyed in for extra spiciness. Fission daughter nuclei zoom outwards, colliding with millions of partly dissociated steam molecules in a compact area, superheating the exhaust. The exhaust expands out generating high thrust at high velocity, enabling missions with tens, hundreds, or even thousands of km/s of Δv.

How much antimatter do we need? It depends on how fast we want to go. For a Starship-class vehicle with 20 km/s of Δv, we need barely a handful of antimatter and U-238 – that’s enough to boil 1000 T of water to 25000 K exhaust temperature. Same vehicle but now we want 100 km/s, enough to fly to Pluto and back in less than twenty years? 10 kg of U-238 and just 45 g of antimatter, both occupying about 500 ccs. 45 g of antimatter too expensive? Mix in 1 kg of U-235 and we can make do with just 0.5 g.

This model is truly the “Heart of Gold” for advanced propulsion.

Here’s a basic spreadsheet that calculates performance characteristics for the antiproton catalyzed fission engine cycle.

Alternatives

There are numerous other ways to potentially chase the dream of high thrust, high Isp engines powered by antimatter. Restricting ourselves to recognizably useful systems that violate no known laws of physics, the following changes are possible.

Delete antimatter

Antimatter production and storage is painful – no two ways about it. Can we do without it?

Yes, but you might not like it. Instead of U-238 we can use mostly U-235 (highly enriched uranium) and drive fissions with a stream of neutrons rather than antiprotons. The energetics are much the same, but now we need a sufficiently bright neutron source. This probably requires a 1 GeV particle accelerator, but there are some ideas for compact particle accelerators that could be launched on rockets. See, for example, the AWAKE experiment.

These are at a similar level of maturity to antimatter production. The key difference is that if we can store antimatter, we can produce it in labs on the ground, whereas a neutron-driven subcritical fission fragment propulsion system would always need an at least shipping container-sized accelerator to generate the neutrons – and AWAKE makes nowhere near enough of them.

Delete antimatter storage

Storing antimatter is highly nontrivial and, if containment fails, beyond catastrophic. Not quite nuclear explosion level but still, the warranty is voided.

If we’re going to design and build a flight-ready 1 GeV-class high intensity particle accelerator, why not just generate anti-protons directly instead of spalling off neutrons?

A just-in-time antiproton production system, combined with a partially enriched uranium target, could potentially scale to much larger sizes. Why? If the impinging antiproton is fast enough, it will relativistically beam the daughter nuclides in the direction of impact, removing the requirement for sufficiently dense steam and/or electrostatic mirrors. This is one potential form of a relativistic interstellar engine.

Superconducting magnets

LK-99 was a bust, but maybe it’s possible to build enormously powerful and compact magnets. In that case, we can delete the expansion bell and use a magnetic nozzle to contain and expand the superheated exhaust to provide thrust. Like particle accelerators, this almost certainly works better at a larger scale and higher Isps better suited to travel to the outer solar system and beyond.

Conclusion

There are a number of very smart people wondering what NASA might do, now that commercial launch and reusable rockets have been effectively incubated and the end is in sight for SLS, Orion, and the ISS. Something deeply technical, requiring deep investment, world leading expertise, and that gives humanity the next big unlock. A Manhattan Project that will give us the entire galaxy. An answer to the ambitions of adversaries who are gaining fast on our existing technology stack.

Even incremental improvements in any one part of antimatter production, storage, and use will deliver enormous benefits to our civilization. Relatively modest improvements across all three will unlock the solar system. Finally, in the limit, having the ability to condense the power of the sun into pure energy in the form of antimatter is about as far as our tech tree is likely to go. The end is in sight!

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