Astrophage may be possible, but it wouldn’t work as it does in Project Hail Mary

Project Hail Mary is a big hit in theatres. Since its release a few weeks ago, it has grossed over $300 million globally, making it Ryan Gosling’s highest-grossing film in which he has been the lead. If you haven’t seen the movie, go see it now. It is excellent. This article does have a few spoilers.

While the movie is primarily focused on the buddy relationship between biologist Ryland Grace and his alien counterpart Rocky, physics plays a big role, too. After all, the only way that either Rocky or Ryland made it to Tau Ceti within their lifetimes was by using the fantastic property of astrophage to collect enormous quantities of energy and release it with near-perfect efficiency to cross vast depths of space in short periods of time.

Although the movie only vaguely describes how astrophage works, the book provides a more detailed explanation. It turns out, however, that this explanation is deeply flawed. Yet, I have figured out a better way that astrophage could, within the realm of known physics, exist with its fantastic properties, although it would change some of the plot points of the story, and even then, it is highly speculative.

A strophage means “star killer” in Latin, and it is a tiny black, dot-like cell that maintains an internal temperature of 96.415 degrees Celcius, just below the boiling point of water at 1 atmosphere of pressure. This temperature is constant no matter where it is, even inside a star, which means that when exposed to temperatures higher than its internal temperature, it absorbs that energy. This is how it destroys stars. It simply absorbs the energy. When it has to release energy, it does so at a particular infrared frequency, which it uses to travel vast interplanetary distances to its breeding grounds, which, in our case, is the CO2-rich atmosphere of Venus.

The astrophage reproduces by seeking out the spectral frequency of CO2 (at 4.26 and 18.31 micrometers). In the book, a Russian researcher, Dimitri Komorov, reports that an astrophage can absorb 1.5 MegaJoules of energy without heating up, becoming 17 nanograms heavier as a result. This kind of direct mass-energy conversion is extremely efficient, approaching that of matter-antimatter reactions.

To collect this much energy, in the book, a large proportion of the Sahara is covered with black panels which absorb enough heat from the sun to feed the astrophage, producing the 2 million kg required to get the Hail Mary and astronauts to Tau Ceti. On Rocky’s planet of Erid, meanwhile, the planet itself is hot enough that they just have to dump the astrophage into their ocean, which is already above 96.415 degrees.

This makes astrophage like any other biological system. It must capture energy, store it, and release it for useful work. Yet, all of these would have to work at a scale and efficiency never seen in the biological world.

How does astrophage capture energy?

Ordinary photosynthesis is very inefficient, with 1–2% of solar energy being stored as chemical energy inside a plant. Moreover, plants generally only absorb a few frequencies of light. Astrophage, on the other hand, is described as completely black and impenetrable to all wavelengths of light, suggesting it absorbs across a very broad spectrum, likely into the gamma rays.

It would need to not only absorb this energy with near-perfect efficiency but also transport it with close to 100% efficiency from end-to-end. Thus, the transition from photon energy to storage would need to have only minimal losses.

How does astrophage store energy?

You may have heard that ATP is the main energy storage medium of biological organisms. These store about 30 kJ/mol, which is nowhere near what astrophage stores.

Even hydrogen, which is the highest-density chemical fuel, wouldn’t come close at 120 MegaJoules/kg. Metallic hydrogen, such as might be found at the core of the planet Jupiter, might come closer to astrophage but would require enormous pressure to store and would not be stable. Chemistry hits a hard wall when it comes to energy storage because of electron energy levels. Chemical energy comes from rearranging electrons in orbitals, which is measured in electronVolts per bond. Astrophage, on the other hand, stores energy in the thousands to millions of electronVolts per particle, which suggests either nuclear physics or high-energy electromagnetic storage.

Astrophage’s storage capacity approaches that of uranium fission or higher, but there is no indication in the book that it requires heavy elements at all.

The book suggests that astrophage stores energy using “neutrinos and a form of pair production”. Neutrinos form from colliding protons together and are able to absorb energy while contained within their astrophage. These neutrinos then release energy at the Petrova line frequency.

This is where I get lost. Andy Weir is a first-rate storyteller, but for whatever reason, he has used dubious physics as the centerpiece of his two most popular novels: Project Hail Mary and The Martian. (I admit that I haven’t read his other novel, Artemis, yet.)

In The Martian, he places a lot of emphasis on Martian storms threatening to blow the MAV over as the reason why the crew needs to leave the surface in a hurry, abandoning Mark Watney on the surface. NASA (the real one, not the one in the book/movie) pointed out at the time that the Martian atmosphere is so thin that a 60 mph wind there would have the force of a 5 mph wind on Earth. Now, I can imagine other great reasons why the crew might have to leave in a hurry. That their getaway vessel is at risk of tipping over (and apparently has no way to stow itself in case of storms) seems awfully weak, given the clearly enormous amount of money and effort spent on the series of Mars missions. The fact that it is also physically unlikely makes it a bit confusing.

In Project Hail Mary, the problem with astrophage is that Weir wants it to be composed only of simple elements. If astrophage needed heavy elements to reproduce, it would be too easy to stop. Heavy elements are rare and tend to be buried in the crusts of rocky planets, produced from ancient supernovae. If all it needs to reproduce is CO2 and hydrogen, then it is much harder to stop. Nevertheless, Weir needs some explanation for how it can store so much energy that each microscopic astrophage can literally propel itself from the Sun to Venus and back again.

Neutrinos seem like a good answer until you actually look at how neutrinos behave.

The idea is, of course, clever in some ways. Neutrinos don’t interact, so they can carry huge amounts of energy without damaging the host organism. Neutrinos only interact via the weak force, which is related to radioactive decay. This means no heating, no structural damage, and no radiation.

Neutrinos can carry sufficient energy in the MeV per particle regime, so that’s another point in their favor.

And there is no need for electromagnetic confinement that you would need with antimatter, because neutrinos have no charge and again don’t interact.

The downside is a big one, however: you can’t confine neutrinos. There is no mirror, trap, or container for them. Neutrino detectors on Earth, like Ice Cube at the South Pole, depend on capturing just a few interactions out of the countless numbers streaming from the Sun.

Suppose you could store them. You run into another problem. You can’t get the energy back because they don’t interact with anything. Neutrinos are hugely reluctant to give up their energy.

Andy Weir waves away the containment issue (maybe physics we don’t know about?) and tries to solve the energy release issue by assuming that neutrinos are their own antiparticle. When a massive particle is its own antiparticle, we call it a Majorana fermion after a guy named Ettore Majorana, who, by the way, disappeared without a trace (likely drowned in a lake in Switzerland).

This turns out not to work either because neutrinos would have just as much trouble interacting with their antiparticles as they would with other matter. Thus, the whole advantage of neutrinos (being safe for biological lifeforms because they don’t interact) makes them impossible as a medium for extracting useful energy. Neutrinos will simply take their energy and spread it through the cosmos for the most part, never being absorbed. Astrophage would have to rely entirely on the weak force to extract energy.

This means that neutrinos have none of the required attributes to be an energy storage and propulsion mechanism, other than the ability to carry a great deal of energy per particle. They cannot be contained, you cannot recover energy from them with the required efficiency, their interaction strength is weak, and there is absolutely no physical experiment that remotely resembles what astrophage is supposedly doing with them.

Mind you, I am open to all kinds of stretches in science fiction. I’m a big Star Trek fan after all, but the problem goes deeper than can be addressed by saying “maybe there is physics we don’t know about in play”. Rather, the book contains an explanation that is based on a contradiction: astrophage wants neutrinos that both don’t interact with things (so they can have a lot of energy but not damage the biology) and do interact with things (so they can be contained and their energy released on demand). These cannot both be true at the same time.

So, how do we, within the intersection of science fiction and fact, rectify this problem? My theory is that astrophage, if one wanted to create it, would not use neutrinos. It also would not use uranium or plutonium, nor would it make antimatter. There are lesser-known mechanisms that work better.

Let’s look, first, at the what astrophage’s requirements are: it needs to store a lot of energy in a stable configuration where it does not randomly explode (although it does explode in the book and movie, killing the science officers intended for the mission [why would you have your main and backup science officers anywhere near each other if this mission was so important, Stratt?], the explosion is triggered accidentally by the science team). It also needs to release the energy slowly in the infrared, not gamma rays, which might be more typical of uncontrolled reactions.

Thus, astrophage sits in a regime that does not occur naturally:

Only one thing that I could find in the literature could come anywhere near satisfying all of these:

A living nuclear isomer capacitor.

You may be familiar with capacitors. You take two pieces of metal and bring them close together, separated by an insulator such as glass, ceramic, plastic, paper, or even air, then provide a current. One side charges up with a positive charge, while the other charges up with a negative. If the current is reversed (as in an alternating current) or stopped, it releases its energy.

There have long been proposals to build batteries out of capacitors. They would charge extremely quickly (potentially seconds) and be able to hold an arbitrary charge since it is largely dependent on how close together the plates are and how big they are. This idea, however, has never been made practical because they don’t approach the energy density of batteries.

Since astrophage is basically an extremely energy-dense, highly efficient battery, we need to find something better than an ordinary capacitor (or chemical battery).

A nuclear isomer is an atomic nucleus that can exist in a long-lived excited state. An example is Halfnium-178m2. The energy storage capacity is approaching that of astrophage and has a charged state that could, in principle, release energy on demand.

(There is a whole episode called the Halfnium Controversy where now discredited experiments purported to demonstrate exactly this happening in the lab. This does not discredit the idea as a whole, however, as physically plausible. Isomers are unique in being able to store nuclear levels of energy stably for a long time.)

Astrophage’s other requirement is that it needs to release energy in the infrared, low frequencies that don’t damage biological cells.

In the case of nuclear isomers, there is a big downside because they mainly produce gamma rays. Gamma rays are far too destructive for a biological lifeform and would destroy most equipment.

Astrophage, to protect themselves, would have to downgrade the energy produced by the isomers to some lower, non-destructive frequency. Infrared wavelengths that they produce in the book and movie are perfect: not so high energy that they would destroy the astrophage (or the Hail Mary for that matter), and not so low that the tiny astrophage would be too small to produce them.

There are ways of downgrading gamma rays. Radiation detectors do this all the time. The gamma rays hit some atoms in a material and produce high-energy electrons, the electrons excite lower-energy states, and the material emits lower-energy photons. In fact, this is typical of any kind of energy absorption. The trouble with it is that it is highly inefficient, and the material the gamma rays hit will be damaged in the process. The damage might not be a problem for a hardy biological lifeform capable of self-repair, but the inefficiency might be.

Thus, the astrophage would need to contain layered nanostructures that gradually absorb gamma rays and distribute the energy step by step in a cascading system. This kind of system is called a *scintillator,* and these are frequently used in medical fields and high-energy physics.

Scintillator crystals (Wikipedia)

Scintillators are pretty standard physics, but they would not approach the efficiency of astrophage in the book. An alternative idea would be that the astrophage uses something called phonon coupling. Basically, in this case, you have a crystal lattice that absorbs gamma rays and turns them into lattice vibrations. These vibrations have characteristics in common with physical particles and are called phonons, essentially particles of vibration.

A highly ordered lattice that can absorb gamma rays could convert the harmful rays into harmless vibrational modes (frequencies). The problem with this kind of engineering is that it is hard to control the energy to prevent it from dissipating. You would need some very clever engineering to prevent diffusion by constraining the energy into specific modes (like tones). Pure tones could carry the energy without dissipating it.

In summary, to be remotely physically plausible, astrophage would need to contain:

1. Heavy elements capable of long-lived excited modes to create the nuclear isomer capacitor, such as Halfnium or similar nuclei that have the right states.
2. An embedded nanostructure energy-conversion matrix that does the gamma downconversion cascade is able to operate at incredible efficiency.
3. Self-repairing ability if the matrix becomes damaged.
4. Ability to control the release of the energy precisely, using, e.g., coherent nuclear excitation (similar to how lasers work but for atomic nuclei). This is basically a gamma ray laser or graser. This would also need to dampen recoil from the isomer since gamma rays produce a lot of energy when released (like a bullet fired from a gun). This has never been achieved in the lab (see Halfnium Controversy).

To date, no one has been able to create a graser, and triggering nuclear isomers is speculative. It’s also a bit sketchy how you would pump the isomers with energy from just about any source of energy, like the Sun or hot water, too.

Despite all this, the plan: living nuclear isomer capacitor + gamma downgrade cascading lattice is probably the best physical approach to astrophage since all this stays within the framework of known physics (although it isn’t clear what the limits in efficiency are).

The rest is engineering.

The downside for the plot of Project Hail Mary is, of course, that the astrophage would not be going to breed in the CO2 atmosphere of Venus, but more likely would want to seek out the crust of a rocky planet to find Zirconium, which would almost certainly contain traces of Halfnium. And cutting astrophage off from sources of Halfnium sounds like something we could accomplish without sending anyone off on a suicide mission to another star system.

Originally published at https://timandersen.substack.com.