3I/ATLAS. A cat on my balcony
(En español aquí)
Summary
Recently, a new interstellar object passing through our solar system has been discovered: 3I/ATLAS. This object has sparked much curiosity and is being intensely studied by astronomers worldwide. A paper uploaded to the arXiv preprint server suggests the possibility that this object might actually be an alien spacecraft secretly preparing to attack us at the end of this year. This peculiar idea is based on a series of coincidences supposedly too unlikely for a natural object. In this article, written for a general audience without requiring prior knowledge, I counter each of these alleged improbabilities. The first sections (Sections 1 and 2) present general reflections on the danger of establishing criteria after the fact to defend an implausible hypothesis. We discuss aspects of the philosophy of science that are important to understand why science must be conducted with rigorous and disciplined methodologies. This first part is also explained in a video I had the pleasure of participating in for Javier Santaolalla’s channel [1]. Section 3 discusses the list of improbabilities presented by the paper’s authors, showing that they are not such. Section 4 dismantles the idea that the trajectory of 3I/ATLAS suggests it is preparing a clandestine Oberth maneuver to change its course at perihelion. This section is somewhat more technical but requires only high school–level knowledge. Finally, the conclusions demonstrate that the narrative of a spacecraft preparing a clandestine Oberth maneuver does not hold up.
1. The game of sought improbabilities
3I/ATLAS is a recently discovered interstellar object being studied with great interest by astronomers worldwide [2–6]. As its name indicates (specifically the “3I” part), it is the third interstellar object we have discovered passing through our solar system. The first was 1I/ʻOumuamua (discovered in 2017), followed by 2I/Borisov (in 2019), and now we are witnessing the passage of 3I/ATLAS.
Everything we know about 3I/ATLAS is perfectly consistent with it being a comet. However, the well-known researcher Abraham (Avi) Loeb, together with his colleagues Adam Hibberd and Adam Crowl, have uploaded a paper (hereafter referred to as HCL, from the initials of the authors) to the arXiv preprint server with the suggestive title Is the Interstellar Object 3I/ATLAS Alien Technology? [7]. In it, they propose the possibility that this object might actually be an alien spacecraft disguised as a comet. They further speculate that such a spacecraft could have malevolent intentions, even planning a “clandestine” reverse Oberth maneuver. This maneuver would be executed when the object is hidden from our view on the far side of the Sun, using it to brake and settle into a closed orbit within our solar system, surprising us in November with a new orbit. Before delving into further discussion, I find it a bit sad that our first reaction to a spacecraft braking to remain in our solar system is to assume a hostile stance. We carry out this type of maneuver routinely with our own probes to planets in our vicinity, and always with the best intentions—to observe, study, and better understand the universe.
The authors themselves admit that the most likely explanation is that 3I/ATLAS is simply a comet (albeit a tremendously interesting one) and justify their speculation as a “fun” pedagogical exercise. Setting aside the debate on the advisability of slipping poorly substantiated sensationalist claims into scientific repositories (we know from prior experience—much of it amassed by Avi Loeb himself—that such claims do little more than misinform the public), I do agree with the playful and educational nature of the exercise. That is why I choose here to focus on other aspects I find very relevant and from which much can be learned. The HCL preprint grounds its peculiar hypothesis in a series of statistical improbabilities. The problem is that these improbabilities are chosen after the fact—that is, once we already know the object’s properties, we select those that appear favorable to our hypothesis.
Below we will discuss the validity of the improbabilities that support HCL’s speculation, but in this section I want to focus on the general problems with this type of approach. To that end, let us consider a concrete example and, as one should lead by example, I will use myself as a case study. By all appearances, my physical features are compatible with those of a human being (there are plenty of videos on YouTube where this can be confirmed, such as [1]). If you were the scientific community of Earth observing me, you would likely conclude, with almost total confidence, that I am human. However, some more free-thinking scientists might come up with the idea that I am actually an alien in disguise, based on a series of statistical improbabilities they have observed in me.
They might begin by noting that my height is 1.94 m (about 6'4"), which is unusual. Only 1% of humans are 1.94 m or taller. Of course, this peculiarity would be consistent with the hypothesis that I am an alien in disguise, since a larger size would be needed to fit the costume around my true alien body. Perhaps they would also notice that I have two uncommon medical conditions that affect my eyes. One is strabismus, which occurs in only 3% of humans, and the other is Gilbert’s syndrome, which appears in only 4%. The eyes are a very complex organ, and it would make sense that an alien disguise might show imperfections in something so difficult to replicate. Finally, our bold scientists might observe that I live on a small island—Tenerife. Only 1% of the human population lives on such islands. They conclude that this is consistent with the idea that I am an alien in disguise trying to remain unnoticed on Earth, since it is easier to hide in such small territories.
When we combine these statistical improbabilities to do a hypotheses contrast, we obtain a probability of 0.00001% that I am human. In other words, there is a 99.99999% probability that I am an alien in disguise. Obviously, this is not true. Despite these numbers, I assure you I am entirely human (though I must confess that, after seeing them, I feel a certain unease and wonder if it has anything to do with my poor performance on CAPTCHAs). The reason we get this result is that, in something as complex as a human being, it is always possible to find a set of parameters that fall outside the norm. The moral of this exercise is that we are all special in one way or another. Obviously, a comet is not as complex as a human being—not even an extraordinarily interesting interstellar comet like 3I/ATLAS—but the game is the same. Therefore, arguments based on criteria selected after the fact, like those used by HCL to support the idea that 3I/ATLAS is a camouflaged alien spacecraft, are not a good way to establish the truth of implausible hypotheses.
2. Aliens vs. (feline) Predators
Loeb et al. are simply suggesting that we keep an open mind, since no one has been able to prove that 3I/ATLAS is not an alien spacecraft disguised as a comet. In other words, everything we see is compatible with it being a comet—but can we refute that it is a disguised spacecraft? The truth is, no, we cannot rule it out. But here we are already stepping more into the realm of the philosophy of science than science itself. And it is precisely for cases like this that I sometimes wish there were more education in the philosophy of science—not only among the general public but also among researchers themselves. To understand it simply, let’s turn again to an everyday situation.
Let’s suppose I am at home, quietly resting after dinner, when suddenly a sound breaks the silence of the night. “Meow!” I clearly hear a meow coming from my balcony. The most natural assumption in this situation would be that there is a cat on the balcony. However, I could also consider a second possibility: that there is an alien on my balcony pretending to be a cat and imitating its meow. Can I prove that this second option is not correct? No, I cannot—but surely we would all stick with the first one. Why? Mainly for two reasons.
First, because from prior, absolutely conclusive observations, we know that cats exist. Second, also from unquestionable prior observations, we know that the abundance of cats on our balconies is overwhelmingly higher than that of aliens. There are on the order of a billion cats in the world. By contrast, even those who believe that creatures from other worlds roam here (not me!) would not estimate their number at more than a few. Therefore, the expected frequency of cats on my balcony is at least hundreds of millions of times higher than that of aliens (in my opinion, in fact, infinitely higher).
Even if we don’t consciously work out these relative abundance calculations, this is the mental process underlying our decision to prefer the feline hypothesis over the alien one as the explanation for the meow on the balcony. With this example, I am not trying to ridicule the possibility of extraterrestrial intelligent life. Not at all. In fact, it is a subject that fascinates me, excites me, and to which I dedicate research work. This explanation is meant to be pedagogical, applying a reductio ad absurdum to expose flaws in logic that may be obscured by scientific jargon but become obvious when brought into everyday contexts, where our intuition operates more comfortably.
And in this case, what does experience tell us about the relative abundances of comets and interstellar spacecraft? Unfortunately, we have absolutely no information about the density of kilometer-sized interstellar spacecraft. However, we do know enough astrophysics to have an idea of how many comets and asteroids are out there, wandering the galaxy. For starters, formation models tell us that the solar system has expelled—over its lifetime, but especially early on—an enormous number of asteroids and comets. No fewer than between trillions (a one followed by 12 zeros) and hundreds of trillions of objects, the vast majority of them comets, launched into interstellar space.
Granted, these estimates are based on theoretical calculations and carry some uncertainty. But it doesn’t really matter. Even if the estimates are off by a factor of two either way, we are still talking about a colossal number of expelled objects. By contrast, our solar system has not produced a single kilometer-sized interstellar spacecraft (and I would argue none of any size—I am very reluctant to count the Voyagers as interstellar probes). Thus, the solar system has produced between trillions and hundreds of trillions of comets and not a single interstellar spacecraft. The ratio is far greater even than that of cats to aliens on terraces. And it likely increases even more if we consider other stars, since we can almost certainly assume that the vast majority of the 200 billion stars in our galaxy are also contributing between trillions and hundreds of trillions of comets to the interstellar medium—leading to mind-boggling figures like thousands of billions of trillions of comets wandering the galaxy, expelled from their parent stars.
The numbers also tell us that it is normal for a few of these objects to enter the solar system each year, and by chance, one is occasionally discovered. We do not see all of them because they are very faint, and it takes some serendipity for a powerful telescope to be observing exactly the patch of sky where such an object happens to be at that moment (large telescopes are usually like a microscope, seeing only a tiny fraction of the sky). Considering this, and the staggering overabundance of interstellar comets compared to kilometer-scale spacecraft, the sensible conclusion is that the meow on the terrace is a cat, and 3I/ATLAS is a comet.
Avi Loeb’s constant call to keep an open mind is fine, and I agree with it. I am totally on board. But we also have to call things by their name. We must be clear about which arguments support our hypotheses. Carl Sagan popularized the phrase that “you should keep an open mind, but not so open that your brains fall out” (the earliest written reference I know of this sentence is by Max Radin, 1937). Loeb has a long track record of publishing papers associating eye-catching phenomena—any intriguing and media-worthy event—with the alien hypothesis. He began with FRBs (fast radio bursts). Now we understand them better, but years ago, when these signals were first detected and still shrouded in mystery, Loeb had already published in the scientific literature the possibility that they were extraterrestrial technology.
Then came the interstellar objects. When the first, 1I/ʻOumuamua, was discovered, Loeb published several articles—both scientific and popular—claiming it was an alien spacecraft. He defended this idea with great conviction and even wrote a book, Interstellar, which became a bestseller. Then came the meteors he labeled IM1 and IM2 (IM for interstellar meteor), which impacted Earth in 2014 and 2017. Although there is legitimate scientific controversy, these meteors might also be of interstellar origin. Of these, he not only claimed they were alien probes but even proposed in a paper that they came from a mothership crossing the solar system. And now he argues that 3I/ATLAS is also a spacecraft. In other words, according to Avi Loeb, all interstellar objects except 2I/Borisov are extraterrestrial technology. Even at a purely speculative level, doing this is not good scientific practice. What sense does it make that all the interstellar objects we see are spacecraft, when we also know that there must be an overwhelmingly large number of comets and asteroids out there that we should be discovering at a rate of about one per year?
But there is a much deeper problem with this way of proceeding—a problem that, again, lies more in the philosophy of science. What if he turns out to be “right”? What would happen if, against all reasonable odds, November comes and 3I/ATLAS suddenly fires its thrusters, performs a reverse Oberth maneuver, and changes its trajectory to position itself for an attack on our planet (whatever that may mean)? Would we say that this researcher was correct? The answer is no—he would not be correct, though few people would understand this. Let’s use another everyday example to clarify.
Imagine a small town where there is a very peculiar doctor—Dr. Death. This doctor always tells all his patients that they will probably die in the coming days. Naturally, the patient leaves the office very worried, but as the days go by, they are relieved to find nothing happens, and they eventually forget the ominous diagnosis. It all becomes an unimportant anecdote. However, from time to time, someone who appeared to be in good health does indeed die in the days following a visit to the macabre doctor. When this happens, the entire town is shaken by the news that, against all odds and against the opinion of the medical establishment, Dr. Death “was right.” Our eccentric doctor thus earns an almost prophetic reputation among the townsfolk. But was the doctor really correct? For science, this is not being correct, because the method lacks any real predictive power. Science must produce a model that can yield quantitative results and make verifiable predictions. Otherwise, it is of no use.
The doctor in our story can demand that we keep an open mind to his claim. He might say, “I’m not telling you for sure that you will die tomorrow, but you must admit that the possibility exists.” And what can I say?—I am very open-minded about that idea. I am perfectly aware that I will die someday, and that day could be tomorrow. That’s just the way it is. But the doctor’s statement is absolutely useless to me. It doesn’t provide me with any helpful information. His diagnosis won’t help me improve my health or allow me to start appropriate treatment if I have a medical problem. I gain nothing from such a generic prediction. In the same way, saying that there are aliens everywhere does nothing to help find them—which is what we aim for, and what many researchers are seriously working on within the disciplined methodology of science.
3. Debunking improbabilities
The paper published by Hibberd, Crowl, and Loeb cites a series of characteristics of 3I/ATLAS’s trajectory as arguments to support the claim that it is an alien spacecraft disguised as a comet:
Ecliptic – The path of this object lies very close to the plane in which the planets of the solar system orbit (the ecliptic), with an inclination of only 5 degrees relative to this plane. In principle, an interstellar object could arrive from any direction in space, so it would be quite a coincidence for it to come from the ecliptic. However, Loeb and his collaborators failed to take into account that ATLAS—the network of telescopes that discovered this object—is tasked with finding potentially hazardous objects in the solar system and observes predominantly along the ecliptic. Therefore, this improbability is neutralized. Even so, one might wonder why these extraterrestrial beings would want to send their craft along the ecliptic to attack Earth, as HCL propose. I am not a fan of making assumptions about alien psychology and motivations because I believe they would be so alien to us that we would be wrong with any guess we made. However, HCL do discuss possible motivations to support their proposal, so let’s play by their rules.
To decide from which direction you want to enter requires a propulsion system much more efficient and powerful than ours. For example, we humans cannot take our probes out of the ecliptic plane with current rocket propulsion technology; we need to perform gravitational assist maneuvers at planets so that probes like Ulysses or Solar Orbiter can change their inclination relative to the ecliptic. If extraterrestrials wanted to intercept Earth, they could have entered from any other direction and taken us more by surprise. Entering along the ecliptic gives them away—first, because scientists on Earth will realize this is an unlikely direction; and second, because our telescopes observe the ecliptic more than other regions. In other words, coming via the ecliptic is very costly in propulsion terms and makes it easier to be spotted by Earth’s astronomers.
In short, the arguments are inconsistent. It appears that if the observation comes up heads, it supports my theory; if it comes up tails, it also supports my theory.
Retrograde orbit – A very illustrative example of the problems with after-the-fact conditions can be seen in the interpretation of 3I/ATLAS’s retrograde orbit. One would expect an interstellar object to be equally likely to be prograde or retrograde. However, I personally would think that an alien spacecraft coming to the solar system with almost any intention would choose, if it could, a direct (prograde) orbit, which would make it easier to approach planets and with a lower relative velocity. As it happens, 3I/ATLAS travels in the opposite direction, yet HCL consider this fact to support the alien hypothesis because it would make it more difficult for us to intercept or approach it. This strikes me as one of those situations where if the coin comes up heads, I’m right; if it comes up tails, I’m also right.
Size – Another argument Loeb and collaborators put forward is that if 3I/ATLAS were an asteroid, it would be too large to fit with the fact that we have not seen other smaller ones. The thing is, no one has said that 3I/ATLAS is an asteroid. Everyone thinks it is a comet; therefore, this argument is entirely irrelevant. Improbability neutralized.
Outgassing – Comets produce gas emissions as they approach the Sun, due to the sublimation of the ices they contain. This gas emission is responsible for the small nongravitational acceleration they experience when passing near the Sun. HCL argue that 3I/ATLAS has shown no signs of this outgassing. But this is normal at these distances: comets we know in the solar system usually do not present observable emissions until they are within about 2.5 AU, with only a few exceptions [5]. Currently, 3I/ATLAS is a little over 3 AU away, so it is normal for it not to exhibit outgassing. At present, results are already emerging with observations showing incipient gas emissions, but even at the time HCL was published, this “evidence” was not such. Another improbability neutralized.
By the way, within the narrative of an alien spacecraft planning a surprise attack, one wonders whether a civilization so advanced would not be capable of releasing vapor to maintain the deception with Earth’s astronomers. In this article I am limiting myself to assessing the HCL paper with the information we had at the time of its publication, July 16. I am not including more recent information, but for completeness I note that there are now two recent papers claiming to have detected water vapor emissions from 3I/ATLAS [5,8]. Avi Loeb, on his blog, questions these results [9].
Planetary encounters – HCL claim that 3I/ATLAS passes very close to Venus, Mars, and Jupiter, and quantify this by calculating the probability that, at the moment of closest approach to each of these planets, the planet’s longitude and the object’s longitude are similar. This way of calculating is incorrect because the moment of closest approach and the longitude are not independent variables—there is obviously a relationship (albeit not a trivial one) between them.
In fact, we can see that in the case of Venus, there is no such close approach: the minimum distance to Venus is 0.63 AU. For comparison, from Earth we approach Venus at 0.28 AU. However, 3I/ATLAS and Venus have their closest approach when both are roughly in the same direction as seen from the Sun. But this is expected—if Venus were on the other side of the Sun, it wouldn’t be the moment of minimum distance.
There are, however, real close approaches to Mars and Jupiter. The Mars encounter is easy to explain: the trajectory of 3I/ATLAS is almost tangential to Mars’s orbit (see Figure 1), so sooner or later they were going to meet along that path, moving in opposite directions. One might argue that it is quite a coincidence that 3I/ATLAS’s trajectory is so similar to the tangent to Mars’s orbit (technically, we would say that the impact parameter relative to the Sun is similar to Mars’s orbital distance). But this is a misuse of statistics because, in reality, any trajectory taken by 3I/ATLAS through the inner solar system—given that it travels along the ecliptic—would have taken it very close to one of the planets. And in any of those cases, it could have been argued that it was quite a coincidence to have approached whichever planet it was.
That is the problem with after-the-fact selection of criteria. In fact, one would think a priori that the alien hypothesis would be reinforced by a close encounter with Earth. In reality, the opposite is true: Earth is actually the farthest away of all. Yet this fact is still used to justify the alien hypothesis. This is another case of tossing the coin—heads supports my hypothesis; tails also supports it.
The only genuine coincidence here is the encounter with Jupiter. I see no obvious reason for it and think it is the only one attributable to chance. Everything else seems explainable by selection effects. I leave the reader with the following exercise: Take a drawing of the inner solar system; draw an almost straight line at a random distance from the Sun; for each of the inner planets, toss a coin—if it comes up heads, put a mark on the same side as the line; if tails, put a mark on the opposite side of the Sun. Given that the interstellar object is moving very fast and the planet is moving in the opposite direction, the planets that end up on the same side of the Sun as the object will meet it. Check in this exercise that planetary encounters at distances similar to 0.2 AU (the one that will occur between Mars and 3I/ATLAS) will usually result. You will probably find some justification for why the extraterrestrials would want to visit the planets they encounter and avoid the ones they don’t. Improbability neutralized (except for Jupiter’s).
Radiant – 3I/ATLAS comes from “near” the galactic center, which, according to HCL, makes it harder to detect because there are many objects in that area. I note that these authors also claim that coming near the ecliptic suggests an artificial origin, when near the ecliptic it is actually much easier to detect. So, are they trying to avoid being detected or not? This seems like another situation where if the coin comes up heads, it supports my theory; if tails, it also supports it.
In any case, the object does not come from so close to the galactic center—it is about 28 degrees away in the sky—and on the other hand, it is to be expected that natural objects like comets would be more likely to come from the more densely populated regions of the galaxy. There is nothing surprising about an interstellar object coming from the general direction of the galactic center. Improbability neutralized.
Hidden perihelion – The remaining peculiar improbabilities of 3I/ATLAS are related to orbital mechanics and, above all, to the possibility of performing an Oberth maneuver to change its trajectory. This maneuver would be carried out at perihelion (the closest point to the Sun), which occurs when Earth is on the opposite side. According to Loeb et al., 3I/ATLAS’s trajectory is carefully calculated so that this maneuver can be done clandestinely, hidden from Earth’s view.
In the next section we will analyze the possible Oberth maneuver in detail and see why it makes no sense. But before delving into orbital mechanics, I would like to point out that the authors have not considered that not all our “eyes” are on Earth. We also have telescopes in space, and some are in locations far from Earth with very different lines of sight. In principle, spacecraft such as STEREO-A, Solar Orbiter, or the Parker Solar Probe could observe the entire perihelion passage of 3I/ATLAS and detect the supposed hidden maneuver. In any case, it seems naïve for aliens to assume that we would have no spacecraft or telescopes on other planets capable of seeing their maneuver when they pass behind the Sun relative to Earth.
4. Oberth maneuver
The key element in the hostile–spacecraft narrative is the clandestine Oberth maneuver, with which the extraterrestrials would supposedly intend to change course to put themselves—by surprise—on the right trajectory for their nefarious purposes, perhaps settling into orbit around the Sun or Jupiter, perhaps preparing a rendezvous with Earth. The Oberth maneuver is something we routinely do when sending probes to other planets and consists of taking advantage of the passage through periapsis (the closest point to the body being orbited—whether the Sun or a planet) to fire the engines. This is the optimal moment to do so: when a spacecraft increases its velocity by a certain amount—say, Δv—the gain in orbital energy is much greater if done at periapsis than in free space.
However, in the case of 3I/ATLAS, there is no sense in doing this—much less trying to synchronize it with passage through perihelion. To see why, we must first understand what the Oberth effect actually is.
There are many published derivations of this effect—for example, Wikipedia provides formulas for a parabolic trajectory. Here, we will do a similar derivation but in a more general way, applicable to a hyperbolic trajectory (the kind an interstellar object would have), which may have a nonzero velocity at infinity. It is straightforward and can be followed with high school–level knowledge. Readers uninterested in these details may skip directly to the Conclusions section below.
Let’s start with the definitions of the symbols we will work with:
Consider an object in a certain orbit that at a given instant receives a velocity increment from firing its engines. The orbit can be elliptical, parabolic, or hyperbolic, and it can have velocity at infinity or not. It could even be a straight line in empty space. The specific kinetic energy (per unit mass) after the burn corresponds to the new velocity, which is the sum of the pre-burn velocity plus the velocity increment from the propulsion:
In other words, the kinetic energy we have after the burn is the sum of three terms:
a) what we had before,
b) what the burn would contribute if starting from rest, and
c) a term that is the product of the pre-burn velocity and the velocity increment.
This last term accounts for the Oberth effect: the kinetic energy added by the burn is greater the higher the velocity at which it is performed. Note that—contrary to common belief—the Oberth effect is not a gravitational effect. We do not gain energy because the burn is done near a massive body, but because it is done when we are moving faster. In the equations above, there is no dependence on the mass of the central body nor does the gravitational constant appear. The reason we do Oberth maneuvers at perihelion (or periapsis, in general) is simply that there we are moving faster.
From perihelion until completely escaping the potential well, we lose a certain amount of kinetic energy equal to the potential energy gained:
If there were no Oberth effect, the last term would not be present, and the specific kinetic energy at infinity would be what we might intuitively expect: the sum of the initial kinetic energy plus that provided by the burn (if starting from rest), minus what is lost to climb out of the well (converted into potential energy).
In reality, however, we exit with more energy than that:
And therefore, we have gained:
Again, as we can see from the formula, the gain is purely a kinetic effect, not gravitational. All of this applies equally to a spacecraft moving in empty space.
Readers with some physical intuition may have already noticed the problem with the idea of a supposed Oberth maneuver by 3I/ATLAS, but before we get to that, let’s turn the screw once more. It is often said that the Oberth maneuver must be performed exactly at perihelion, and HCL point out that 3I/ATLAS will reach perihelion precisely when hidden behind the Sun from Earth’s point of view. It is true that the maneuver’s efficiency is highest the closer it is done to perihelion because, as we have seen, the energy gain depends on velocity, which is maximal there. But how critical is this requirement? What happens if we don’t do it exactly at perihelion but a little before or after?
Every engineer—and certainly those in the space field—knows that there are no exact values and that any engineering requirement must be given with some tolerance. When mission engineers plan an event to execute, they must consider tolerances—allowable error margins so that the mission remains a success. This is what gives us, for example, launch or burn windows: the amount of deviation we can allow without falling outside the acceptable margin for the final result.
When we see illustrations of Oberth maneuvers in astronautics books, the trajectory is always shown as very curved, with a sharply defined periapsis. In contrast, when we look at 3I/ATLAS’s trajectory in Figure 1, we see that it is almost straight—barely curving at all. This is an effect of its high velocity relative to the Sun and the great distance at which it passes. This means the perihelion is “weak”—there is very little difference (especially in velocity) between the exact perihelion and points somewhat before or after. Let’s quantify this to determine the acceptable positional window for an Oberth maneuver.
In the previous formulas, we have been working with pre-burn velocity, leaving open the possibility that it is not the same as the velocity at perihelion. Let’s now make this explicit: suppose we do not burn at the optimal point (perihelion) but at some other point where the velocity is slightly lower. Let’s call this difference δv (it is negative, because the velocity will be lower at other points than at perihelion):
If we substitute this into the previous formulas, we can propagate δv and calculate how much we lose by burning away from perihelion. The energy lost compared to an optimal maneuver is:
And in relative terms:
Conclusions
Let’s apply all this to the case of 3I/ATLAS and see some numbers. We said that it seems like quite a coincidence for perihelion to occur just when Earth is on the other side and thus a good opportunity for a hidden Oberth maneuver. But how far could the aliens deviate from perihelion if their timing were not very precise and still do the maneuver clandestinely?
The velocity at perihelion will be about 68.34 km/s. One day before or after: 68.33 km/s. One week before or after: 68.17 km/s. One month before or after: 66.2 km/s. Two months before or after: 63.61 km/s. Since the maneuver’s power depends only on the velocity difference, doing it a month from perihelion would reduce the energy gained by a meager 3%. With a two-month margin, the difference would be only 4%. In other words, the aliens would not need to synchronize the perihelion precisely with being hidden behind the Sun. They have a four-month window in which they would still have 96% of the maneuver’s effectiveness, and there would be no special reason to synchronize it exactly. This lack of sensitivity is a direct consequence of the comet’s nearly straight trajectory, as we noted earlier.
But the Oberth absurdity goes even further. Precisely because this object is so little deflected by the Sun’s gravity, an Oberth maneuver provides no significant gain. The confusion arises because people usually consider the Oberth gain of an object at perihelion compared to being at rest at infinity. But 3I/ATLAS is far from being at rest at infinity—in free space, it was traveling at 58 km/s. As it approaches the Sun, it accelerates slightly (not much) to 68 km/s at perihelion. Recall that the Oberth effect is purely kinetic, so the relevant comparison is between burning at 58 km/s versus burning at 68 km/s. Suppose we want to make a burn for Δv = 10 km/s. The difference in energy gained between doing it at infinity or at perihelion is only a factor of 1.12—that is, just a 12% energy increase by doing it at perihelion. That’s a pitiful gain, hardly worth the risk of revealing our presence—especially if we are a highly advanced civilization with the tremendous propulsion capabilities needed to choose to come via the ecliptic or from the galactic center in the first place.
With this, and the explanation of all the supposed improbabilities (except the close pass to Jupiter), it is clear that to date there is nothing peculiar about 3I/ATLAS compared to what we would expect from an interstellar comet. We cannot prove that it is not an alien spacecraft disguised as a comet with intentions to attack us in November—but there is no reason to think it is. Someday, we will discover intelligent extraterrestrials—biological or artificial, living or (more likely) extinct. I do not know whether we will live to see that day or if it lies in the distant future. But I am certain that this achievement will not come thanks to those who see aliens in every meow, but in spite of them. In the uninformed public’s imagination, however, the names of those who make noise will remain as the champions of alien-hunting, while the serious work is—as usual—done anonymously, chipping away at the rock.
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