Which body in the Solar System is the best candidate for being an artificial construct, placed there by a Kardashev Type II Civilization?

Some notes:

  • The purpose of this body is to monitor earth, with as little likelihood of detection as possible until Earth reaches Type I status.
  • "Major" in this case means well known to the astronomical community and somewhat to the public in general (a planet, moon, dwarf planet, or large asteroid or comet). I am looking for a specific, named body.
  • This must be a body that we can observe right now. No fictional planets or ninth planets allowed
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    $\begingroup$ Surely the advanced aliens have microelectronics and whatever they use to monitor us is tiny. A few devices on the moon that take pictures of earth, monitor radio signals and beam the data back using lasers. The devices could be small and well disguised. see cubesat $\endgroup$ Commented Mar 18, 2017 at 15:12
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    $\begingroup$ This is a GREAT question. I nominate it for Question of the Year. Specific, on topic, short, interesting, drawing on science for a hypothetical result. THIS is a great WB question in my opinion. $\endgroup$
    – SRM
    Commented Mar 18, 2017 at 16:10
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    $\begingroup$ What about the sun? I'd say it would need to be in the last place we could ever explore. $\endgroup$
    – n00dles
    Commented Mar 19, 2017 at 15:24
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    $\begingroup$ "No ... ninth planets allowed" Poor, poor Pluto. How quickly they forget. $\endgroup$
    – Steve-O
    Commented Mar 20, 2017 at 20:43
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    $\begingroup$ "All these worlds are yours - except Europa. Attempt no landings there." $\endgroup$ Commented Mar 21, 2017 at 9:42

17 Answers 17


They could use any Near-Earth Asteroid. These are asteroids that are in the vicinity of the Earth.They only last for a few million years, but such a civilization could easily install some stealthier thrusters (like cold gas thrusters) to use when not in view of the Earth to increase that lifespan.

Being near to the Earth means that whatever tools they use to monitor the Earth would have an easier time seeing it, while there are so many Near-Earth Asteroids that this would not draw too much attention.

Note that there is a risk of people attempting to use it for materials and discovering that it has advanced science equipment on there, but this risk would be small if it looks like a normal Near-Earth Asteroid until people actually start trying to build a dyson swarm.

  • 6
    $\begingroup$ This made me wonder. If they added a new Near-Earth Asteroid right now, what are the chances we'll notice it is new, and not just assume we haven't noticed it before yet? $\endgroup$
    – William
    Commented Mar 19, 2017 at 10:12
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    $\begingroup$ @WilliamMariager We find new ones all the time. We would just assume it was one that we hadn't noticed before, unless it especially large $\endgroup$
    – James K
    Commented Mar 19, 2017 at 12:07
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    $\begingroup$ @JarredAllen: Such an object would be ripe for study. It would be the single most attention-grabbing object in the Sun's Hill sphere. Expect humans to send a probe to such an object as soon as possible. $\endgroup$
    – dotancohen
    Commented Mar 19, 2017 at 16:08
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    $\begingroup$ @kingledion The page that I linked to contains several such bodies that the OP could choose from. I figured that it would be better for the OP if I just listed the entire category and let the OP choose which one would work best for the story. $\endgroup$ Commented Mar 19, 2017 at 21:06
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    $\begingroup$ I feel like we're getting lost on the topic of would a newly added NEA go unnoticed, but it seems like the answer is just saying to "wire-up" a preexisting one? $\endgroup$
    – DasBeasto
    Commented Mar 21, 2017 at 12:26

Earth itself

There are lot of unique planet properties that are too good to be conincidence and are usually explained via weak anthroptic principle, such as:

  1. Lots of water in its best chemical state, thanks to optimal distance to the Sun
  2. Magnetic field protecting life and water from solar winds
  3. Just enough volcano activity to provide chemical factory for life, but not for destroying it.

and many others

So many unique properties makes more realistic synthesizing habitable planets than searching them for single-galaxy civilization.

We could imagine that some of the unexplored places at ocean bottoms or under earth crust have some signs of planet synthesis or designed for collecting and transmitting information about the current life state. Some of that signs could be right before our eyes, yet we could not distinguish them from natural, due to lack of knowledge of planet design on cosmological level.

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    $\begingroup$ I like this idea. It's the last thing the fictional earthlings, or the audience would expect. Any solar system body moon or planet is familiar in name only, but still 'alien' from the perspective and experience of audience members. $\endgroup$
    – Tom
    Commented Mar 18, 2017 at 21:29
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    $\begingroup$ @Tom Well, we wouldn't have expected it until Douglas Adams spilled the beans. $\endgroup$ Commented Mar 19, 2017 at 7:03
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    $\begingroup$ Once the Silurians wake up from their cryo sleep, we will have a lot of problems to deal with. $\endgroup$ Commented Mar 20, 2017 at 3:06
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    $\begingroup$ @MarkRogers they weren't an alien race though. They were just dinosaurs like any other dinosaur. Space faring? yes. But they were from Earth originally. $\endgroup$
    – user64742
    Commented Mar 22, 2017 at 2:48

L4 would be a good place. Asteroids at L4 are called Trojans. Earth trojans could maintain an orbit that doesn't go behind the sun relative to the Earth.

There is one Earth Trojan: 2010 TK7 (wiki). It orbits the sun taking one year, but is ahead of the Earth, allowing this orbit to be stable. Its actual position, relative to the Earth does change, it loops in a "tadpole" shape relative to the Earth (its actual orbit, relative to the sun is elliptical, only relative to the orbiting Earth is the orbit tadpole-shaped). Bringing it closer and further from the Earth, but never nearer than 50 lunar distances.

Good points: it stays roughly the same distance from Earth, not getting too close, or too far; continuous monitoring should be possible. It is a good size, about 300m. Big enough for a data centre and transmission equipment. Small enough to be hidden and not take an unreasonable amount of time to build.

Bad points: Its orbit is not very stable, in the longer term it is chaotic, it is possible for it to flip from ahead of the Earth to behind the Earth (with periods behind the sun) In the much longer term it could get too close to the Earth and get ejected (or worse) however, station keeping every 100 years or so could keep it stable. It doesn't get close enough for detailed observations, though this would depend on the type of equipment you installed there.

Another object, with a somewhat similar orbit is 3753 Cruithne. It is in a 1:1 resonant orbit with Earth, and has been for quite a long time. Its orbit takes it further from Earth, but it never goes completely behind the sun, so Earth stays visible at all times. Its orbit is likely to be more stable than 2010 TK7, and it has some reputation (it has a proper name and has been mentioned at least twice on QI - satisfying the "somewhat known to the general public" criteria) It is also larger, about 5km across.

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    $\begingroup$ And bonus points if the secret observation asteroid is later used to sneak troops into the solar systems. (Trojan Horse) $\endgroup$
    – Valthek
    Commented Mar 20, 2017 at 15:15

Consider a short-period comet (specifically, Comet Encke).


  • Very short-period comets can have orbital periods of only a few years, with approaches relatively close to Earth ($\sim0.1\text{ AU}$) happening every couple of decades.
  • Comets will spend some time away from Earth, and may thus escape the watchful eye of people worrying about asteroids that could hit the Earth. This is an advantage over Jarred Allen's excellent choice.
  • Comets can fragment, so if the species wanted to send a scout ship to Earth, it wouldn't appear to be much different form normal cometary behavior. An asteroid spontaneously breaking up, though, would look suspicious.

It might be harder to observe Earth when the "comet" is further away, but this may be an advantage, as it would be much harder for people on Earth to see that the "comet" isn't actually a comet. In addition, using multiple "comets" could mitigate this.

Other notes

It seems that all of the answerers to date agree that the object in question should be small. There are a few good reasons for this:

  • Small objects may be harder to observe from Earth unless they're much closer. They can tumble erratically, for instance, so tracking a point on the surface isn't easy.
  • It's not easy to land probes on them. We've done it before - you may recall the Philae lander recently - but you can't put a rover on them like we can on mars or other large bodies. These two criteria mean it would be harder for us to figure out that these bodies are artificial.
  • They can be closer to Earth than larger bodies can. This is why you don't pick, say, Jupiter, which would be much farther away than any of the choices we're picked so far.
  • It's easier to build a small object than a large one.
  • $\begingroup$ Mainly the last one. Building Pluto would take about 100.000.000 times more material than building Encke. $\endgroup$
    – Tgr
    Commented Mar 19, 2017 at 20:49
  • $\begingroup$ Perhaps these comets are simply probes which report back to a hidden interstellar transmitter on the edge of the solar system? $\endgroup$
    – Kys
    Commented Mar 21, 2017 at 20:34

That's No Moon

Use Earth's moon. When earthlings first try to land on the Moon, lay out some fresh dust and wait for them to leave. The novelty of the Moon will die down. The Moon, while a constant companion, is still really far off. Earth would be none the wiser until they started mining. That won't happen until there is an efficient way to get resources back and forth from Earth. This probably won't happen until Earth is Type I.

Extra Credit

If the earthlings get to close to fast, here are some strategies the Type II civilization might employ:

  • Spread some doubt (conspiracy theories) to force the mainstream media/leading scientists to keep repeating "Yes, we've gone to the Moon". The general public will become disinterested more quickly this way.
  • De-fund agencies with intentions of returning to the Moon to prevent any awkward mishaps.
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    $\begingroup$ baen.com/mutineer-s-moon.html, much? $\endgroup$
    – nijineko
    Commented Mar 19, 2017 at 19:41
  • $\begingroup$ @nijneko:that was exactly what i was thinking when I read the Question $\endgroup$ Commented Mar 21, 2017 at 7:35

3753 Cruithne is an asteroid about 5 kilometers in diameter with a solar orbital period of almost exactly 1 Earth year. It is about 12 million kilometers from Earth at closest approach, and is never in a position where Sol is directly between it and Earth.


The science-fiction author Arthur C Clarke had already proposed a suitable candidate for an artificial object that is an artificial construct. Namely, the Jovian moon Jupiter Five. This has a nearly circular orbit around the planet. Its albedo is bright enough to be consistent to its surface being polished metal. The diameter of Jupiter Five is thirty-five kilometres.

enter image description here

Clarke employed the concept in his short-story, appropriately titled, "Jupiter Five" where it is discovered that Jupiter Five was a spaceship that had brought members of what was called "Culture X" to the solar system, possibly, millions of years ago.

Why go past the work of a master of the art. Jupiter Five is a suitable candidate to be a monitoring artefact installed in the solar system by Kardashev Type Two civilization. In all probability, this would be their base of monitoring vessels and surveillance systems to observe life and activity of the human species on planet Earth. Jupiter Five is an object well known to astronomers.


Thanks to kingdelion's comment about Jupiter Five being the Jovian moon Amalthea. Further information about Amalthea can be found here. This suggests that Amalthea is less of a metallic spheroid, as in Clarke's story, but more of a pile of rubble and icy material. Still there is no requirement for Kardashev Type 2 civilizations to be neat builders. Generally what may have been a good idea in 1953 may not be so brilliant now in 2017. Orbiting piles of rubble could be still the perfect cover for galactic monitoring stations.

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    $\begingroup$ Jupiter V is Amalthea. $\endgroup$
    – kingledion
    Commented Mar 19, 2017 at 20:31
  • $\begingroup$ @kingledion Good to know. Clarke called it Jupiter Five throughout his story, but didn't mention its name. I'm surprised he didn't. I'll check your source and edit my answer appropriately. Much appreciated. $\endgroup$
    – a4android
    Commented Mar 20, 2017 at 0:17
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    $\begingroup$ It was named in 1976, Clarke wouldn't have had anything else to call it when he wrote in 1953. $\endgroup$
    – kingledion
    Commented Mar 20, 2017 at 0:27
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    $\begingroup$ @kingledion That's good to know because Clarke was usually careful about astronomical details. Even simple things like names. $\endgroup$
    – a4android
    Commented Mar 20, 2017 at 0:38
  • $\begingroup$ That`s no moon... $\endgroup$
    – xDaizu
    Commented Mar 21, 2017 at 8:19

No major solar system body could realistically be said to be artificial.

However, Iapetus looks artificial. It is a major body (you accept comets and asteroids but I'm setting the bar higher), has odd features that could be explained by an artificial origin and is not nearly as large as the planets and largest moons or dwarf planets, making construction easier.

(Also it is the Death Star.)

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    $\begingroup$ How do we puny humans know if solar system bodies could be 'realistically' artificial? $\endgroup$
    – kingledion
    Commented Mar 19, 2017 at 20:32
  • $\begingroup$ All Bodies could be artificial - and we could not know beacuse we dont have anything to compare. Interesting Idea - a whole Solar System could be formed artificially with far advanced tech. That does not mean that it would not be formed of stone, ice etc. $\endgroup$ Commented Mar 21, 2017 at 7:39
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    $\begingroup$ The seam is a dead giveaway. $\endgroup$
    – Spencer
    Commented Mar 21, 2017 at 23:39

One overlooked possibility is to hide your satellite right under our nose(s) - among human produced Earth satellites. While not considered "major" bodies, these fit the question as they are regularly tracked and catalogued by professional and amateur astronomers alike, and any unknown satellite would be automatically considered a secret military bird (perhaps belonging to the opponent from the Cold War era). You do not even need to hide the artificial signatures of the satellite (tough it is advisable to paint it with the same spectral characteristics as produced by Earthling paints).

Wikipedia has a category for reconnaissance satellites, of course. Those that were declassified or those where the information leaked,anyway.

  • $\begingroup$ Even if it deorbits and components crash intact, super-intelligent aliens should have no problem making a satellite look like one of hours. Remember, anyone claiming the crashed satellite was made by aliens will be labelled a crackpot. $\endgroup$
    – gerrit
    Commented Mar 20, 2017 at 11:14
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    $\begingroup$ @gerrit and there are measures against reverse engineering possible, if they suspect the satellite might deorbit and survive the reentry (nothing is foolproof, not even alien technology) - e.g. coat the electronics in phosphorus. But anyway, do not design it able to survive the reentry (re- is probably not an appropriate prefix here :-)) For added misinformation value, label some dummy parts in Arabic. $\endgroup$ Commented Mar 20, 2017 at 16:03
  • $\begingroup$ This answer dodges the question by trying to answer the story goal of the question instead of the question asked. The question is "Which body of solar system could be artificial?" and not "How could aliens spy on Earth?" $\endgroup$
    – SRM
    Commented Mar 21, 2017 at 12:34

Deimos, moon of Mars? Its origin is somewhat unknown, it isn't too big and has been known for quite a while, 1870 or something. It's close to Earth so it can study us in relative detail.

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    $\begingroup$ "there is little alternative to the hypothesis that it is hollow and therefore Martian made." -- Hollow Phobos. $\endgroup$
    – David Cary
    Commented Mar 18, 2017 at 17:14
  • $\begingroup$ @DavidCary: You're cherry picking your quotes. That subsection goes on to say "Current observations are consistent with Phobos being a rubble pile.[57] In addition, images obtained by the Viking probes in the 1970s clearly showed a natural object, not an artificial one". $\endgroup$ Commented Mar 18, 2017 at 17:33
  • $\begingroup$ @BoundaryImposition: Assuming stealth was desired (as the OP does), a "natural" appearance would be expected. $\endgroup$ Commented Mar 18, 2017 at 23:52
  • $\begingroup$ @WilliamMcBrine: I agree but that doesn't change the fact that David is quoting a statement that is no longer operative within the scientific community $\endgroup$ Commented Mar 18, 2017 at 23:59
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    $\begingroup$ @DavidCary The full quotation is: "If the satellite is indeed spiraling inward as deduced from astronomical observation, then there is little alternative to the hypothesis that it is hollow and therefore Martian made. The big 'if' lies in the astronomical observations; they may well be in error. Since they are based on several independent sets of measurements taken decades apart by different observers with different instruments, systematic errors may have influenced them." The same Wikipedia entry confirms Singer was right about systematic errors. Hollow Phobos is scientifically null and void. $\endgroup$
    – a4android
    Commented Mar 19, 2017 at 1:04

In Clifford Simak's story "Construction Shack," humans arrive at Pluto and find it is hollow. They go in and find the blueprints for the Solar System! Apparently, whoever had the job of building the Solar System built Pluto first, as the base from which they would work. My father was a civil engineer with the New York State Department of Transportation. When they were constructing a new superhighway, they would park a mobile home on the site; this would serve as the office for the engineers, break room, etc.


Working up Wikipedia's list of solar system bodies in size order, S/2009 S 1 seems to be the smallest body that meets your criteria (it's a moon of Saturn), and therefore the most likely to be an alien artifact (it's only 300 metres across). If you want bodies that people have heard of, Icarus is a reasonably well-known asteroid and is only 700 metres across.


This is already in one of your answers, but Luna is, in my opinion, an excellent candidate for several reasons:

1) The placement and size of the moon is perfect for keeping the earth's axis from wobbling. This prevents, for example, half the planet from entering permanent day or night when the axis wobbles to face the sun. This has happened to Mars in the past and is theorized to be one of many reasons the red planet doesn't have large amounts of life.

2) It's large enough to house any number of highly technological constructs without risking their detection.

3) The capabilities of a Type II civilization are so far beyond what we are currently capable of that we don't even know if such a civilization would need to have actual monitoring equipment as we know it. For example, they may have forged a deep connection with the universe in order to "borrow" data storage and processing power from the universe itself, such that their thoughts don't necessarily have to happen within the confines of their own brains. This would render them effectively omniscient, so monitoring equipment would be beyond redundant. If this were the case, the only evidence of their interference with the earth might just be in the odd coincidences that resulted in life on Earth.

4) The moon represents an interesting candidate because one side always faces the earth, so any technological constructs could easily be hidden on the "dark" side of the moon under a small layer of dust without damaging broadcasting capabilities, and it could also serve as a plot device allowing governments to keep the discovery of such a facility a secret from the general public. Excellent fodder for the conspiracy theory crowd.

5) So much of our biology and culture is related to lunar cycles and changes that discovering that it is artificial or that it was placed there as some sort of extraterrestrial intervention would send shock waves throughout all human society. Gestational periods of human women, mathematical conventions for measuring angles and other geometric constructs, architecture both ancient and modern, and pretty much all life on earth reflects strongly the influence of our primary satellite.

If you dig into it, some happy accidents of mathematics occurred because of the influence of the moon on human culture. If an artificial satellite created or at least influenced in some way by a type II civilization is a central plot point for you, I believe the moon would be an excellent choice because of how powerful such a revelation would be to the whole of mankind.


Let me give a different answer from the others.


Why Pluto? Simply because it is by far the least explored of the major astronomical bodies, and the least likely to have had it's artificial status discovered. It's also surprisingly small by comparison with it's almost-brethren. Sure you can go with an obscure asteroid, but where's the drama in that?

It's distance from Earth makes it's monitoring job harder, but that shouldn't be a problem for the kind of civilization we are talking about. Or you can go down the route of "The Sentinel", where the discovery of the artifact is the event that means Human civilization is ready to be contacted.


If by "realistically be artificial" you mean easiest to construct and place then the answer (which admittedly is a cop-out) is whatever the lightest object that meets your criteria for major solar system body is.

The energy expenditure to place even a small asteroid into solar orbit from outside the solar system is going to dwarf any other design considerations.


If the real thing that would give you away would be heat production (of whatever technology you leave behind, plus power production), then maybe the best bet is one of the moons of Jupiter with interior oceans. There you have enough natural heating that you could possibly hide the heat of clever technology and perhaps even be able to use the natural heating to provide power. And the ice could act as radiation shielding for the more sensitive gear.

Maybe an advanced enough civilization could use nanobots injected into the geysers as sensors and/or once and a while drill up and stick out a telescope (or whatever) to take a quick glance at Earth.


A black hole.

Impersonating a black hole would provide a drool-worthy level of secrecy; however there are several problems associated with it.

Problem: Black holes bend light which requires stupid-levels of gravitational pull, likely making the ship itself an inhabitable pressure-cooker.
  • Unless: The civilization piloting the vessel has especially kick-ass refraction and/or reflection technology to emulate the phenomenon.
  • Or Unless: said vessel had slightly-less-kick-ass-,-but-still-pretty-kick-ass refraction/reflection tech, and only a directed illusion was required.
    • For example: If the cloaked vessel was traveling towards a planet, it would have an easier time directing its illusion at the inhabitants of that planet than it would appearing as a black hole to the entire galaxy.
Problem: A few issues:
  • A.) I'm being vague about the aforementioned "kick-ass" light-bending tech.
  • B.) Even in my "Or Unless:" example, the difficulty of maintaining such an illusion would scale with distance the from the target.
  • C.) Black holes do not simply pop into existence. If the target planet or vessel was pre-space-flight, there's a good chance they'd still pick up on a black hole appearing out of nowhere.
    • A.) Unless: I am being vague about potential optics-tech on purpose. Future tech b crazy B. Get creative!
    • B.) Unless: Yeah this would be an issue...Ideally a probe could be sent ahead of schedule to maintain the illusion from a fixed point relatively close to the target.
    • C.) Unless: Maybe they do?! Wouldn't it be a spectacle to witness a never-before seen cosmic-occurrence? Assuming that the target is stationary & less technologically advanced, they could be studying the illusionist's decoy right up to the point when a bunch of warships pop out behind it.
Problem: Gravitational waves dude.
  • Unless: um...yeah. It's science fiction! Maybe people can project gravitational waves now? It's up to you.
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    $\begingroup$ I really hope we don't have a black hole as a major solar system body! $\endgroup$
    – Bobson
    Commented Mar 20, 2017 at 21:36
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    $\begingroup$ How does a black hole keep a functional monitoring equipment. And how does it transmit any monitoring information? And how is it powered? $\endgroup$
    – xDaizu
    Commented Mar 21, 2017 at 8:15
  • $\begingroup$ Hahaha -3. Awesome. The idea I proposed involved the illusion of a black hole. That is, it would appear as a black hole to someone looking at it from a certain perspective, but would be achieved by an impressive ability to bend light. The illusion would require bending light around some planar circle, & the surface of the illusory-plane would need to be oriented perpendicular to the target. The plane itself would ideally be a blackbody, absorbing any photons impacting its surface. This would preclude the projection of monitoring signals, but it would be a perfect passive monitor. $\endgroup$
    – Rob Truxal
    Commented Mar 21, 2017 at 20:50
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    $\begingroup$ There is no black hole in the Solar System, and the question specifically includes the phrase "This must be a body that we can observe right now". I don't think this is an answer. $\endgroup$
    – HDE 226868
    Commented Mar 22, 2017 at 18:02

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