I'm interested in our recent interstellar visitor 'Oumuamua.

Whilst I'm happy to accept the official explanation of what it is, I'm curious about what state it would be in if it was actually alien space junk that's been drifting for hundreds of millions of years.

Assuming it started as a recognizably artificial structure, after 100 million years would it have been affected by micro-meteors? Would it have accumulated a coating of 'space dust' in its travels? Would it still be recognizable as an artificial structure after all this time?

The NASA site I linked suggests it's about 400m long, 40m across, irregular in shape, has a rocky surface and is reddish in colour. Wouldn't this description also apply to a chunk of space junk that has collected accumulations of space dust held together by micro-gravity?

If it was alien space-junk how close would it have had to pass to earth for us to recognize it as such? Apparently it passed at a range of 200 million kilometers, so I'm guessing no one really managed to get a very good look at it.

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    $\begingroup$ We don' know the size of Oumuamua precisely. Its length may be 100 to 1000 m and its width 35 to 167 m. We only observed it as a single pixel with intensity variations. We could neither observe its size nor its shape. The elongation of about 10:1 is only the best model found to explain the measured light curve. We assumed a typical albedo of 10% to deduct its size from measured light intensity. $\endgroup$
    – Uwe
    Dec 14, 2018 at 11:04
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    $\begingroup$ close voters please hold your horses! This paper contains a substantial amount of information from which I would like to write a science and fact-based answer. It does not make sense to insta-close this question claiming that answers would be primarily opinion-based when a fact-based answer is so easy to write. This close reason should only be used when you are fairly certain that nobody can post a reasonable answer to the question without it being primarily opinion based, not just because you can't think of an answer yourself. $\endgroup$
    – uhoh
    Dec 14, 2018 at 11:26
  • $\begingroup$ @Uwe If we've only observed it as is a single pixel, how can (its elongated) shape be deduced? $\endgroup$ Nov 11, 2019 at 22:21
  • $\begingroup$ @Coldblackice Please read my full comment, the explanation is there. $\endgroup$
    – Uwe
    Nov 12, 2019 at 8:59
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    $\begingroup$ @Coldblackice it would be a nice question for astronomy stackexchange: If we've only observed Oumuamua as a single pixel, how can (its elongated) shape be deduced? $\endgroup$
    – Uwe
    Nov 13, 2019 at 21:50

1 Answer 1


To address the tl;dr request, for an answer to "would we be able to tell", the answer is "possibly", using thing like I've mentioned in the bullets following

For example, things like:

The paper Could Solar Radiation Pressure Explain 'Oumuamua's Peculiar Acceleration? published in The Astrophysical Journal Letters (Shmuel Bialy and Abraham Loeb, 868, 1, ) can also be viewed and downloaded in preprint form from ArXiv.

We explore the possibility that the excess acceleration results from Solar radiation pressure. The required mass-to-area ratio is (m/A) ≈ 0.1 g cm−2. For a thin sheet this requires a thickness of ≈ 0.3−0.9 mm. We find that although extremely thin, such an object would survive an interstellar travel over Galactic distances of ∼ 5 kpc, withstanding collisions with gas and dust-grains as well as stresses from rotation and tidal forces. We discuss the possible origins of such an object. Our general results apply to any light probes designed for interstellar travel.

In order to quantitatively explore the possibility that 'Oumuamua could be a thin artificial structure such as a solar sail, the authors have carried out thorough scientific analysis of the question. I've captured bits of their analysis below. From this it is clear that even for a wide variety of material types considered, and for both natural and artificial object, there would be some clear evidence of interstellar travel. These would include

  • sputtering from interstellar hydrogen and cosmic rays (primarily protons)
  • micro-cratering from collisions with interstellar dust

The article does not describe the amount of accreted material on the surface, nor the resulting optical properties. Therefore visual cues like color would be a poor way to try to distinguish an artificial from a natural body after an interstellar trip like this.

However even for thin shells of material ≈ 0.3−0.9 mm considered here, the paper concludes that the underlying material would remain intact. This means that if one could explore bulk properties, a lot could be determined.

For example, things like:

  • solar X-ray induced X-ray fluorescence spectroscopy; see this answer
  • cosmic ray-induced X-ray fluorescence spectroscopy
  • cosmic neutron-induced gamma ray spectroscopy
  • interaction with and reflections of radar beams at a wide range of frequencies (HF to THz) to try to get a handle on electrical resistivity
  • A long-distance impactor could also be used to induce hot vaporization of material that could then be analyzed spectroscopically. This technique has been demonstrated in several cases with both natural and artificial impactors.

might be just a few (mostly) non-contact methods that could be used to address the composition of 'Oumuamua and to address the question of it more likely being of natural or artificial origin.

NOTE: This Astrophysical Journal Letter is densely written, full of math, and heavily sourced. I've just captured a few "sound bites" from it, but the reader is encouraged to dive in to the ArXiv version first, before asking questions about the paper's conclusions.

3. Maximum Distance for Interstellar Travel

3.1. Momentum Transfer - Slow Down

Evidently, ‘Oumuamua can travel Galactic distances before encountering appreciable slow-down.

enter image description here

3.2. Energy Transfer - Collisions with Dust-Grains

For our constrained value for the mass-to-area ratio, ‘Oumuamua can travel through the entire galaxy before a significant fraction of its mass is evaporated. Evaporation becomes important at higher speeds.

3.3. Energy Transfer - Collisions with Gas Particles

When an object travels at a high speed, collisions with atoms in the ISM can potentially transfer sufficient energy to produce sputtering... At higher speeds, the yield increases but typically remains below 0.01 (Tielens et al. 1994), thus at any velocity, vaporization and slow-down remain the dominating processes limiting the allowed distance an object can travel through the ISM.

Cosmic-rays are expected to cause even less damage. Although their energy density is comparable to that of the ISM gas, they deposit only a very small fraction of their energy as they penetrate through the thin object.

4. Tensile Stresses:

4.1. Rotation

Thus, even when self-gravity is ignored, ‘Oumuamua can easily withstand its centrifugal force.

4.2. Tidal Forces

Thus, unless ‘Oumuamua encountered an extremely close approach to a star in its past, it is unlikely that tidal forces played any significant role.

5. Summary and Discussion:

For a planar geometry and typical mass densities of 1–3 g cm−2 this gives an effective thickness of only 0.9–0.3 mm, respectively. For a material with lower mass density, the inferred effective thickness is proportionally larger. We find that although very thin, such an object can travel over galactic distances, maintaining its momentum and withstanding collisional destruction by dust grains and gas, as well as centrifugal and tidal forces.

enter image description here

Source: Washington Post's Harvard’s top astronomer says an alien ship may be among us — and he doesn’t care what his colleagues think

Avi Loeb poses in the observatory near his office in Cambridge, Mass. His theory about an alien spaceship has made the rounds in the media and caused controversy in the academic community. (Adam Glanzman/For The Washington Post)

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    $\begingroup$ This is pretty much all over my head, can you provide a tl:dr section? $\endgroup$ Dec 14, 2018 at 15:14
  • $\begingroup$ @JamesJenkins I've read your questions and answers and so I know that it's not exactly over your head ;-) However it might be a bit on the detailed side, which was to counter the two close votes for "primarily opinion-based". I've added a proxy for a tl:dr at the top. I'll check back tomorrow, let's see what other answers appear in the mean time. Thanks! $\endgroup$
    – uhoh
    Dec 14, 2018 at 15:20
  • $\begingroup$ Why a thin solar sail rather than a cylindrical tank (based on the commonly accepted idea of a cylindrical rock?) $\endgroup$
    – ikrase
    May 13, 2020 at 1:46

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