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Edit: Edited the title, following Mazura's suggestion, for a better wording of the question.

NASA and other space agencies have considered installing radio telescopes on the far side of the Moon. The sheer mass of the Moon blocks any signals coming from Earth which may interfere with radio signals coming from space. The Moon is outside the orbit of all communication satellites, and the far side is an ideal place for placing such telescopes.

My world involves a planet which has been colonized by humans abducted by aliens and placed there. The planet is orbited by a large asteroid, not a moon. It is large enough to be visible from their planet as a bright star and is used in their lunar calendar. Assuming an almost round shape (being an aggregate of smaller particles), and having enough gravity to hold the telescope in place, what is the minimum size the asteroid must be to block radio signals from the planet?

Some planetary information - the aliens have selected a planet with roughly the same mass as Earth, to make the humans feel comfortable with the gravity. They diverted asteroids from the asteroid belt and locked them in an orbit around the planet. They set the orbital period to divide their "year" into 12 months because they are familiar with the humans' use of the lunar calendar. The planetary year is almost 40 days longer than ours.

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  • $\begingroup$ do you have a diagram? $\endgroup$
    – user6760
    Commented Feb 16 at 2:04
  • $\begingroup$ Presumably you would also want the asteroid to be tidally-locked to the planet so that one side was always facing away from it? $\endgroup$
    – RBarryYoung
    Commented Feb 16 at 18:42
  • $\begingroup$ Technically I believe "The planet is orbited by a large asteroid, not a moon" cannot be correct; an asteroid orbits a star, while a moon orbits a smaller body (such as a planet, dwarf planet, or even an asteroid). Perhaps the body in question was once an asteroid in orbit around its star, but once the aliens placed it in orbit around a planet, it became a moon (or perhaps a "moonlet", although your description suggests not). Our solar system has many small moons which are hypothesized to have originated as captured asteroids, but they are now considered moons, not asteroids. $\endgroup$
    – brendan
    Commented Feb 17 at 10:03

2 Answers 2

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An object creates a region of shadow when placed in the path of an electromagnetic wave once its parallel dimension is larger than the wavelength of the electromagnetic wave. (image source)

enter image description here

This is why a person can block sunlight and cast a shadow but won't block radio waves.

In your case it depends on the wavelength used: the longer it is, the larger the asteroid must be to block it.

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    $\begingroup$ Upvoted. For clarity, I would suggest to explicitly say that it must be much larger than the wavelengths of interest. Two or three times larger is clearly not enough, a hundred times larger is sometimes enough, a thousand times larger is usually enough, ten thousand times larger is always enough. $\endgroup$
    – AlexP
    Commented Feb 15 at 21:09
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    $\begingroup$ Microwaves have Faraday cages to prevent it. Unless this is an artificial asteroid, a better analogy is that some stone buildings prevent cell phone calls. $\endgroup$
    – Mary
    Commented Feb 15 at 23:33
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    $\begingroup$ Depends what you're trying to block. For reference, the US Navy comms. with its submarines at 74 Hz, the wavelength being 4,000 Km. 10,000 times that would be a moon the diameter nearly a quarter the diameter of the Earth's orbit. That would be a black hole or suppermassive star. Let's settle for attenuation rather than blocking. $\endgroup$
    – Escaped dental patient.
    Commented Feb 15 at 23:58
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    $\begingroup$ @TheSquare-CubeLaw er no. Not warming your eyeballs is a feature. Not a bug. $\endgroup$
    – candied_orange
    Commented Feb 16 at 13:37
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    $\begingroup$ @Mary it’s not an analogy. I’m talking about why the microwaves Faraday cage can have holes in it. You can see your food because the wavelength of light is small enough to fit through the holes. The wave length of microwaves is too large for the same holes so it gets reflected. $\endgroup$
    – candied_orange
    Commented Feb 16 at 13:46
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A radio telescope is not sensitive to wavelengths much greater than its antenna. A wave won't pass through an insulating material much thicker than its wavelength. Adding those up, if the asteroid's radius is several times the antenna dish, it's going to block all radiation that could interfere with the telescope.

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