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I know that detecting asteroids is difficult because many of them we find are the ones that reflect sunlight but can we use something like RADAR to detect others?

CuteKItty_pleaseStopBArking
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Ch.Siva Ram Kishore
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    Radar is more useful for observing space debris in low-Earth orbit. Imagine a radar as a flashlight that illuminates something in the dark. The most powerful space transmitters are incomparable to the "brightness" of the Sun. Therefore, we can see what reflects bright sunlight earlier than the "reflection" of a weak radio signal from non-metallic asteroids. – A. Rumlin Jul 15 '21 at 15:31
  • @BrendanLuke15, definitively "detecting", according to this link Radar Detectability of NEA – Ng Ph Jul 15 '21 at 15:49
  • @NgPh ambiguously "observing"/"detecting" according to Lists of Objects Recently Detected with Arecibo from the same source – BrendanLuke15 Jul 15 '21 at 17:02
  • @NgPh I am taking "detect" to mean "discover" in the question – BrendanLuke15 Jul 15 '21 at 17:08
  • @Ch.SivaRamKishore by "detect" do you mean "discover" or "observe"? – BrendanLuke15 Jul 15 '21 at 17:23
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    @BrendanLuke15, this clarification changes everything. I don't think using radar for discovery (searching for unknown) is a good approach. Radar is powerful because it is directional. It is better used post-discovery, to get more details than telescopes could do. With telescopes, we have a powerful omnidirectional transmitter. – Ng Ph Jul 15 '21 at 19:28
  • @NgPh The question isn't about the quality of detection via radar, but "can we use something like RADAR to detect others" which is unequivocally technically possible. – L0j1k Jul 16 '21 at 02:26
  • See: https://en.wikipedia.org/wiki/Radar_astronomy#Asteroids_and_comets – Nilay Ghosh Jul 17 '21 at 12:28

2 Answers2

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Yes, radar is one of the useful tools for detecting and observing asteroids.

It is however most effective at closer ranges, like near-earth asteroid, since sending out a radio wave and bouncing it back scales with the inverse fourth power of distance.

The most capable telescope for radar observation of asteroids was the Arecibo telescope, until it collapsed in December 2020. Now it's probably Goldstone.

SE - stop firing the good guys
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    Was radar ever used to detect asteroids? – uhoh Jul 15 '21 at 19:51
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    @uhoh so far I've found that answer to be no – BrendanLuke15 Jul 15 '21 at 20:02
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    Could you passively detect reflections of radar waves from the sun, the way you do with visible light, or is it too "dim" at those frequencies? – Cadence Jul 15 '21 at 20:02
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    @Cadence That would no longer be radar though, just plain radio astronomy. – SE - stop firing the good guys Jul 15 '21 at 20:13
  • @SE-stopfiringthegoodguys Yeah, but it's like radar, we've just hired out the transmitting bit. – Don Branson Jul 15 '21 at 20:48
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    @Cadence the Sun and stars like it are poor emitters of radio waves compared to the power emitted in visible light How far have individual stars been seen by radio telescopes? The best scheme for asteroid detection is to let the Sun's visible light warm them, then look for their thermal infrared radiation against a backdrop of cold space. This way we make use of the Sun's full power (1361 watts per square meter at 1 AU) and yet look for conspicuous signals from moving objects. Radar can't compete with that amount of power per unit area. – uhoh Jul 15 '21 at 21:47
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    -1 because the question is "Can we use something like RADAR to detect asteroids?" and the answer to that question is mostly no (several comments here and below the question confirm this sentiment). This is a nice answer to a different question that was not asked and that's not how Stack Exchange is supposed to work. – uhoh Jul 15 '21 at 21:49
  • @uhoh a fair amount of resources online seem to use "detect" to mean "observe", I did ask the question OP to clarify this – BrendanLuke15 Jul 15 '21 at 22:32
  • Comments are not for extended discussion; this conversation has been moved to chat. – called2voyage Jul 19 '21 at 14:48
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The way the question is formulated can lead to contradictory answers.

It can be understood as: since a RADAR can "see" in the dark, can it be used to detect objects that are not illuminated by the Sun (or illuminated but with low albedo)? The answer is YES. Has it been ever used for this purpose? YES. For technical details, cf [Radar detectability of Near-Earth Asteroids] (https://www.naic.edu/~pradar/detect.php)

It can be understood as: Has a NEO been detected by RADAR (and not discovered before)? The answer is also YES. Ex: The moon of asteroid (285263) 1998 QE2

It can be understood as: Can RADAR be used to systematically detect NEOs that escape telescopes' detection? The answer is: YES, in principle, but it would be very very costly. RADARs have to rely on very directive transmitters. It can illuminate only a small portion of the sky at each time.

Ng Ph
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