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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?

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    $\begingroup$ 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. $\endgroup$
    – A. Rumlin
    Jul 15, 2021 at 15:31
  • $\begingroup$ @BrendanLuke15, definitively "detecting", according to this link Radar Detectability of NEA $\endgroup$
    – Ng Ph
    Jul 15, 2021 at 15:49
  • $\begingroup$ @NgPh ambiguously "observing"/"detecting" according to Lists of Objects Recently Detected with Arecibo from the same source $\endgroup$ Jul 15, 2021 at 17:02
  • $\begingroup$ @NgPh I am taking "detect" to mean "discover" in the question $\endgroup$ Jul 15, 2021 at 17:08
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    $\begingroup$ @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. $\endgroup$
    – Ng Ph
    Jul 15, 2021 at 19:28

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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.

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    $\begingroup$ The problem with using radar to actively* detect asteroids in deep space is that SPACE IS BIG (much much bigger than Han Solo can image), while radar only works like a spotlight. $\endgroup$
    – RonJohn
    Jul 15, 2021 at 22:01
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    $\begingroup$ You absolutely want your NEOs to be low albedo so that they warm up and radiate efficiently in the thermal infrared - which has been firmly established as the best way to detect new NEOs. see comment This is why NEO Surveyor will also use thermal IR. $\endgroup$
    – uhoh
    Jul 15, 2021 at 22:40
  • $\begingroup$ moons of minor-planets are discovered by radar $\endgroup$
    – uhoh
    Jul 17, 2021 at 12:51
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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.

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    $\begingroup$ Was radar ever used to detect asteroids? $\endgroup$
    – uhoh
    Jul 15, 2021 at 19:51
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    $\begingroup$ @uhoh so far I've found that answer to be no $\endgroup$ Jul 15, 2021 at 20:02
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    $\begingroup$ 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? $\endgroup$
    – Cadence
    Jul 15, 2021 at 20:02
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    $\begingroup$ @Cadence That would no longer be radar though, just plain radio astronomy. $\endgroup$ Jul 15, 2021 at 20:13
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    $\begingroup$ @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. $\endgroup$
    – uhoh
    Jul 15, 2021 at 21:47

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