From Wikipedia's "Asteroid impact prediction" article:

Performance is improving in detecting smaller objects as existing systems are upgraded and new ones come on line, but the blind spot issue which all current systems face around the Sun can only be overcome by a dedicated space based system or by discovering objects on a previous approach to Earth many years before a potential impact.

What is the nature of this "blind spot issue"? Is it primarily optical in nature, having to do with the attenuation of signal by atmosphere, or does it have to do with the angles of reflection of light, meaning that a highly-effective system would have to be not only space-based, but far from Earth as well?

More information:

The Sentinel Space Telescope was planned to orbit at Venus L3. It was scrapped in favor of NEOCam / NEO Surveyor infrared telescope (currently in preliminary design phase), which is planned to orbit at Earth-Sun L1. (Will this be effective?)

  • $\begingroup$ Much like spotting a bird that is crossing the sky towards the sun $\endgroup$
    – Alchimista
    Sep 30, 2021 at 8:06

1 Answer 1


I will have to look for a source for this, but basically the best way to search for small NEOs is to use image at thermal infrared wavelengths.

At circa 1 AU the Sun illuminates them with about 1600 watts per square meter. They are generally quite dark, which makes them hard to spot looking at reflected sunlight, and since they are against a background of dim stars they are even harder to identify.

However almost all of that power absorbed by the dark objects is radiated back again in thermal infrared. So they are brighter in thermal IR than visible. The bonus is that there are far fewer background objects to confuse them with.

Thermal IR telescopes and image sensor arrays must be kept at cryogenic temperatures so they don't overwhelm themselves with their own thermal radiation and electronic noise.

This means you have to be very careful not to point them anywhere near the Sun or they will warm and boil off their stored cryogen or overwhelm their cooling system.

So the "blind spot" is almost certainly the limits on safe directions you can point at cryogenic, thermal infrared imaging telescope and not start to get warmed by sunlight.

  • $\begingroup$ OK. So basically, putting one of these at Sun-Earth L1 is NOT going to fix the blind spot issue. $\endgroup$
    – DJG
    Sep 29, 2021 at 14:49
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    $\begingroup$ The vast majority of current NEO discovery is done with optical telescopes (apart from a few 10s/year (discovery stats)found by NEOWISE working at 3.6 and 4.5um) so the "Sun blind spot" mentioned in the quoted Wikipedia article is simply due to the fact you can't point optical telescopes close to the Sun. Your answer is not wrong but not really a cause of the existing Sun blind spot yet since there aren't thermal IR NEO surveys operating. $\endgroup$ Sep 29, 2021 at 16:14
  • $\begingroup$ @astrosnapper yes all telescopes have to avoid pointing close to the Sun but for cryogenic/thermal IR telescopes that exclusion zone must be a lot larger? Also, looking the quote in question it's about detecting the small ones that may be overlooked by current methods, and I think the performance for the smallest significant NEOs is better for thermal IR; it can find ones that are important to know about but will likely be missed in visible light. $\endgroup$
    – uhoh
    Sep 30, 2021 at 2:51
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    $\begingroup$ @DJG No, it will help with the blind-spot issue, because from L1 you can point at parts of the solar system that you can’t from Earth while still avoiding the Sun. $\endgroup$ Sep 30, 2021 at 17:36
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    $\begingroup$ The “blind spot” references in the article are for all ground-based observations, not just IR. $\endgroup$ Sep 30, 2021 at 17:42

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