I'm writing an SF story in which a main belt asteroid is diverted into a collision course trajectory with earth (long story), and unknown to us here on earth.

Its H magnitude is 12. As it nears earth, I would assume it gets brighter in V--apparent magnitude. Considering all the eyes on the sky these days, how bright, apparent mag V, would it get before being discovered here on earth? 9 or 10 seem reasonable?

And if you could give its distance in AU, at discovery, that would be super (trying to find out how much time we have left ;) )!

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    $\begingroup$ Sky surveys do pick them up at mag 9 or 10: google.com/… Actually this non-main belt rock was identified at a magnitude greater than 11.4. $\endgroup$ Commented Feb 12, 2017 at 15:48
  • $\begingroup$ Yeah, with all the scopes out there, amateur and professional, plus operations like this... I was looking for an informed opinion. $\endgroup$ Commented Feb 12, 2017 at 16:34
  • $\begingroup$ I'm not an expert, but as far as intentional searches for potential NEOs, there are definitely parts of the sky receiving substantially more attention than other parts. For example, they may concentrate on the ecliptic, and the ones using thermal IR would tend to favor directions roughly towards but not into the sun. Sky surveys on the other hand may not be completely uniform, but they would likely be be closer to it averaged out over any given year, and at any given time they'd probably be looking away from the sun. $\endgroup$
    – uhoh
    Commented Feb 13, 2017 at 3:37
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    $\begingroup$ @uhoh Uhoh, copied wrong link. This one: universetoday.com/122924/… 2015 TB145 $\endgroup$ Commented Feb 13, 2017 at 5:37
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    $\begingroup$ @uhoh, come out of the sun, like a fighter pilot would do, eh? Good idea. Arrange that the asteroid was in the milky way too, maybe lost in the star clouds there! $\endgroup$ Commented Feb 13, 2017 at 15:30

1 Answer 1


That's a big asteroid!

Recent discoveries of objects of this size are rare, one example was 2013UQ4. We believe that all objects of this size in the main asteroid belt have been discovered. (The discovery of 2013US10, which appeared to be main belt object turned out to be an error, the combination of observations of two different objects) This object has a highly inclined and retrograde orbit. It turned out to be a comet not an asteroid.

For context US10 it was at magnitude 19 when discovered. It is unlikely that a large body would remain undiscovered until it was at mag. 10 or 9, especially if it was in the main belt. A very small object that is rapidly brightening could reach these magnitudes before discovery, but not a 10-20km sized object that your story implies.

  • $\begingroup$ Correct, James, its the size of Manhattan. I had a feeling that something brightening ~6-15 times would catch someone's eye, amateur or professional. What if it came out of the Milky Way? $\endgroup$ Commented Feb 13, 2017 at 15:54
  • $\begingroup$ James, do you think a numbered asteroid brightening from mag 13.5 to 12 (when it closes to 1 AU away) would be detected? $\endgroup$ Commented Feb 14, 2017 at 20:23
  • $\begingroup$ It is much less likely that brightness changes in a known asteroid would be noticed. Known asteroids aren't routinely surveyed. But if the asteroid is in a new, or unexpected place, that will be picked up, by the near earth object systems. There is the all sky supernova searches astronomy.ohio-state.edu/~assassin/index.shtml These will detect your asteroid, but are likely to flag it as "known" providing it is in a known location. $\endgroup$
    – James K
    Commented Feb 14, 2017 at 21:18
  • $\begingroup$ If they're not routinely surveyed, how would they know its in a new or unexpected place? How much do you think the discovery of US10 or UQ4 was serendipity? $\endgroup$ Commented Feb 14, 2017 at 22:01
  • $\begingroup$ There are systematic and whole sky searches for asteroids. Each object that is found is checked to see if it is a known star or asteroid. If an object appears in the expected place, even if the brightness is "wrong", the system will probably identify it as an existing known object, and move on. If the object is in the "wrong" place, it will treat it as a new asteroid and flag it for human investigation. US10 etc was found by systematic search. While there is some degree of "luck", I wouldn't call it serendipitous. $\endgroup$
    – James K
    Commented Feb 14, 2017 at 22:34

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