<p>That's a big asteroid!</p>

<p>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. </p>

<p>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. </p>

CC BY-SA 3.0
2017-02-13T15:54:36.463Z
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?
catsteevens
CC BY-SA 3.0
2017-02-14T20:23:54.440Z
James, do you think a numbered asteroid brightening from mag 13.5 to 12 (when it closes to 1 AU away) would be detected?
catsteevens
CC BY-SA 3.0
2017-02-14T21:18:22.067Z
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 http://www.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.
James K
CC BY-SA 3.0
2017-02-14T22:01:20.203Z
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?
catsteevens
CC BY-SA 3.0
2017-02-14T22:34:53.530Z
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.
James K
CC BY-SA 3.0
2017-02-14T23:48:50.623Z
Yes, I guess all they would need to do would be to compare plates from 2 different dates for any out-of-place points of light.
catsteevens
CC BY-SA 3.0
2017-02-13T07:37:03.437Z
2017-02-13T07:37:03.437Z
James K
<p>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. </p>

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

<p>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 ;) )!</p>

CC BY-SA 3.0
2017-02-12T15:48:42.413Z
Sky surveys do pick them up at mag 9 or 10: https://www.google.com/search?q=asteroid+flyby+magnitude&ie=utf-8&oe=utf-8 Actually this non-main belt rock was identified at a magnitude greater than 11.4.
Wayfaring Stranger
CC BY-SA 3.0
2017-02-12T16:34:47.753Z
Yeah, with all the scopes out there, amateur and professional, plus operations like [this](http://neo.jpl.nasa.gov/programs/)... I was looking for an informed opinion.
catsteevens
CC BY-SA 3.0
2017-02-13T03:37:53.657Z
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*.
uhoh
CC BY-SA 3.0
2017-02-13T03:40:25.540Z
So you may want to figure out how to help the bad guy(s) use temperature and direction to find a "blind spot", although there may be no guarantee it would be missed.
uhoh
CC BY-SA 3.0
2017-02-13T05:37:26.300Z
@uhoh Uhoh, copied wrong link. This one: http://www.universetoday.com/122924/how-to-see-the-spooktacular-halloween-flyby-of-asteroid-2015-tb145/ 2015 TB145
Wayfaring Stranger
CC BY-SA 3.0
2017-02-13T15:30:50.207Z
@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!
catsteevens
CC BY-SA 3.0
2017-02-13T15:43:45.063Z
@Wayfaring Stranger, good link! And downloaded Stellarium which is very helpful.
catsteevens
<asteroids><apparent-magnitude>
Asteroid apparent magnitude
catsteevens
# Asteroid apparent magnitude

(Asked by: username_0 on 2017-02-12)

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 ;) )!

**Tags:** asteroids, apparent-magnitude
> **Comments:**
> **username_1**: Sky surveys do pick them up at mag 9 or 10: https://www.google.com/search?q=asteroid+flyby+magnitude&ie=utf-8&oe=utf-8 Actually this non-main belt rock was identified at a magnitude greater than 11.4.  
> **username_0**: Yeah, with all the scopes out there, amateur and professional, plus operations like [this](http://neo.jpl.nasa.gov/programs/)... I was looking for an informed opinion.  
> **username_2**: 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*.  
> **username_2**: So you may want to figure out how to help the bad guy(s) use temperature and direction to find a "blind spot", although there may be no guarantee it would be missed.  
> **username_1**: @uhoh Uhoh, copied wrong link. This one: http://www.universetoday.com/122924/how-to-see-the-spooktacular-halloween-flyby-of-asteroid-2015-tb145/ 2015 TB145  
> **username_0**: @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!  
> **username_0**: @Wayfaring Stranger, good link! And downloaded Stellarium which is very helpful.  

---

## 1 answers

### Answer by username_3 on 2017-02-13. Score: 2 | Accepted answer

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.

> **Comments:**
> **username_0**: 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?  
> **username_0**: James, do you think a numbered asteroid brightening from mag 13.5 to 12 (when it closes to 1 AU away) would be detected?  
> **username_3**: 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 http://www.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.  
> **username_0**: 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?  
> **username_3**: 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.  
> **username_0**: Yes, I guess all they would need to do would be to compare plates from 2 different dates for any out-of-place points of light.  

---

