Now if we move this 20 km asteroid from 0.03939 AU in to a distance of 0.0367 AU, it will keep its Absolute Visual Magnitude of 11.6, but will decrease its Apparent Visual Magnitude from 4.7 to 4.5 which is the limiting magnitude of a human dark adapted eye. In other words, by moving the asteroid further in, we increase the asteroid's luminocity by a factor of (0.0367 / 0.03939)$$² = 0.87. This agrees well with the change in luminosity calculated from the change in apparent magnitude, which is reduced by a factor of (2.512)$$^{(4. 5 - 4. 7)} = 0.83.  The 0.04 difference between these two calculations being due to the fact that changes in luminosity follow power relationships while magnitude calculations follow logrithmic relationships which though close, are not identical.
At its original distance the asteroid's Apparent Visual Magnitude of 4.7 makes it slightly dimmer than a dark adapted human eye can see. The above move to a distance of 0.0367 AU would make this asteroid as visible as the faintest star a dark adapted eye can see. The asteroid would then appear as bright as Alcor, the companion star of Mizar at the bend on the Big Dipper's handle.
So, from how far away can it be seen as a point source of light using a 12" clear aperture telescope with a Limiting Magnitude of 15.45 ?  A quick first approximation of this distance would be:
(2. 512$$^{( 15. 45 - 4. 7 )} )$$^{0. 5} * 0.03939411 = 5.565918358 AU = 832,650,254.5 km.
This is the greatest distance from which any 12" Clear aperture telescope can see a 20 km asteroid with an albedo of 0.1 as a point source of light.  How far out is this ? Adding the distance we can detect these astroids to our present distance from the Sun, we can detect asteroids of this size out to 6.57 AU. This is well beyond the Main Asteroid Belt at 2.8 AU and Jupiter at 5.2 AU.  Obviously we are not missing some of these colliders for lack of the ability to detect them.