Number of undiscovered near-Earth asteroids revised downward
https://phys.org/news/2017-10-undiscovered-near-earth-asteroids-downward.html
Observers have been cataloging potentially hazardous asteroids for decades. Based on the number of finds, the area of sky explored, and the limiting brightness our telescopes
and cameras can reach, researchers can estimate what fraction of the NEA population has been detected so far and how many more objects lurk undiscovered. Harris has published
numerous such estimates over the years. Recently he realized that his estimates have been plagued by a seemingly innocuous but nonetheless consequential round-off error. Once
corrected, the estimated number of large (diameter > 1 kilometer) NEAs remaining to be discovered decreases from more than 100 to less than 40.
The population ("size-frequency distribution") of NEAs is usually given in terms of number versus brightness, since most discovery surveys operate in visible (reflected) light.
Brightness isn't a reliable proxy for size, though, because asteroid surfaces don't all have the same albedo, or reflectivity. NEA brightnesses are expressed in units of absolute
magnitude H, with lower numbers indicating brighter objects. The IAU Minor Planet Center—the world's clearinghouse for asteroid measurements—rounds off reported values of H to
the nearest 0.1 magnitude. While this is mostly unimportant, amounting to a reduction in the estimated NEA population N ( less than H ) of only about 7%, it becomes significant in
assessing the completion of surveys for the largest objects.