The population of near-earth asteroids revisited and updated

In this paper we update, extend, and improve upon the recent paper on Near-Earth Asteroid (NEA) population by Harris and D'Abramo (2015). We update the population estimate taking into account discoveries to August 3, 2020. Shortly after the previous paper was published, we identified a problem in our previous studies due to rounding off of absolute magnitude H by the Minor Planet Center to 0.1 magnitude that implicitly shifted our bin boundaries by 0.05 magnitude. Here we correct the problem by choosing H bin boundaries at 0.25-0.75 magnitude, rather than 0.00-0.50 magnitude thereby eliminating the round-off shift. We also introduce an updated model distribution of NEA orbits (Granvik et al. 2018) in our survey simulations. This new population model includes orbital distributions as a function of size, allowing us to test our presumption that distributions are homologous with respect to size. The change in distribution of orbits between that used by Harris and D'Abramo (2015) and the newer Granvik et al. (2018) distributions was substantially greater than the range over size given in the latter, but made almost no difference in population estimates, suggesting that the re-detection algorithm is robust and does not depend strongly on the detailed orbit distribution, and also validates our presumption of homologous distribution over size. For the current analysis, we have added ATLAS and Zwicky Transient Facility (ZTF) to our analysis. We have separately analyzed the re-detection ratio versus size (H magnitude) for each of four surveys (Catalina + Mt. Lemmon, Pan-STARRS, ATLAS and ZTF) to investigate any differences between the various systems and cadences, and found none of significance, thus again, the re-detection algorithm appears very robust and mostly independent of survey parameters. With these changes and more recent discovery statistics to 2020, we estimate the number of NEAs of absolute magnitude H < 17.75 (nominally D > 1 km) to be 940 ± 10. Following the updated population estimate, we examine more closely the rates of discovery of the largest NEAs (absolute magnitude H < 17.75, nominally diameter D > 1 km) to make a more detailed estimate of the numbers of these large objects that remain undiscovered. Lastly we use the distribution of albedos of NEAs from NEOWISE observations (Mainzer et al. 2011) to transform our population estimate from number versus H, N(<H), to number versus D, N(>D). Unfortunately this transformation depends substantially on only minor uncertainties in the albedo distribution, so N(>D) remains much less certain than N(<H).