Number of undiscovered near-Earth asteroids revised downward

October 19, 2017, MoreData!
An artist's impression of an asteroid breaking up. Credit: NASA/JPL-Caltech

Fewer large near-Earth asteroids (NEAs) remain to be discovered than astronomers thought, according to a new analysis by planetary scientist Alan W. Harris of MoreData! in La Canada, California. Harris is presenting his results this week at the 49th annual meeting of the American Astronomical Society's Division for Planetary Sciences in Provo, Utah.

Observers have been cataloging 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 ( <H ) of only about 7%, it becomes significant in assessing the completion of surveys for the largest objects.

In a 2015 study, Harris and Italian astronomer Germano D'Abramo estimated that there exist 990 NEAs brighter than H = 17.75, which is considered equivalent on average to a diameter D = 1 km. After correcting for the round-off problem, that estimate decreases to 921 ± 20, consistent with a recent estimate by Pasquale Tricarico (Planetary Science Institute), who used similar data but a different computational approach. We know how many NEAs of H < 17.75 have been discovered: 884 according to the latest tally by the Jet Propulsion Laboratory. The previous population estimate of 990 implied 89% completion and 106 yet to be found. The new estimate of 921 implies 96% completion and only 37 left to be found, almost three times fewer.

The recent NEOWISE survey, which more directly measured NEA diameters from their thermal infrared emission, opens up the possibility to transform the population estimate from brightness units, N(less than H), to sizes, N(>D). The transformation method resembles a game of Sudoku, but the rules are different. We can make a table, nine columns wide like Sudoku, with each column representing a range of albedos, for example 0.0703 to 0.1114, and vertically with 25 rows, each row representing a range of diameter, for example from 0.794 to 1.000 km. These bin sizes are chosen so that each range in diameter or albedo corresponds to 0.5 magnitude in brightness H. With this arrangement a diagonal sum across the boxes follows a constant value of H, and this sum should equal the known n(H) for that value of H. Horizontally, the sum of the boxes across represent the desired number n(D), and the best solution in each box needs to follow the albedo distribution defined by NEOWISE.

Unfortunately the solution depends rather strongly on the albedo distribution, so if one chooses the distribution for all NEAs, or just "Earth-crossing asteroids" that actually have any probability of impacting, or just big ones, or only small ones, one can get quite different answers. Harris solved the "Sudoku" puzzle for a variety of reasonable albedo distributions and derived estimates of N(D > 1 km) ranging from about 750 to 900. The good news is that this seemingly large uncertainty in the total number does not much affect the completion fraction of 96%. So the number of large (D > 1 km) NEAs yet to be discovered is still limited to around 30 to 40.

Explore further: Earth's new traveling buddy is definitely an asteroid, not space junk

Related Stories

Close approach of asteroid 2012 TC4 poses no danger to Earth

October 10, 2017

The house-sized asteroid 2012 TC4 is slated to give Earth a close shave on Thursday, October 12, swooshing by our planet at approximately 5:41 UTC (1:41 a.m. EDT) at a distance of about 31,000 miles (50,000 kilometers). Although ...

What do we need to know to mine an asteroid?

September 19, 2017

The mining of resources contained in asteroids, for use as propellant, building materials or in life-support systems, has the potential to revolutionise exploration of our Solar System. To make this concept a reality, we ...

The hustle and bustle of our solar system

July 31, 2012

( -- This diagram illustrates the differences between orbits of a typical near-Earth asteroid (blue) and a potentially hazardous asteroid, or PHA (orange). PHAs are a subset of the near-Earth asteroids (NEAs) and ...

Recommended for you

In colliding galaxies, a pipsqueak shines bright

February 20, 2019

In the nearby Whirlpool galaxy and its companion galaxy, M51b, two supermassive black holes heat up and devour surrounding material. These two monsters should be the most luminous X-ray sources in sight, but a new study using ...

Physicists 'flash-freeze' crystal of 150 ions

February 20, 2019

Physicists at the National Institute of Standards and Technology (NIST) have "flash-frozen" a flat crystal of 150 beryllium ions (electrically charged atoms), opening new possibilities for simulating magnetism at the quantum ...

When does one of the central ideas in economics work?

February 20, 2019

The concept of equilibrium is one of the most central ideas in economics. It is one of the core assumptions in the vast majority of economic models, including models used by policymakers on issues ranging from monetary policy ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.