How shiny are near-Earth objects?

How shiny are near-Earth objects?
A radar image of the Near Earth Object Toutatis. Astronomers studying the sizes of NEOs have concluded that an apparent unexpected excess of optically high albedo (bright) NEOs could be real, or could be the result of incompletely correcting for their rotational lightcurves. Credit: NASA and Steve Ostro, JPL

Near-Earth objects (NEOs) are small solar system bodies whose orbits sometimes bring them close to the Earth. NEOs are consequently potential collision threats, but scientists are also interested in them because they offer keys to the composition, dynamics and environmental conditions of solar system and its evolution. Most meteorites for example, one of the key sources of knowledge about the early solar system, come from NEOs. The large majority NEOs were discovered in optical searches, and today the total number of known NEOs exceeds 20,000. The crucial NEO parameter of interest for most problems, including the possible dangers from an impact, is the size, but unfortunately optical detections usually cannot determine size. This is because an NEO's optical light is reflected sunlight, and the object could be bright either because it is large or because it has a high reflectivity (albedo).

CfA astronomers Joe Hora, Howard Smith, and Giovanni Fazio helped lead the team that was the first to undertake the systematic measurement of NEO sizes using their infrared brightnesses. An NEO's is the result of its thermal emission, and that provides an independent measure of its size. The team used Spitzer IRAC infrared observations of NEOs together with optical data and their sophisticated thermal model to break the size/albedo degeneracy and determine the sizes of NEOs. (The NASA WISE mission and its NEOWISE team subsequently also undertook infrared size determinations.) So far, infrared measurements have been made on over 3000 NEOs, the vast majority of them using IRAC. The smallest NEO characterized this way, so far, is only about twelve meters in diameter (with about a 20 percent uncertainty). But strangely, the results also suggest an abundance of high-albedo objects, nearly eight times more than had been expected based on the current thinking about the population distribution.

The scientists had previously analyzed and published the variations of NEO brightness that resulted as their non-spherical bodies rotated in space (their light-curves). They wondered whether the large apparent excess of high-albedo objects was the result of an inadequate correction for light-curve variations. They performed a using Monte-Carlo simulations to estimate what might be expected of a population of rotating, non-spherical NEOs. They conclude that while light-curve variations could indeed be the cause of the large high-albedo excess, the excess is also consistent with a real—and still unexplained—overabundance of shiny objects. They also concluded that whatever the explanation, it is unlikely that NEOs have albedos exceeding 50 percent. Additional observations of full NEO light-curves are need to resolve the uncertainties.

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Near Earth Objects

More information: Annika Gustafsson, et al. Spitzer Albedos of Near-Earth Objects. arXiv:1906.07284v1 [astro-ph.EP]:
Citation: How shiny are near-Earth objects? (2019, July 1) retrieved 24 July 2019 from
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Jul 01, 2019
So we know that large majority of asteroids are very dark---darker than a lump of coal. But how did they get there? Why such dark, dense material? How did they get orbiting around the sun? How did the planets all line up so perfectly in the same direction around the sun? How did the massive sun even start rotating? All of these are perfectly legitimate questions of which evolutionary scientists have no adequate answers. "It just happened to get like that..." is their reasoning. I think we can do much better.

Jul 01, 2019
"Why such dark, dense material"

What you are seeing is a coating residue, also known as lag. It is likely carbonaceous in make up and it comes from the sublimation of ice on the surface. It is a left over and is likely not as dense as you perceive. Also, when an asteroid has low albedo (light reflection), it does not mean that it is devoid of ice. You have the entire subsurface and interior of the asteroid to consider as well.

A good example of the ice sublimation and leftover coating residue dynamic is the Saturnian moon Iapetus. From Iapetus's Wikipedia:

The pattern of coloration is analogous to a spherical yin-yang symbol or the two sections of a tennis ball. The dark region is named Cassini Regio, and the bright region is divided into Roncevaux Terra north of the equator, and Saragossa Terra south of it.


Jul 01, 2019
Renfield, interesting postulation. What about density?

BTW, I work in an industry that is very closely tied to the reflectivity of materials, and I hope we can ban some materials from normal usage because these kind of objects wreck havoc on AI algorithms!

Jul 01, 2019
The densities of asteroids, planetesimals, and planets are intertwined with gravitational fields during their formations and location from their parent star. If you have the right viewing access (in this case I believe it is Amazon Prime) I highly recommend watching the latest episode of Brian Cox's "The Planets" documentary series episode 4. Here is its IMDB entry (It's got a whopping 9.2 on ratings), and trust me, it is mesmerizing to say the least:

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