Physical Properties of two tiny Asteroids from Spitzer Observations

Little is known about the physical properties of the smallest NEOs with diameters of less than 10 meters. Due to their small sizes, they are usually very faint and hard to observe. Hence, only a small number of asteroids in this size regime are known and for only very few of those physical properties like diameter and albedo have been measured. However, small NEOs are much more frequent than larger NEOs, making some of these objects easily accessible spacecraft targets and potential impactors. Traditionally, people believed that these small NEOs have formed through collisions and that they are individual slabs of rock, i.e,. monolithic bodies.

We have used Spitzer Space Telescope observations of two tiny NEOs, 2009 BD and 2011 MD,  to constrain the physical properties of these objects. Based on available astrometric observations, it was known before that the orbits of both objects are subject to so-called non-gravitational forces, i.e., their orbits are not only shaped by gravity alone, but also by solar radiation pressure and Yarkovsky forces. By combining an asteroid thermophysical model, modeling the thermal flux emitted by the surface of an asteroid, and an orbital model taking into account non-gravitational forces, we were able to constrain much more than only diameter and albedo for both objects.

tinyNEOs_schematicWhat we found for both objects was somewhat unexpected. Our results show 2011 MD to have a diameter of about 6 meters and an intermediate albedo surface. More interestingly, we were able to derive the bulk density of this object, which is only slightly higher than that of water, telling us that at least two thirds of the volume of this object consists of void space – 2011 MD is not a monolithic asteroid, it is a rubble pile asteroid! In the case of 2009 BD we found two equally possible solutions for its physical properties: the object either has a diameter of about 4 meters and an intermediate surface albedo (solution 1) or it has a diameter of 3 meters and a very high albedo (solution 2). Both solutions have different implications for the interior structure of this asteroid. Solution 1 shows 2009 BD as a care-rock rubble-pile asteroid, whereas solution 2 implies the object to be monolithic and to be covered with a layer of dusty material like regolith. Both solutions are very extraordinary and haven’t been observed in larger asteroids. The diagram shows the different possible configurations for both 2009 BD and 2011 MD; in the case of the latter, thermal inertia could not be constrained as part of our analysis.

The results of this analysis are published in two papers: 2009 BD and 2011 MD, and there has been a NASA/JPL press release.

Addendum (August 2015): Our Spitzer observations were originally performed in support for NASA’s Asteroid Redirect Mission, which aimed on retrieving an asteroid into an orbit in the Earth-Moon system. Unfortunately, NASA decided to change its strategy and instead grab a boulder from the surface of a larger asteroid and bring that back into the Earth-Moon system. But on the bright side: our team has been awarded a NASA Group Achievement Award for ‘exemplary science implementation, analysis and execution of the Spitzer 2011 MD and 2009 BD near-Earth asteroid observations for NASA’s Asteroid Redirect Mission’.

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Detection of Cometary Activity in NEO Don Quixote

Part of the NEO population is considered to consist of so-called dead comets. Dead comets are comets that have spent a long time as NEOs and have been depleted their volatile inventories in numerous, close encounters with the Sun, i.e., they are extinct comets. They can be identified through their distinctive comet-like orbits and their low, comet-like albedos.

One of the dead comet prototypes is NEO (3552) Don Quixote. We observed Don Quixote as part of the ExploreNEOs program, in which ~600 NEOs were observed to derive the diameters and albedos of these objects. Our observations at 3.6 and 4.5 µm were saturated because the target was brighter than expect, revealing something rather unexpected.

donquixote1The subtraction of the Spitzer IRAC point-spread-function (psf) from our observations show some kind of extended emission around the object at 4.5 µm, but not so at 3.6 µm. We went into some effort to show that the extended emission is not an image artifact, it is not a background source, not a latency effect, and not a result of the saturation of the object. The latter is obvious after applying the same psf-subtraction technique to a saturated image of an even brighter calibration star (HD 149661).

donquixote2The nature of the extended emission becomes clear after plotting the radial brightness distribution around the object. At 3.6 µm, the distribution is rather noisy and basically in agreement with a null result, whereas at 4.5 µm, the radial brightness distribution is clearly proportional to the reciprocal distance from the target center. This behavior is typical for cometary comae, consisting of optically thin dust and gas that is ejected from the surface as ices sublimate in the warming sunlight. After subtracting the model for a cometary coma from our observations, even a faint tail becomes obvious.

NASA/JPL

The fact that we observe cometary activity at 4.5 but not at 3.6 µm provides constraints on the composition of the coma. If significant amounts of dust would be ejected together with the gas, the emission would be visible in both bands, more likely so at 3.6 µm, which is not the case. The most likely explanation is emission from molecular bands: both CO and CO2 have molecular band emission lines that fall into the 4.5 µm bandpass, but not into the 3.6 µm bandpass. Both materials have been found in cometary spectra, but CO2 is usually more likely. The activity we find in Don Quixote is rather minute and comparable to the weakest activities found in active comets. Still, the fact that this object does show activity, which has not been discovered in 30 years after its discovery as an asteroid, is extraordinary.

Another interesting fact we could derive from its taxonomic classification as a D-type asteroid and meteorite analog material is that Don Quixote is likely to hold a large amount of water. Based on the diameter we derived (18.4 km), Don Quixote is likely to hold 100 billion tons of water, which is about the same amount of water as in Lake Tahoe, California.

Our study has been published here and press releases have been issued by several institutions including NASA JPL and the European Planetary Science Congress, where the results have been presented.