Marco Delbo

Thermal inertia of near-Earth asteroids and implications for the magnitude of the Yarkovsky effect

Abstract Thermal inertia determines the temperature distribution over the surface of an asteroid and therefore governs the magnitude the Yarkovsky effect. The latter causes gradual drifting of the orbits of km-sized asteroids and plays an important role in the delivery of near-Earth asteroids (NEAs) from the main belt and in the dynamical spreading of asteroid families. At present, very little is known about the thermal inertia of asteroids in the km size range. Here we show that the average thermal inertia of a sample of NEAs in the km-size range is 200 ± 40 J m −2 s −0.5 K −1 . Furthermore, we identify a trend of increasing thermal inertia with decreasing asteroid diameter, D . This indicates that the dependence of the drift rate of the orbital semimajor axis on the size of asteroids due to the Yarkovsky effect is a more complex function than the generally adopted D −1 dependence, and that the size distribution of objects injected by Yarkovsky-driven orbital mobility into the NEA source regions is less skewed to smaller sizes than generally assumed. We discuss how this fact may help to explain the small difference in the slope of the size distribution of km-sized NEAs and main-belt asteroids.

Thermal fatigue as the origin of regolith on small asteroids

Space missions and thermal infrared observations have shown that small asteroids (kilometre-sized or smaller) are covered by a layer of centimetre-sized or smaller particles, which constitute the regolith. Regolith generation has traditionally been attributed to the fall back of impact ejecta and by the break-up of boulders by micrometeoroid impact. Laboratory experiments and impact models, however, show that crater ejecta velocities are typically greater than several tens of centimetres per second, which corresponds to the gravitational escape velocity of kilometre-sized asteroids. Therefore, impact debris cannot be the main source of regolith on small asteroids. Here we report that thermal fatigue, a mechanism of rock weathering and fragmentation with no subsequent ejection, is the dominant process governing regolith generation on small asteroids. We find that thermal fragmentation induced by the diurnal temperature variations breaks up rocks larger than a few centimetres more quickly than do micrometeoroid impacts. Because thermal fragmentation is independent of asteroid size, this process can also contribute to regolith production on larger asteroids. Production of fresh regolith originating in thermal fatigue fragmentation may be an important process for the rejuvenation of the surfaces of near-Earth asteroids, and may explain the observed lack of low-perihelion, carbonaceous, near-Earth asteroids.

Super-catastrophic disruption of asteroids at small perihelion distances

Most near-Earth objects came from the asteroid belt and drifted via non-gravitational thermal forces into resonant escape routes that, in turn, pushed them onto planet-crossing orbits. Models predict that numerous asteroids should be found on orbits that closely approach the Sun, but few have been seen. In addition, even though the near-Earth-object population in general is an even mix of low-albedo (less than ten per cent of incident radiation is reflected) and high-albedo (more than ten per cent of incident radiation is reflected) asteroids, the characterized asteroids near the Sun typically have high albedos. Here we report a quantitative comparison of actual asteroid detections and a near-Earth-object model (which accounts for observational selection effects). We conclude that the deficit of low-albedo objects near the Sun arises from the super-catastrophic breakup (that is, almost complete disintegration) of a substantial fraction of asteroids when they achieve perihelion distances of a few tens of solar radii. The distance at which destruction occurs is greater for smaller asteroids, and their temperatures during perihelion passages are too low for evaporation to explain their disappearance. Although both bright and dark (high- and low-albedo) asteroids eventually break up, we find that low-albedo asteroids are more likely to be destroyed farther from the Sun, which explains the apparent excess of high-albedo near-Earth objects and suggests that low-albedo asteroids break up more easily as a result of thermal effects.

ExploreNEOs. I. Description and First Results from the Warm Spitzer Near-Earth Object Survey

We have begun the ExploreNEOs project in which we observe some 700 Near-Earth Objects (NEOs) at 3.6 and 4.5 μm with the Spitzer Space Telescope in its Warm Spitzer mode. From these measurements and catalog optical photometry we derive albedos and diameters of the observed targets. The overall goal of our ExploreNEOs program is to study the history of near-Earth space by deriving the physical properties of a large number of NEOs. In this paper, we describe both the scientific and technical construction of our ExploreNEOs program. We present our observational, photometric, and thermal modeling techniques. We present results from the first 101 targets observed in this program. We find that the distribution of albedos in this first sample is quite broad, probably indicating a wide range of compositions within the NEO population. Many objects smaller than 1 km have high albedos (0.35), but few objects larger than 1 km have high albedos. This result is consistent with the idea that these larger objects are collisionally older, and therefore possess surfaces that are more space weathered and therefore darker, or are not subject to other surface rejuvenating events as frequently as smaller NEOs.

Spitzer observations of spacecraft target 162173 (1999 JU3)

Context. Near-Earth asteroid 162173 (1999 JU3) is the primary target of the Japanese Aerospace Exploration Agency (JAXA) Hayabusa-2 sample return mission, and is also on the list of potential targets for the European Space Agency (ESA) Marco Polo sample return mission. Earth-based studies of this object are fundamental to these missions. Aims. Our aim is to provide new constraints on the surface properties of this asteroid. Methods. We present a mid-infrared spectrum (5–38 μm) obtained with NASA’s Spitzer Space Telescope in May 2008 and results from the application of thermal models. Results. These observations place new constraints on the surface properties of this asteroid. To fit our spectrum we used the near-Earth asteroid thermal model (NEATM) and the more complex thermophysical model (TPM). However, the position of the spin-pole, which is uncertain, is a crucial input parameter for constraining the thermal inertia with the TPM; hence, we consider two pole orientations. First is the extreme case of an equatorial retrograde geometry from which we derive a rigorous lower limit to the thermal inertia of 150 Jm −2 s −0.5 K −1 . Second, when we adopt the pole orientation of Abe et al. (2008a, 37 th COSPAR Scientific Assembly) our best-fit thermal model yields a value for the thermal inertia of 700 ± 200 Jm −2 s −0.5 K −1 and even higher values are allowed by the uncertainty in the spectral shape due to the absolute flux calibration. Our best estimates of the diameter (0.90 ± 0.14 km) and geometric albedo (0.07 ± 0.01) of asteroid 162173 are consistent with values based on previous mid-infrared observations. Conclusions. We establish a rigorous lower limit to the thermal inertia, which is unlikely but possible, and would be consistent with a fine regolith similar to wthat is found for asteroid 433 Eros. However, the thermal inertia is expected to be higher, possibly similar to or greater than that on asteroid 25143 Itokawa. An Accurately determining the spin-pole of asteroid 162173 will narrow the range of possible values for its thermal inertia.

Heating of near-Earth objects and meteoroids due to close approaches to the Sun

It is known that near-Earth objects (NEOs) during their orbital evolution may often undergo close approaches to the Sun. Indeed it is estimated that up to ∼70 per cent of them end their orbital evolution colliding with the Sun. Starting from the present orbital properties, it is possible to compute the most likely past evolution for every NEO, and to trace its distance from the Sun. We find that a large fraction of the population may have experienced in the past frequent close approaches, and thus, as a consequence, a considerable Sun-driven heating, not trivially correlated to the present orbits. The detailed dynamical behaviour, the rotational and the thermal properties of NEOs determine the exact amount of the resulting heating due to the Sun. In the present paper, we discuss the general features of the process, providing estimates of the surface temperature reached by NEOs during their evolution. Moreover, we investigate the effects of this process on meteor-size bodies, analysing possible differences with the NEO population. We also discuss some possible effects of the heating which can be observed through remote sensing by ground-based surveys or space missions.

RUBBLE-PILE RESHAPING REPRODUCES OVERALL ASTEROID SHAPES

There have been attempts in the past to fit the observed bulk shapes (axial ratios) of asteroids to theoretical equilibrium figures for fluids, but these attempts have not been successful in many cases, evidently because asteroids are not fluid bodies. So far, however, the observed distribution of asteroid macroscopic shapes has never been attributed to a common cause. Here, we show that a general mechanism exists, capable of producing the observed shape distribution. We base our approach on the idea that aggregates of coherent blocks held together mostly by gravity (gravitational aggregates) can change their shape under the action of external factors, such as minor collisions, that break the interlocking of the constituent blocks, thus allowing them to asymptotically evolve toward fluid equilibrium. We show by numerical simulations that this behavior can produce a shape distribution compatible with the observations. Our results are shown to be consistent with a simple interpretation based on the topology of the potential energy field for rotating bodies. Also, they suggest that most asteroids have an internal structure that is at least partially fragmented, consistent with constraints derived from large asteroids (diameters >100 km) with satellites.

Thermophysical properties of near-Earth asteroid (341843) 2008 EV5 from WISE data

Aims. We derive the thermal inertia of 2008 EV5, the baseline target for the Marco Polo-R mission proposal, and infer information about the size of the particles on its surface. Methods. Values of thermal inertia were obtained by fitting an asteroid thermophysical model to NASA’s Wide-field Infrared Survey Explorer (WISE) infrared data. Grain size was derived from the constrained thermal inertia and a model of heat conductivity that accounts for different values of the packing fraction (a measure of the degree of compaction of the regolith particles). Results. We obtain an effective diameter D = 370 ± 6 m, geometric visible albedo pV = 0.13 ± 0.05 (assuming H = 20.0 ± 0.4), and thermal inertia Γ= 450 ± 60 J m −2 s −1/2 K −1 at the 1σ level of significance for its retrograde spin-pole solution. The regolith particles radius is r = 6.6 +1.3 −1.3 mm for low degrees of compaction and r = 12.5 +2.7 −2.6 mm for the highest packing densities.

FIRST VLTI-MIDI DIRECT DETERMINATIONS OF ASTEROID SIZES*

We have obtained the first successful interferometric measurements of asteroid sizes and shapes by means of the Very Large Telescope Interferometer-Mid-Infrared Interferometric Instrument (VLTI-MIDI). The VLTI can spatially resolve asteroids in a range of sizes and heliocentric distances that are not accessible to other techniques such as adaptive optics and radar. We have observed, as a typical bench mark, the asteroid (951) Gaspra, visited in the past by the Galileo space probe, and we derive a size in good agreement with the ground truth coming from the in situ measurements by the Galileo mission. Moreover, we have also observed the asteroid (234) Barbara, known to exhibit unusual polarimetric properties, and we found evidence of a potential binary nature. In particular, our data are best fit by a system of two bodies of 37 and 21 km in diameter, separated by a center-to-center distance of ~24 km (projected along the direction of the baseline at the epoch of our observations).

Thermophysical modeling of asteroids from WISE thermal infrared data – Significance of the shape model and the pole orientation uncertainties

Abstract In the analysis of thermal infrared data of asteroids by means of thermophysical models (TPMs) it is a common practice to neglect the uncertainty of the shape model and the rotational state, which are taken as an input for the model. Here, we present a novel method of investigating the importance of the shape model and the pole orientation uncertainties in the thermophysical modeling – the varied shape TPM (VS-TPM). Our method uses optical photometric data to generate various shape models that map the uncertainty in the shape and the rotational state. The TPM procedure is then run for all these shape models. We apply the implementation of the classical TPM as well as our VS-TPM to the convex shape models of several asteroids together with their thermal infrared data acquired by the NASA’s Wide-field Infrared Survey Explorer (WISE) and compare the results. These show that the uncertainties of the shape model and the pole orientation can be very important (e.g., for the determination of the thermal inertia) and should be considered in the thermophysical analyses. We present thermophysical properties for six asteroids – (624) Hektor, (771) Libera, (1036) Ganymed, (1472) Muonio, (1627) Ivar, and (2606) Odessa.