Context
.
The space weathering timescales of near-Earth S-type asteroids have been investigated using several approaches (i.e., experiments, sample analyses, and theoretical approaches), yet there ...are orders of magnitude differences.
Aims
.
We aim to examine the space weathering timescale on a near-Earth S-type asteroid, Itokawa, using Hayabusa/AMICA images, and further investigate the evolutional process of the asteroid.
Methods
.
We focused on bright mottles on the boulder surfaces generated via impacts with interplanetary dust particles (IDPs). We compared the bright mottle size distribution with an IDP flux model to determine the space weathering timescale.
Results
.
We found that the space weathering timescale on Itokawa’s boulder surfaces is 10
3
yr (in the range of 10
2
–10
4
yr), which is consistent with the timescales of space weathering by light ions from the solar wind.
Conclusions
.
From this result, we conclude that Itokawa’s surface has been weathered rapidly in 10
3
yr but portions of the surface are exposed due to seismic shaking that was triggered by a recent impact, which created the Kamoi crater.
Context.
Recent observations of dust ejections from active asteroids, including (3200) Phaethon, have drawn considerable interest from planetary astronomers studying the generation and removal of ...small dust particles on asteroids.
Aims.
In this work, we aim to investigate the importance of thermal radiation pressure from asteroid regolith (AR) acting on small dust particles over the surface of the AR. In particular, we aim to understand the role of thermal radiation in the near-Sun environment.
Methods.
We describe the acceleration of particles over the AR within the radiation fields (direct solar, reflected (scattered) solar, and thermal radiation) in addition to the asteroid’s rotation and gravitational field. Mie theory is used because the particles of interest have sizes comparable to thermal wavelengths (~1–100 μm), and thus the geometric approximation is not applicable. A new set of formalisms is developed for the purpose.
Results.
We find that the acceleration of particles with spherical radius ≲1 μm to ~10 μm is dominated by the thermal radiation from the AR when the asteroid is in the near-Sun environment (heliocentric distance
r
h
≲ 0.8 au). Under thermal radiation dominance, the net acceleration is towards space, that is, outwards from the AR. This outward acceleration is the strongest for particles of ~1 μm in radius, regardless of other parameters. A preliminary trajectory integration using the Phaethon-like model shows that such particles escape from the gravitational field within about 10 min. Our results are consistent with the previous observational studies on Phaethon in that the ejected dust particles have a spherical radius of ~1 μm.
The Hayabusa spacecraft rendezvoused with its target asteroid 25143 Itokawa in 2005 and brought an asteroidal sample back to the Earth in 2010. The onboard camera, AMICA, took more than 1400 images ...of Itokawa during the rendezvous phase. It was reported that the AMICA images were severely contaminated by light scatter inside the optics. The effect made it difficult to produce the color maps at longer wavelengths (> 800 nm). In this paper, we demonstrate a method to subtract the scattered light by investigating the dim halos of Itokawa and the Moon taken by AMICA during the inflight operation. As a result, we found that the overall data reduction scheme including the scattered light correction enables the recognition of 3% regional differences in the relative reflectance spectra of Itokawa. We confirmed that the color variation in Itokawa was largely attributed to space weathering.
Abstract
We revisited a mass ejection phenomenon that occurred in asteroid P/2010 A2 in terms of the dynamical properties of the dust particles and large fragments. We constructed a model assuming ...anisotropic ejection within a solid cone-shaped jet and succeeded in reproducing the time-variant features in archival observational images over ∼3 years from 2010 January to 2012 October. We assumed that the dust particles and fragments were ejected in the same direction from a point where no object had been detected in any observations, and the anisotropic model explains all of the observations including (i) the unique dust cloud morphology, (ii) the trail surface brightness, and (iii) the motions of the fragments. Our results suggest that the original body was shattered by an impact with specific energy
J kg
−1
, and remnants of slow antipodal ejecta (i.e., anisotropic ejection in our model) were observed as P/2010 A2. The observed quantities are consistent with those obtained through laboratory impact experiments, supporting the idea that the P/2010 A2 event is the first evidence of the impact shattering that occurred in the present main asteroid belt.
Abstract
Two extremely red main-belt asteroids, 203 Pompeja and 269 Justitia, were identified from combined visible and near-infrared spectroscopic observations collected at the Infra-red Telescope ...Facility (IRTF) and Seoul National University Astronomical Observatory (SAO). These two asteroids have a redder spectral slope than any other
D
-type body, which are the reddest objects in the asteroid belt, and similar to RR and IR-class objects found in the outer Solar System among trans-Neptunian objects (TNOs) and Centaurs. Spectroscopic results suggest the presence of complex organic materials on the surface layer of these asteroids, implying that they could have formed in the vicinity of Neptune and been transplanted to the main belt region during a phase of planetary migration. 203 Pompeja is the only very red asteroid known so far among the ∼250 bodies with diameter larger than 110 km (i.e., presumably structurally intact) found in the asteroid belt. These discoveries provide more evidence that the main asteroid belt hosts a population of bodies that were formed in the outskirts of the Solar System.
Context. To investigate the physical properties of the surface layers of small Solar System objects, we take advantage of the phase-angle dependence of the linear polarization degree, which varies ...with albedo, composition, and other factors. In particular, the angle at which the positive and negative are reversed is a polarimetric parameter known as inversion angle. A group with large inversion angles (the so-called Barbarians) is unusual and is also noteworthy because of their association with meteorites. Aims. We identified an object with such a large inversion angle in the near-Earth asteroids and derived previously unknown parameters, including the maximum polarization degree. By comparing the polarimetric parameters and spectral properties of meteorites, we inferred the surface conditions based on the degree of polarization of the asteroid. Methods. We carried out multiband polarization imaging observations of an L-type near-Earth asteroid, (85989) 1999 JD6, at the Hokkaido University Observatory in 2015 and 2019, covering a wide range of phase angles 30°–105°. Of the polarimetric parameters, we derived the maximum value, inversion angle, and the slope at that angle from the phase angle-polarization degree curve. Results. We found that the inversion angle and the maximum polarization degree of 1999 JD6 are 27° and 13%. The inversion angle is significantly larger than those of the majority of all asteroids observed before, but is consistent with that of Barbarian asteroids. 1999 JD6 is the first example belonging to this group that is also among the near-Earth asteroids. Conclusions. We claim that 1999 JD6 is a member of the Barbarians and has rougher surface particles than lunar regolith.
The near-Earth asteroid (3200) Phaethon is the parent body of the Geminid meteor stream. Phaethon is also an active asteroid with a very blue spectrum. We conducted polarimetric observations of this ...asteroid over a wide range of solar phase angles α during its close approach to the Earth in autumn 2016. Our observation revealed that Phaethon exhibits extremely large linear polarization: P = 50.0 ± 1.1% at α = 106.5°, and its maximum is even larger. The strong polarization implies that Phaethon's geometric albedo is lower than the current estimate obtained through radiometric observation. This possibility stems from the potential uncertainty in Phaethon's absolute magnitude. An alternative possibility is that relatively large grains (~300 μm in diameter, presumably due to extensive heating near its perihelion) dominate this asteroid's surface. In addition, the asteroid's surface porosity, if it is substantially large, can also be an effective cause of this polarization.
We report new observations of the active asteroid P/2010 A2 taken when it made its closest approach to Earth (1.06 au in 2017 January) after its first discovery in 2010. Despite a crucial role of the ...rotational period in clarifying its ejection mechanism, the rotational property of P/2010 A2 has not yet been studied due to the extreme faintness of this tiny object (∼120 m in diameter). Taking advantage of the best observing geometry since the discovery, we succeed in obtaining the rotational light curve of the largest fragment with Gemini/GMOS-N. We find that (1) the largest fragment has a double-peaked period of 11.36 0.02 hr spinning much slower than its critical spin period; (2) the largest fragment is a highly elongated object (a/b ≥ 1.94) with an effective radius of m; (3) the size distribution of the ejecta follows a broken power law (the power indices of the cumulative size distributions of the dust and fragments are 2.5 0.1 and 5.2 0.1, respectively); (4) the mass ratio of the largest fragment to the total ejecta is around 0.8; and (5) the dust cloud morphology is in agreement with the anisotropic ejection model in Kim et al. These new characteristics of the ejecta obtained in this work are favorable to the impact shattering hypothesis.
Studying small near-Earth asteroids is important in order to understand their dynamical histories and origins as well as to mitigate the damage caused by asteroid impacts on Earth. We report the ...results of multicolor photometry of the tiny near-Earth asteroid 2015 RN35 using the 3.8 m Seimei telescope in Japan and the TRAPPIST-South telescope in Chile over 17 nights in 2022 December and 2023 January. We observed 2015 RN35 across a wide range of phase angles from 2° to 30° in the g, r, i, and z bands in the Pan-STARRS system. These lightcurves show that 2015 RN35 is in a nonprincipal axis spin state with two characteristic periods of 1149.7 ± 0.3 s and 896.01 ± 0.01 s. We found that the slope of the visible spectrum of 2015 RN35 is as red as asteroid (269) Justitia, one of the very red objects in the main belt, which indicates that 2015 RN35 can be classified as an A- or Z-type asteroid. In conjunction with the shallow slope of the phase curve, we suppose that 2015 RN35 is a high-albedo A-type asteroid. We demonstrated that surface properties of tiny asteroids could be well constrained by intensive observations across a wide range of phase angles. 2015 RN35 is a possible mission-accessible A-type near-Earth asteroid with a small Δv of 11.801 km s−1 in the launch window between 2030 and 2035.
Abstract Large terrestrial bodies in our solar system like the Earth, Mars, Mercury, and the Moon exhibit geologically complex surfaces with compositional heterogeneity. From past studies using large ...telescopes and spacecraft, it was shown that asteroids with diameters larger than 100 km also show surface heterogeneity at hemispheric scales, while on smaller objects, such features remain to be detected. Here, we investigate candidates for surface heterogeneity in a sample of 130 main-belt asteroids using multiepoch spectroscopic data from the MIT–Hawaii Near-Earth Object Spectroscopic Survey, which has been observing asteroids for about 20 yr using a self-consistent observation technique. Twelve conservative candidates with spectra more than 3 σ apart from each other at 2.4 μ m and 52 optimistic candidates for surface heterogeneity are detected. These candidates include eight objects already reported as being heterogeneous. Our study suggests that the size boundary between small homogeneous asteroids and larger heterogeneous objects, if it exists, is lower than 100 km. A-type asteroids have a higher proportion of heterogeneous candidates than other asteroids. This may be because olivine, which is the main surface constituent of these objects, reacts more efficiently to space weathering with respect to pyroxene, such that a similar range of surface ages will translate into a wider range of optical-to-near-infrared spectral slopes in the case of A-type bodies.
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