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Register a new userDetection of radiation pressure acting on 2009 BD
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We report the direct detection of radiation pressure on the asteroid 2009 BD, one of the smallest multi-opposition near-Earth objects currently known, with H ~ 28.4. Under the purely gravitational model of NEODyS the object is currently considered a possible future impactor, with impact solutions starting in 2071. The detection of a radiation-related acceleration allows us to estimate an Area to Mass Ratio (AMR) for the object, that can be converted (under some assumptions) into a range of possible values for its average density. Our result AMR = (2.97 pm 0.33) x 10^(-4) m^2 kg^(-1) is compatible with the object being of natural origin, and it is narrow enough to exclude a man-made nature. The possible origin of this object, its future observability, and the importance of radiation pressure in the impact monitoring process, are also discussed.
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We present our astrometric observations of the small near-Earth object 2011~MD ($H sim 28.0$), obtained after its very close fly-by to Earth in June 2011. Our set of observations extends the observational arc to $73$ days, and together with the published astrometry obtained around the Earth fly-by allows a direct detection of the effect of radiation pressure on the object, with a confidence of $5sigma$. The detection can be used to put constraints on the density of the object, pointing to either an unexpectedly low value of $rho = (640 pm 330) mbox{ kg} / mbox{m} ^3$ ($68%$ confidence interval) if we assume a typical probability distribution for the unknown albedo, or to an unusually high reflectivity of its surface. This result may have important implications both in terms of impact hazard from small objects and in light of a possible retrieval of this target.
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 asteroids rotation and gravitational field. Mie theory is used because the particles of interest have sizes comparable to thermal wavelengths (~1-100 {mu}m), 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 {mu}m to ~10 {mu}m is dominated by the thermal radiation from the AR when the asteroid is in the near-Sun environment (heliocentric distance rh < 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 {mu}m 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 minutes. Our results are consistent with the previous observational studies on Phaethon in that the ejected dust particles have a spherical radius of ~1 {mu}m.
We investigate the pressure torque between the fluid core and the solid mantle arising from magnetohydrodynamic modes in a rapidly rotating planetary core. A two-dimensional reduced model of the core fluid dynamics is developed to account for the non-spherical core-mantle boundary. The simplification of such a quasi-geostrophic model rests on the assumption of invariance of the equatorial components of the fluid velocity along the rotation axis. We use this model to investigate and quantify the axial torques of linear modes, focusing on the torsional Alfven modes (TM) in an ellipsoid. We verify that the periods of these modes do not depend on the rotation frequency. Furthermore, they possess angular momentum resulting in a net pressure torque acting on the mantle. This torque scales linearly with the equatorial ellipticity. We estimate that for the TM calculated here topographic coupling to the mantle is too weak to account for the variations in the Earths length-of-day.
Impacts of micrometeoroids on the surfaces of Nix and Hydra can produced dust particles and form a ring around Pluto. However, dissipative forces, such as the solar radiation pressure, can lead the particles into collisions in a very short period of time. In this work we investigate the orbital evolution of escaping ejecta under the effects of the radiation pressure force combined with the gravitational effects of Pluto,Charon, Nix and Hydra. The mass production rate from the surfaces of Nix and Hydra was obtained from analytical models. By comparing the lifetime of the survived particles, derived from our numerical simulations, and the mass of a putative ring mainly formed by the particles released from the surfaces of Nix and Hydra we could estimate the ring normal optical depth. The released particles, encompassing the orbits of Nix and Hydra, temporarily form a 16000 km wide ring. Collisions with the massive bodies, mainly due to the effects of the radiation pressure force, remove about 50% of the $1mu$m particles in 1 year. A tenuous ring with a normal optical depth of $6 times 10^{-11}$ can be maintained by the dust particles released from the surfaces of Nix and Hydra.
The light scattered from dust grains in debris disks is typically modeled as compact spheres using Lorenz-Mie theory or as porous spheres by incorporating an effective medium theory. In this work we examine the effect of incorporating a more realistic particle morphology on estimated radiation-pressure blowout sizes. To calculate the scattering and absorption cross sections of irregularly shaped dust grains, we use the discrete dipole approximation. These cross sections are necessary to calculate the $beta$-ratio, which determines whether dust grains can remain gravitationally bound to their star. We calculate blowout sizes for a range of stellar spectral types corresponding with stars known to host debris disks. As with compact spheres, more luminous stars blow out larger irregularly shaped dust grains. We also find that dust grain composition influences blowout size such that absorptive grains are more readily removed from the disk. Moreover, the difference between blowout sizes calculated assuming spherical particles versus particle morphologies more representative of real dust particles is compositionally dependent as well, with blowout size estimates diverging further for transparent grains. We find that the blowout sizes calculated have a strong dependence on the particle model used, with differences in the blowout size calculated being as large as an order of magnitude for particles of similar porosities.
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