Barrel Instability in Binary Asteroids

Most close-in planetary satellites are in synchronous rotation, which is usually the stable end-point of tidal despinning. Saturn’s moon Hyperion is a notable exception by having a chaotic rotation. Hyperion’s dynamical state is a consequence of its high eccentricity and its highly prolate shape. As many binary asteroids also have elongated secondaries, chaotic rotation is expected for moons in eccentric binaries, and a minority of asteroidal secondaries may be in that state. The question of secondary rotation is also important for the action of the binary Yarkovsky–O’Keefe–Radzievskii–Paddack (BYORP) effect, which can quickly evolve orbits of synchronous (but not nonsynchronous) secondaries. Here we report results of a large set of short numerical simulations which indicate that, apart from synchronous and classic chaotic rotation, close-in irregularly shaped asteroidal secondaries can occupy an additional, intermediate rotational state. In this “barrel instability” the secondary slowly rolls along its long axis, while the longest axis is staying largely aligned with the primary–secondary line. This behavior may be more difficult to detect through lightcurves than a fully chaotic rotation, but would likewise shut down BYORP. We show that the binary’s eccentricity, separation measured in secondary’s radii and the secondary’s shape are all important for determining whether the system settles in synchronous rotation, chaotic tumbling, or barrel instability. We compare our results for synthetic asteroids with known binary pairs to determine which of these behaviors may be present in the near-Earth asteroid binary population.

Thermal infrared imaging experiment of S-type binary asteroids in the Hera mission

<p>The thermal infrared imager TIRI onboard the ESA Hera spacecraft is being developed to investigate thermophysical properties of the S-type asteroid 65803 Didymos and its moon Dimorphos by mapping thermal inertia and compositional variations of them. TIRI is based on an uncooled micro-bolometer array of 1024 x 768 effective pixels and covers the field of view of 13.3° x 10.0°, with the resolution of 0.23 mrad per pixel. TIRI has an eight-position filter wheel to be used as one wide bandpath at 8-14 µm for thermal imaging, six narrow bands peaked at 7.8, 8.6, 9.6, 10.6, 11.65, and 13.1 µm for compositional information, and one closed plate both for a shutter and a temperature reference.</p> <p>TIRI will be mounted on the top panel of the Hera spacecraft to point the target asteroids in the same direction with other instruments AFC, PALT, and Hyperscout-H, for the simultaneous observations. The asteroid surface temperature will change day and night according to thermal inertia and roughness of the surface layer, which will be consequently derived from the diurnal temperature profile. The maximum temperature in a day will also change according to the solar distance of the asteroid from ~1 to ~2 au at the beginning to the end of the nominal mission. During the early characterization phase (ECP) at 20 to 30 km from the asteroid, TIRI will take images from large solar phase angles from 50° to 70° with the spatial resolution of ~4.6 to 6.9 m per pixel to construct the asteroid shape model even in the night side and map the thermal inertia and composition. During the detailed characterization phase (DCP) at 10 to 20 km from the asteroid, TIRI will take images from the noon with the spatial resolution of ~2.3 m per pixel for more detailed thermal properties and compositional mapping. During the close-up operation phase (COP) at < 5 km from the asteroid, TIRI will take images from the noon with the spatial resolution of ~1 m per pixel. Higher spatial resolution will be achieved during the further close observations.</p> <p>In the Hayabusa2 mission, thermal imaging has revealed the highly porous nature of C-type asteroid from global to local scales (Okada et al, 2020; Shimaki et al, 2020), but nobody knows the surface properties of S-type asteroids so that this is a unique opportunity to investigate the S-type asteroid Didymos in comparison with the C-type asteroid Ryugu. For the moon Dimorphos, it will be the smallest asteroid ever explored so that it is also a unique opportunity to investigate the small-sized asteroid, especially for the strength and porosity. TIRI will contribute to verifying Yarkovsky and YORP (B-YORP) effects, orbital and rotational evolution in relation to thermophysical modeling. The temperature profile and compositional difference between the inside and outside of the artificial crater formed by the kinetic impact of the NASA DART spacecraft will be the important target both for the purpose of planetary defense and science.</p>

New estimates of the mass and density of asteroid (16) Psyche

<p>Asteroid mass determination is performed by analyzing an asteroid's gravitational interaction with another object, such as a spacecraft, Mars, a companion in the case of binary asteroids, or a separate asteroid during a close encounter. During asteroid-asteroid close encounters, perturbations caused by the masses of larger asteroids can be detected in the post-encounter orbits of the smaller test asteroid involved in such an encounter. This can be described as an inverse problem where the aim is to fit six orbital elements for each asteroid and mass(es) for the perturbing asteroid(s), for a total of 13 parameters at minimum unless more asteroid-asteroid encounters are modeled simultaneously.<br /><br />To solve this inverse problem, which is traditionally done with least-squares methods, we have implemented a Markov-chain Monte Carlo (MCMC) based solution and recently (Siltala & Granvik 2020) reported, among others, significantly lower than expected masses and densities for the asteroid (16) Psyche in particular. Psyche is an interesting, and topical, object as it is the target of NASA's eponymous Psyche mission and is commonly thought to be of metallic or stony-iron composition, which our previous density estimates disagreed with. In our previous work our two separate mass estimates for Psyche were based on modeling encounters with two separate test asteroids in both cases. Since then we have further refined our mass estimate for Psyche by simultaneously using eight separate test asteroids for this object, significantly increasing the amount of observational data included on the model which, in turn, will narrow down the uncertainties of our results at the cost of additional model complexity. Here we report and discuss our latest results for the mass of Psyche based on this case and compute corresponding densities based on existing literature values for the volume. We obtain a mass of (0.972 ± 0.148) * 10^-11 solar masses for Psyche corresponding to a bulk density of (3.37 ± 0.58) g/cm³ which is higher than our previous results while remaining consistent with them considering the uncertainties involved. It still remains lower than other previous literature values. We compare our results to these previous literature values and briefly discuss possible physical implications of these results.<br /><br />In addition, due to previous interest from the scientific community, we have also computed mass estimates for Ceres and Vesta, both of which already have very precisely known masses from the Dawn mission. As such, our results for these two asteroids are not of direct scientific interest but they serve as an useful benchmark to verify that our algorithm provides realistic results as we have 'ground truth' values to compare our results to. We find that for both cases, our results are in line with those of Dawn.</p>

Janus: A NASA SIMPLEx mission to explore two NEO Binary Asteroids

<p>Janus is a NASA SIMPLEx mission currently in Phase B. The SIMPLEx program is designed around the idea of using secondary launch opportunities to explore interplanetary destinations. The Janus mission concept plans to take advantage of the NASA Psyche launch to send two spacecraft to fly by Near Earth Objects of interest. A specific point design has been developed that sends two spacecraft to two binary asteroid systems, (175706) 1996 FG3 and (35107) 1991 VH, both of which have been observed repeatedly with photometry, spectrometry and radar.<span class="Apple-converted-space"> </span></p> <p>The Janus mission sends light-weight, low-cost spacecraft built by Lockheed Martin to encounter these high-science value small body targets. The science instruments are a visible and IR imager, from Malin Space Science Systems. The spacecraft will perform a rigorous remote sensing campaign when the object is a point source, and when resolved. The spacecraft will track the binary asteroid systems through closest approach, allowing for a combination of absolute surface resolution, relative resolution across the target asteroids and phase angle coverage unparalleled in previous asteroid flyby missions.<span class="Apple-converted-space"> </span></p> <p>Janus science will combine flyby observations of the target binary asteroids with ground-based observations, enabling the high resolution imaging and thermal data to be placed into a global context and leveraging all available data to construct an accurate topographical and morphological model of these bodies. Based on these measurements, the formation and evolutionary implications for small rubble pile asteroids will be studied.<span class="Apple-converted-space"> </span></p> <p>The science team members all have experience on asteroid missions or have made extensive ground based observations of NEAs. The industry team has extensive experience in the design, fabrication and operation of interplanetary spacecraft and instrumentation.</p> <p><strong>Acknowledgements:</strong> The Janus mission is supported by NASA under a contract from the SIMPLEx Program Office. Part of this research was conducted at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA.</p>

"Barrel Instability" for Elongated Secondaries in Binary Asteroids

<p>Most close-in planetary satellites are in synchronous rotation, which is usully the stable end-point of tidal despinning. Saturn's moon Hyperion is a notable exception by having a chaotic rotation. Hyperion's dynamical state is a consequence of its high eccentricity and its highly prolate shape (Wisdom et al. 1984). As many binary asteroids also have elongated secondaries, chaotic rotation is expected for moons in eccentric binaries (Cuk & Nesvorny 2010), and a minority of asteroidal secondaries may be in that state (Pravec et al. 2016). The question of the secondary's rotation is importrant for the action of the BYORP effect, which can quickly evolve orbits of synchrnous (but not non-synchronous) secondaries (Cuk & Burns 2005). Here we report preliminary numerical simulations which indicate that in binary systems with a large secondary and significant spin-orbit coupling a different kind of non-synchronous rotation may arise. In this "barrel instability" the secondary slowly rolls along its long axis, while the longest diameter is staying largelly aligned with the primary-secondary line. This behavior  may be more difficult to detect through lightcurves than a fully chaotic rotation, but would likewise shut down BYORP. Unlike fully chaotic rotation, barrel instability can happen even at low eccentricties. In our presentation we will discuss our theoretical results and their implications for the evolution of binary asteroids, such as the Didymos-Dimorphos pair.</p>