Orbital evolution of small binary asteroids

The Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect is a thermal recoil force that is experienced by small bodies when they re-emit energy absorbed from sunlight. Due to the complex shapes and surfaces of asteroids, this re-emission is anisotropic, causing a torque that can modify an asteroid's rotation rate and spin-axis orientation. This effect, which is loosely analogous to a windmill, has a profound influence on the evolution of the Solar System. In order to better understand the YORP effect, it must be measured for a large number of asteroids. Almost 25 years since it was first predicted, the YORP effect has been observationally detected with twelve asteroids. Measuring the YORP effect requires that an asteroid is monitored over a significant period of time, and the asteroid must be well-characterised before it can be assessed for YORP. This means that the shapes, spin rates, and spin-axis orientations of asteroids must be measured, often requiring the use of both optical and radar observations. This thesis presents the results of a project to characterise four asteroids and search for evidence of the YORP effect acting upon them. This work forms part of a larger research collaboration, originating with a European Southern Observatory (ESO) Large Programme (LP) to monitor 40 near-Earth asteroids. The asteroids considered in this work are (23187) 2000 PN9, (29075) 1950 DA, (85275) 1994 LY, and (159402) 1999 AP10. The results presented here include an independently-developed shape model for each of these asteroids, and a comprehensive search for signs of YORP-driven rotational acceleration. For (23187) 2000 PN9, a high-resolution shape model was developed using a combination of optical lightcurves and planetary radar observations. A spin-state analysis shows that the asteroid is in a state of constant-period rotation, with any undetected YORP acceleration being significantly constrained. The asteroid is found to be YORP-evolved, and bears a striking resemblance to other asteroids with 'YORPoid' morphologies. Following a previously reported indication of YORP acting upon (29075) 1950 DA, this work includes an extensive campaign to follow up and confirm a detection of rapid rotational deceleration. The previously reported measurement is verified and constrained by this work, using both previously published and newly developed shape models with a mix of old and new datasets. Two possible explanations are offered for the peculiar result: either there is a previously overlooked flaw in the tools and methods used to characterise asteroids, or this is the first detection of a hitherto unknown physical phenomenon. Finally, analyses of (85275) 1994 LY and (159402) 1999 AP10 fail to detect YORP, but the asteroids are characterised and constraints are placed on any potential YORP acceleration. 1994 LY provides a good example of YORP false positives, and the results of this work will be beneficial to future studies of the asteroid system. The model of 1999 AP10 developed for this work has already contributed to significant results, and proves the viability of developing physical models using small optical telescopes over relatively short timescales. Like many theses, this work ultimately raises more questions than it answers. The results for 2000 PN9 are conclusive, while an exhaustive analysis of 1950 DA has only deepened the mystery surrounding its rapid spin-down. 1994 LY and 1999 AP10 have been analysed as far as is sensible with existing data, but are shown to be worthy of further study when they are next observable.

The Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect is a net torque caused by solar radiation directly reflected and thermally re-emitted from the surface of small asteroids and is considered to be crucial in their dynamical evolution. By long-term photometric observations of selected near-Earth asteroids, it is hoped to enlarge asteroid samples with a detected YORP effect to facilitate the development of a theoretical framework. Archived light-curve data are collected and photometric observations are made for (1685) Toro and (85989) 1999 JD6, which enables measurement of their YORP effect by inverting the light curve to fit observations from a convex shape model. For (1685) Toro, a YORP acceleration υ = (3.2 ± 0.3) × 10−9 rad · day−2 (1σ error) is updated, which is consistent with previous YORP detection based on different light-curve data; for (85989) 1999 JD6, it is determined that the sidereal period is 7.667 749 ± 0.000009 hr, the rotation pole direction is located at λ = 232° ± 2°, β = − 59° ± 1°, the acceleration is detected to be υ = (2.4 ± 0.3) × 10−8 rad · day−2 (1σ error) and in addition to obtaining an excellent agreement between the observations and model. YORP should produce both spin-up and spin-down cases. However, including (85989) 1999 JD6, the dω/dt values of 11 near-Earth asteroids are positive totally, which suggests that there is either a bias in the sample of YORP detections or a real feature needs to be explained.

<p>We report 491 new near-infrared spectroscopic measurements of 420 Near-Earth Objects (NEOs) collected on NASA’s IRTF in the context of MITHNEOS (PI: DeMeo). The measurements were combined with previously published data (Binzel et al. 2019) and bias-corrected for albedo variations to derive the intrinsic compositional distribution of the overall NEO population. We also investigated individual subpopulations coming from various escape routes (ERs) in the asteroid belt by use of the dynamical model of Granvik et al. (2018). The resulting distributions reflect well the compositional gradient of the asteroid belt, with decreasing fractions of silicate-rich (S- and Q-type) bodies and increasing fractions of carbonaceous (B-, C-, D- and P-type) bodies as a function of increasing ER distance from the Sun. The compositional match between NEOs and their predicted source populations validates dynamical models used to identify ERs and argues against strong composition change in the main belt between approximately 5 km and 100 m. An exception comes from the overabundance of D-type NEOs from the 5:2J and, to a lesser extent, the 3:1J and ν6 ERs, hinting at the presence of a large population of small D-type asteroids in the main belt. Alternatively, this excess may indicate spectral evolution from D-type surfaces to C and P types due to space weathering or point to preferential fragmentation of D-types in the NEO space. No further evidence for the existence of collisional families in the main belt, below the detection limit of current main-belt surveys, was found in this work.</p>

Gravity is the most important force to affect the motion of bodies in the Solar system. At small sizes, however, additional forces must be taken into account to explain fine details of their translational and rotational motion, as well as parameters of their populations. This is because the strength of the non-gravitational perturbations typically increases as . The principal perturbation acting on macroscopic main-belt bodies (sizes up to several kilometers for timescales of about billion years) is due to the anisotropic thermal re-radiation of the absorbed sunlight. In orbital dynamics this is known as the Yarkovsky effect, while in rotational dynamics the same physical phenomenon is called the YORP (Yarkovsky-O'Keefe-Radzievskii-Paddack) effect. We review the main observational implications of the Yarkovsky/YORP effects as understood and evidenced today.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Absolute photometry of small main-belt asteroids in 2007–2009

The YORP effect on the GOES 8 and GOES 10 satellites: A case study

Evolution of angular velocity for defunct satellites as a result of YORP: An initial study

We estimate the total population of near-Earth objects (NEOs) in the solar system using an extensive, “solar-system-to-pixels” fake-asteroid simulation to debias detections of real NEOs by the ATLAS survey. Down to absolute magnitudes H = 25 and 27.6 (diameters of ∼34 and 10 m, respectively, for 15% albedo), we find total populations of (3.72 ± 0.49) × 105 and (1.59 ± 0.45) × 107 NEOs, respectively. Most of the plausible sources of error tend toward underestimation, so the true populations are likely larger. We find the distribution of H magnitudes steepens for NEOs fainter than H ∼ 22.5, making small asteroids more common than extrapolation from brighter H mags would predict. Our simulation indicates a strong bias against detecting small but dangerous asteroids that encounter Earth with high relative velocities—i.e., asteroids in highly inclined and/or eccentric orbits. Worldwide NEO discovery statistics indicate this bias affects global NEO detection capability to the point that an observational census of small asteroids in such orbits is probably not currently feasible. Prompt and aggressive followup of NEO candidates, combined with closer collaborations between segments of the global NEO community, can increase detection rates for these dangerous objects.

The distribution of heavy elements is anomalously low in the asteroid main belt region compared with elsewhere in the solar system. Observational surveys also indicate a deficit in the number of small (≲50 km size) asteroids, which is two orders of magnitude lower than what is expected from the single power-law distribution that results from a collisional coagulation and fragmentation equilibrium. Here, we consider the possibility that a major fraction of the original asteroid population may have been cleared out by Jupiter’s secular resonance, as it swept through the main asteroid belt during the depletion of the solar nebula. This effect leads to the excitation of the asteroids’ orbital eccentricities. Concurrently, hydrodynamic drag and planet–disk tidal interaction effectively damp the eccentricities of sub-100 km-size and of super-lunar-size planetesimals, respectively. These combined effects lead to the asteroids’ orbital decay and clearing from the present-day main belt region (∼2.1–3.3 au). Eccentricity damping for the intermediate-size (50 to several hundreds of kilometers) planetesimals is less efficient than for small or large planetesimals. These objects therefore preferentially remain as main belt asteroids near their birthplaces, with modest asymptotic eccentricities. The smaller asteroids are the fragments of subsequent disruptive collisions at later times as suggested by the present-day asteroid families. This scenario provides a natural explanation for both the observed low surface density and the size distribution of asteroids in the main belt, without the need to invoke special planetesimal formation mechanisms. It also offers an explanation for the confined spatial extent of the terrestrial planet building blocks without the requirement of extensive migration of Jupiter, which is required in the grand-tack scenario.

Context. The asteroid 25143 Itokawa is one of the candidates for the detection of the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect in the rotation period. Previous studies were carried out up to the 196 608 facets triangulation model and were not able to provide a good theoretical estimate of this effect, raising questions about the influence of the mesh resolution and the centre of mass location on the evolution the rotation period. Aims. The YORP effect on Itokawa is computed for different topography models up to the highest resolution Gaskell mesh of 3 145 728 triangular faces in an attempt to find the best possible YORP estimate. Other, lower resolution models are also studied and the question of the dependence of the rotation period drift on the density distribution inhomogeneities is reexamined. A comparison is made with 433 Eros models possessing a similar resolution. Methods. The Rubincam approximation (zero conductivity) is assumed in the numerical simulation of the YORP effect in rotation period. The mean thermal radiation torques are summed over triangular facets assuming Keplerian heliocentric motion and uniform rotation around a body-fixed axis. Results. There is no evidence of YORP convergence in Gaskell model family. Differently simplified meshes may converge quickly to their parent models, but this does not prove the quality of YORP computed from the latter. We confirm the high sensitivity of the YORP effect to the fine details of the surface for 25 143 Itokawa and 433 Eros. The sensitivity of the Itokawa YORP to the centre of mass shift is weaker than in earlier works, but instead the results prove to be sensitive to the spin axis orientation in the body frame. Conclusions. Either the sensitivity of the YORP effect is a physical phenomenon and all present predictions are questionable, or the present thermal models are too simplified.

Context. Basaltic V-type asteroids play a crucial role in studies of Solar System evolution and planetesimal formation. Comprehensive studies of their physical, dynamical, and statistical properties provide insight into these processes. Thanks to wide surveys, currently there are numerous known V-type and putative V-type asteroids, allowing a detailed statistical analysis. Aims. Our main goal is to analyze the currently available large sample of V-type spin rates, to find signatures of the non-gravitational Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect among the different V-type populations, and to estimate the spin barrier and critical density for V-type asteroids. Our intention is to increase the pool of information about the intriguing V-types. Methods. We collected rotational periods from the literature for spectrally confirmed V-types, putative V-types, and Vesta family members. Through spectroscopic observations we confirmed their taxonomic type and verified the high confirmation rates of the putative V-types. We combined the collected periods with periods estimated in this manuscript and produced rotational frequency distributions. We determined the spin barrier in the frequency–light curve amplitude space for V-type asteroids. Results. We analyzed rotational periods of 536 asteroids in our sample. As expected, due to the small size of the objects analyzed, the frequency distributions for the Vesta family and the V-types outside the family are inconsistent with a Maxwellian shape. The Vesta family shows an excess of slow-rotators. V-types outside the family show an excess of both slow and fast rotators. Interestingly, we found that the population of V-types outside the Vesta family shows a significant excess of fast rotators compared to the Vesta family. The estimated critical density for V-type asteroids exceeds ρc = 2.0 g cm−3, which surpasses the previous estimates. Conclusions. We demonstrated that V-type asteroids have been influenced by the thermal radiation YORP effect and that their critical spin rate is higher than for C-type asteroids. The population of V-types outside the Vesta family shows a significant excess of fast rotators compared to the Vesta family. We hypothesize that the objects that evolved from the Vesta family though the Yarkovsky drift are also more susceptible to the YORP effect. Objects for which YORP has not yet had enough time to act and those that are more YORP resistant will be left in the family, which explains the relatively small proportion of fast rotators being left. The YORP timescale must thus be similar to the migration timescale for those objects.

Binary asteroid population: 1. Angular momentum content

The distribution of rotation rates of small (D = 3–15 km) main-belt asteroids is analyzed in the context of the probable influence of solar radiation on the rotation of asteroids (YORP effect). It is demonstrated that the evolution of this distribution with heliocentric distance (i.e., the presence of well-marked slow and fast rotation excesses in the inner part of the belt, which become less noticeable in the middle part and are almost indistinguishable in the outer part) is well accounted for by the YORP effect. The influence of this effect on the studied asteroid population is rather weak compared to its influence on near-Earth asteroids, which are 3–4 times smaller on average and with ~3 times more intense solar irradiation; however, it is still sufficiently significant to be identified reliably in the available observational data.

Here, we report on results of statistical analyses on spin periods and shapes of main-belt asteroids (MBAs) smaller than $\ \sim\ $1 km in diameter, which were derived from lightcurve observations of a single night with the 8.2 m Subaru telescope. It is shown that, among our detected 68 MBAs giving reliable lightcurve periods, the fraction of fast-rotating asteroids (with spin periods of $\lt\ \sim\ $2.3 hr, candidates of monolithic bodies) is about 49%. In particular, statistical tests reveal with a high confidence level that, in terms of the shape (deduced from the light variation amplitude), the fast rotators belong to a population quite different from that for non fast-rotators; in other words, most of the fast-rotators show a strong trend that they are more spherical in shape than the non fast-rotator group. Consideration of several orbital and rotational evolution timescales for small asteroids in the main asteroid belt seems to indicate that the above-mentioned trend is not by coincidence, but primordial. Therefore, referring to the shape distributions of impact fragments produced in laboratory experiments, our discovered sphericity preference of small fast-rotating asteroids probably requires some spin deceleration mechanisms, which selectively worked on all elongated objects during their impact formation and/or subsequent evolution. We also note that our fast rotators are much more spherical compared to known small MBAs and near-Earth objects.

Any population of asteroids, like asteroid families, will disperse in semimajor axis due to the Yarkovsky effect. The amount of drift is modulated by the asteroid spin state evolution, which determines the balance between the diurnal and seasonal Yarkovsky forces. The asteroid’s spin state is, in turn, controlled in part by the Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect. The otherwise smooth evolution of an asteroid can be abruptly altered by collisions, which can cause impulsive changes in the spin state and can move the asteroid onto a different YORP track. In addition, collisions may also alter the YORP parameters by changing the superficial features and overall shape of the asteroid. Thus, the coupling between YORP and Yarkovsky is also strongly affected by the impact history of each body. To investigate this coupling, we developed a statistical code modeling the time evolution of semimajor axis under YORP–Yarkovsky coupling. It includes the contributions of NYORP (normal YORP), TYORP (tangential YORP), and collisions whose effects are deterministically calculated and not added in a statistical way. We find that both collisions and TYORP increase the dispersion of a family in semimajor axis by making the spin axis evolution less smooth and regular. We show that the evolution of a family’s structure with time is complex and collisions randomize the YORP evolution. In our test families, we do not observe the formation of a “YORP-eye” in the semimajor axis versus diameter distribution, even after a long period of time. If present, the “YORP-eye” might be a relic of an initial ejection velocity pattern of the collisional fragments.