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David Polishook – Research Projects

Flybys of Near-Earth Asteroids Applying tidal forces, the Earth can modify asteroids that pass very close to it: the asteroid can spin-up, shakes can roll boulders and rocks, sub-surface material can be exposed, and its entire shape can change. Only by observing asteroids before and after their flybys, and measure the change (or non-change) in their parameters we can study the strength and elasticity of the internal structure. This allows us to predict what will happen to a future impactor before colliding with the Earth and what measures should be taken in order to destroy or deflect it. Image: The changed orbit of 2012 DA14, during its flyby on February 15, 2013. Polishook et al. 2012, Icarus 221. Moskovitz et al. 2014 submitted to Nature (cool results from the 2012 DA14’s flyby)
	Asteroids disintegration by rotational fission The newly discovered category of asteroid pairs consists of gravitationally unbound pairs that once belonged to a single body. Studies showed that asteroid pairs’ progenitors were spun-up by the YORP effect, until they gain sufficient angular momentum to cross the breakup limit for a strength-less object, known as the “rubble pile spin barrier”, and these asteroids split into two components. The study of asteroid pairs and the way asteroids can disintegrate has manifold applications: while it teaches us on their internal structure, it can also demonstrate the strong forces that shaped the early solar system at the early stages of planet formation. Image: Rotation periods of asteroid pairs is correlated to the size ratio between the small and large members of each pair. Polishook 2014. Accepted to Icarus. arXiv:1406.3359 – spin axes of asteroid pairs were modified by the YORP effect suggesting the YORP effect spin them up and not collisions. Also – the first linkage found that asteroid pairs have low density values as expected from “rubble-pile” asteroids. Polishook et al. 2014. Icarus 233 – the secondary member of a pair might split due to a secondary fission. Pravec, Vokrouhlicky, Polishook, et al. 2010. Nature 466, and its Press Release – asteroid pairs are formed by the rotational-fission mechanism. Polishook et al. 2011. Icarus 212 – binary asteroids with high separation were formed by the YORP effect. Vokrouhlicky, Durech, Polishook, et al., 2011, AJ 142 – the spin state of the youngest asteroid pair 6070 Rheinland.
Thermal forces modify asteroids Re-emission of sunlight from atmosphere-less bodies can modify their orbit (the Yarkovsky effect), impose a torque on their spin, and change their rotational axis. This mechanism has significant role in transporting small bodies in the Solar System, forming near-Earth asteroids; splitting fractured bodies by spinning them up; determining the special and size distribution of small bodies in the Solar System through its history. We study the parameters relevant for this mechanism, such as size and spin, and its effects on asteroids such as spin distribution and formation of binaries. Image: The effect of asteroid’s spin on the Yarkovsky’s efficiency. Polishook et al. 2011. Icarus 212 – binary asteroids with high separation were formed by the YORP effect. Polishook et al. 2010. DPS meeting #42, p. 1055 – Yarkovsky effect dependent on asteroid’s spin. Polishook & Brosch 2009, Icarus 199 – Spin distribution of small main-belt asteroids is controlled by the YORP effect.
	Space Weathering Surfaces of atmosphere-less bodies are modified with time by the “space weathering” effect. This mechanism, caused by solar wind, cosmic rays and micrometeorite bombardment alter the top layer on planetary surfaces, causing it to display a “weathered”, darker and redder reflectance spectrum. Space weathering mechanism is not yet understood: different bodies and materials present different amount of weathering, and current estimations of its timescale differ dramatically from one another, and range between 10^4 to 10^9 years. The effect of space weathering on asteroids obscures their true nature. Understanding it will help us determine their true composition, origins, and their role in planet formation. Image: lunar sample that shows the formation of a “weathered” coating on the surface. Clark et al. 2002 Asteroids III, 585-599. Polishook et al. 2014. Icarus 233 - Fresh Surfaces Observed in the Main Belt on asteroid pairs! Polishook et al. 2009, M&PS 44 – looking for fresh surfaces by rotational spectroscopy.
Mining Astronomy Asteroidal data were “mined” from the Palomar Transients Factory (PTF), a survey with exceptionally wide field of view (7.2 square degrees) on a 48’’ telescope, dedicated for transient search. Our pipeline detects asteroids in the PTF images, constructs their lightcurves, and calculates their rotation periods. The pipeline was tested on data from four nights covering an area of ~21 deg^2, and was able to detect 624 asteroids, of which 145 were previously unknown. Rotation periods for 173 asteroids were derived. 3 of the asteroids are probably binary asteroids. We estimate that implementing our search for all existing high-cadence PTF data will provide rotation periods for thousands of asteroids. Image: Asteroids tracks on the PTF’s large field of view. Polishook et al., 2012, MNRAS 421.