Some Short Topics AS3141 Benda Kecil dalam Tata Surya Prodi Astronomi 2007/2008 B. Dermawan

Topics Space Weathering Non-gravitational forces Rotation & Internal Structure Planetary Companions Dynamical evolution

Space Weathering (SW): Terminology Clark et al. 2002: Any surface modification process(es) that may tend to change the apparent traits (optical properties, physical structure, chemical or mineralogical properties) of the immediate, remotely sensed surface of an airless body from analogous traits of the body’s inherent bulk material Chapman 2004: The observed phenomena caused by the processes (accretion or erosion of particular materials, modification of material in situ by energetic impacts or irradiation) operating at or near the surface of an airless Solar System body that modify the remotely sensed properties of the body’s surface from those of the unmodified, intrinsic, subsurface bulk of the body Nesvorný et al. 2005: Processes that alter optical properties of surfaces or airless bodies (such as solar wind sputtering, micrometeorites impacts, etc.)

SW Evidences  Although many S-type asteroids are probably similar in bulk composition to OC meteorites, surface of S-type asteroids are significantly ‘redder’ than colors of OC meteorites, and have much shallower olivine/pyroxene absorption band at 1  m  Color variations on surfaces of S-type asteroids Ida, Gaspra, and Eros mimic the sense of the color differences observed for lunar soils with older surfaces being darker and redder in appearance. Conversely, it is believed that other common asteroid types (e.g., the V- and C-types) show little evidence of optical alteration with time

Spectral Features The depth of the 1.0  m band & slopes

The Prime Question:  Given early (post-Apollo) demonstration that the lunar surface is space weathered…  Why has it taken so long for it to become accepted that asteroid surfaces are space weathered?  Indeed, is it even yet accepted?

SW Evidence from SDSS Nesvorný et al. 2005

Laboratory Experiments (1) Abundant nanophase-reduced Fe on the rims

Laboratory Experiments (2) Effect of adding a % SMFe to a pulverized OC Clark et al Lazzarin et al Reflectance spectra of three CC meteorites before and after laser and ion radiation

Non-gravitational Forces Outgassing (cometary activity) Thermal Radiation Radiation Pressure Poynting-Robertson Drag Solar Wind, Lorentz Force, Plasma Drag Acceleration: > – (r) – (r) – (t) < Thermal Radiation Acceleration  Yarkovsky & YORP Effects 10 cm – 10 km

Yarkovsky & YORP Effects  Orbit  Size and shape  Spin period and axis orientation  Mass  Density of surface layers  Albedo  Conductivity

Bottke et al Yarkovsky Effect Seasonal Important for smaller fragments of 1–100 m A force felt by a body caused by the anisotropic emission of thermal photons, which carry momentum Diurnal Dominant for larger bodies  100 m

Orbital Drift Brož et al. 2005

Farinella et al. 1998, Vokrouhlický & Farinella 2000, Bottke et al. 2000) Yarkovsky Effect  Meteorite Delivery Long Cosmic Ray Exposure Ages of Meteorites

Bottke et al Koronis Family Dispersal evolution

Yarkovsky Effect on Evolutional Tracks Nesvorný & Bottke 2004 Karin Family members Nesvorný et al. 2002: 39 members

YORP Effect Yarkovsky-O’Keefe-Radzievskii-Paddack Second-order variation on the Yarkovsky effect which causes an asteroid to spin up or down (Rubincam 2000) Distribution of rotation rates of small asteroids (sizes < 50 km) shows a clear excess of very fast and slow rotators Time-scales: Sizes of 10 km: 10 2 Myr < 10 km: much faster

Koronis Family Bimodal obliquity distribution Slivan et al Slivan 2002

Merxia Family Vel. Dispersion, c YORP, Age, K Brož et al. 2005

Three Major Asteroid Size Ranges Large asteroids – rotations collisional ｌ y evolved Small asteroids – rotations driven by YORP Spin barrier at sizes D = 0.2 to 10 km – suggesting cohesionless structure from 0.2 up to 3 km Superfast rotators below D = 0.2 km – cohesion implied Binary population among asteroids with D = km – related to critical spins near the spin barrier 1.Large asteroids, D > 60 km 2.Small asteroids, D = 0.2 – 60 km 3.Very small asteroids, D < 0.2 km Asteroid population splits according to properties related to their rotations into three major ranges at D~60 km and 0.2 km:

Internal Characteristic 

Spin Barrier

Planetary Companions Quasi-satellites Co-orbital (tadpole & horseshoe orbits, Trojans)

Planetary Companions

2002 AA29 has a horseshoe orbit, approaching Earth and being perturbed to move away This is a classic example of Kepler’s third law with change in a The full orbit is not shown, it passes the other side of the Sun. Libration period ca. 190 yr

2002 AA Finding Co-orbitals and Earth Trojans Co-orbitals are currently usually found when near Earth (by LINEAR). Scanning high latitudes could be a good place to look and currently undersurveyed. For Trojans, the search region is smaller. CFHT searches for 1º/day objects in this region could find both types of object.

2003 YN107 has a similar horseshoe behavior at times but lower inclination It is currently trapped as a quasi-satellite near Earth

Dynamical Evolution Orbital integration up to several Gyr Concepts: Symplectic + handle close encounter with planets