General Introduction to Asteroids

Kinoshita Daisuke

Institute of Astronomy, National Central University, Taiwan

27 June 2016 Astronomy Summer Camp 2016

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Outline

General Introduction to Asteroids Nomenclature Asteroid search Structure of the Solar System Orbits Spatial Distribution Classiﬁcation of asteroids Size measurement of an asteroid Mass measurement of an asteroid Asteroid families Asteroid surface

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Importance of study of asteroids

asteroids: remnants of planetesimals primordial properties for asteroids which have not been diﬀerentiated key objects to understand the formation and evolution of the solar system.

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Nomenclature

Examples of asteroid names (1) Ceres (19470) Wenpingchen (155140) 2005 UD 2016 AB123 2016 LA Designations of asteroids asteroid number asteroid name provisional designation Evolution of asteroid names just after the discovery: provisional designation only after having well-determined orbit: number + provisional designation after the oﬃcial name is given: number + name

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Provisional Designations of Asteroids

How to give a provisional designation to a newly discovered asteroid? 2014 AA: the first asteroid discovered in 2014 four-digit number year of discovery first letter after a space→ half-month of discovery → A: first half of January B: second half of January C: first half of February ..... M: second half of June ..... Y: second half of December (I is not used.) second letter order of discovery in a half-month → 2014 AA, 2014 AB, 2014 AC, ... (I is not used.) If there are more than 25 new asteroid discoveries within a half-month, then a subscript is used after the second letter.

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Provisional Designations of Asteroids (continued...)

How to give a provisional designation to a newly discovered asteroid? 2016 LA50: an asteroid discovered in the first half of June in 2016 asteroids discovered in the first half of June in 2016 2016 LA, 2016 LB, 2016 LC, ... , 2016 LZ, 2016 LA1, 2016 LB1, 2016 LC1, ..., 2016 LZ1, 2016 LA2, 2016 LB2, 2016 LC2, ..., 2016 LZ2, ... 2016 LA49, 2016 LB49, 2016 LC49, ..., 2016 LZ49, 2016 LA50, ... 2016 LA50: the 1251st asteroid discovered in the first half of June in 2016

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Moving Object Search

Classical blinking method (shallow search) comparison of multiple images taken at diﬀerent time

2001 DR106

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Moving Object Search

“Shift & Add” method (deep search) acquisition of many images of the same ﬁeld shifting + co-adding + subtraction of static image

“Shift & Add” “Shift & Add” + subtraction

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Shift & Add Method

Shift & Add method and assumed heliocentric distances RH = 40AU RH = 55AU RH = 70AU

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Detection of Comet 1P/Halley at 28 au

mR = 28.2 mag, 9-hr integration by 8-m VLT of ESO Hainaut, Delsanti, Kinoshita, Meech, Pompei, West

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Terminology

planets satellites dwarf planets small solar system bodies asteroids near-Earth asteroids mainbelt asteroids Trojan asteroids centaurs trans-Neptunian objects (Kuiper belt objects) comets

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Solar System Formation

For details, see the talk by Prof. Chen Wen-Ping.

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Structure of the Solar System

Outer Solar System

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Structure of the Solar System

Inner Solar System

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Orbits of Asteroids

Keplerian elements

For details, see the talk by Prof. Jiang Ing-Guey.

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Orbits of Asteroids

Orbital elements (Keplerian elements) 6 parameters (a, e, i, Ω, ω, M) semimajor axis a (in au) eccentricity e inclination i (in deg) longitude of ascending node Ω (in deg) argument of perihelion ω (in deg) mean anomaly M (in deg) Orbital elements in heliocentric Ecliptic coordinate (X , Y , Z, X˙ , Y˙ , Z˙ ) positions (X , Y , Z) velocities (X˙ , Y˙ , Z˙ ) astrometry orbital elements → number of unknown parameters: 6 position measurement (RA, Dec) 3 position measurements→ at diﬀerent time epoch orbital elements →

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Spatial Distribution of Asteroids

0.9

0.8

0.7

0.6

0.5 Eccentricity 0.4

0.3

0.2

0.1

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 Semimajor axis [AU] semimajor axis and eccentricity 27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Surface Density in Solar System

minimum mass solar nebula model 10000 Dust Gas

1000 ) 2 100

Venus Earth 10 Jupiter Surface Density (g/cm Saturn

1 Mercury

Neptune Mars Uranus 0.1 0.1 1 10 100 Heliocentric Distance (AU) snow-line at 3 AU from the Sun ∼ 27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Snow-Line

Equilibrium temperature S 2 2 4 (1 Ab) 2 πr = 4πr ǫσTeq − RH 280 1000 Teq → ∼ √RH sublimation temperature of H2O: T 170K ∼ 100 T 170K R 3 au

H Temperature [K] ∼ → ∼ snow-line at RH 3 au ∼ igneous asteroids at inner mainbelt 10 primitive asteroids 0.1 1 10 100 Heliocentric Distance [AU] at outer mainbelt

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 A little bit more about blackbody radiation

Blackbody: an ideal body which absorbs all the radiation at all frequencies and all the angle of incidence Planck’s radiation law 2hν3 1 B T ν( )= 2 hν c e kT 1 − Bν: speciﬁc intensity or brightness (energy per unit area per unit time per unit frequency range per unit solid angle)

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Rayleigh-Jeans Law

hν kT limit (low frequency limit) ≪ Taylor expansion

∞ f (n)(a) f (x)= (x a)n n! − Xn=0 Maclaurin expansion

∞ f (n)(0) f (x)= xn n! Xn=0

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Rayleigh-Jeans Law

Using Maclaurin expansion

2 3 hν hν 1 hν 1 hν e kT =1+ + + + kT 2 kT 6 kT · · ·

Then, hν hν e kT 1 − ∼ kT Rayleigh-Jeans law

2hν3 kT 2ν2 Bν(T ) = kT ∼ c2 hν c2

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Wien Law

hν kT limit (high frequency limit) ≫ 3 2hν − hν Bν(T ) e kT ∼ c2

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Blackbody Radiation

Spectrum of Blackbody Radiation (T=300K)

Wavelength [m] 105 104 103 102 101 100 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 Planck -10

] 10 Rayleigh-Jeans

-1 Wien 10-12 -14 strad 10 -1 -16

Hz 10 -2 -18

m 10 -1 10-20 10-22 10-24 10-26

Brightness [J sec 10-28 10-30 103 104 105 106 107 108 109 1010 1011 1012 1013 1014 1015 1016 Frequency [Hz]

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Equilibrium Temperature of Asteroids

Asteroids in main-belt

albedo: Ab =0.1 heliocentric distance: RH = 3 AU emissivity ǫ =0.9 Equilibrium temperature of asteroids

1 1370(1 0.1) 4 Teq,asteroids = − 4 0.9 5.67 10−8 32 · · × · 160 [K] ∼

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Equilibrium Temperature of KBOs

Kuiper belt objects

albedo: Ab =0.1 heliocentric distance: RH = 40 AU emissivity ǫ =0.9 Equilibrium temperature of KBOs

1 1370(1 0.1) 4 Teq,KBOs = − 4 0.9 5.67 10−8 402 · · × · 45 [K] ∼

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Spatial Distribution of Asteroids

4:1 3:1 5:2 2:1 5:3 3:2 4:3 1:1 600 7:2 7:3

500

400

300 Number of asteroids

200

100

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 Semimajor axis [AU] location of gap location of mean motion resonance → 27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Mean Motion Resonances

Mean motion resonance two orbiting bodies have orbital periods that are in a simple integer ratio e.g.: Pluto is locked in a 2:3 mean motion resonance with Neptune Some MMRs stabilize orbits of asteroids. asteroids in 1:1 mean motion resonance with Jupiter (Trojan asteroids) asteroids in 3:2 mean motion resonance with Jupiter (Hilda asteroids) Some MMRs unstabilize orbits of asteroids. nearly no asteroid at the location of 3:1 mean motion resonance with Jupiter nearly no asteroid at the location of 5:2 mean motion resonance with Jupiter

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Kepler’s 3rd Law

Kepler’s third law eq. of motion for circular orbit Mm G = mrω2 a2 now, angular velocity ω is 2π ω = . P hence, M 2π 2 G = a a2 P ﬁnally, we get GM a3 = P2. 4π2

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Locations of Mean Motion Resonances

Kepler’s third law a (semimajor axis) and P (orbital period):

a3 P2 ∝ 3:2 resonance (Hilda)

semimajor axis of Jupiter: aJ =5.2026 au P 3:2 resonance ast =2/3 → PJ semimajor axis of 3:2 resonance object with Jupiter is

2 2 2 3 2 3 aast = aJ =5.2026 3.97 au 3 × 3 ∼

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Classiﬁcation of Asteroids

Aten a < 1.0 AU, Q < 0.983 AU Earth crosser Apollo a > 1.0 AU, q < 1.017 AU Earth crosser Amor 1.017 < q < 1.3 AU Mars crosser Note that eccentricity of the Earth is 0.017.

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Classiﬁcation of Asteroids

Inner mainbelt 1.8-2.5 au many S-type asteroids Middle mainbelt 2.5-3.1 au S and C and M Outer mainbelt 3.1-4.1 au many C-type asteroids

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Classiﬁcation of Asteroids

Hilda 3:2 resonance with Jupiter C and P types Thule 4:3 resonance with Jupiter many D type asteroids Trojan 1:1 resonance with Jupiter L4 and L5 Lagrange points spectral types: D, P, and C types

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Surface Composition and Taxonomy

Surface composition: constraints on the formation conditions and evolution Taxonomy: classification of asteroids into sub-groups Important propersites Reflectance spectra Overall shape of spectra (or colors) Absorption features Albedo (or reflectivity) Major classes: C-type, S-type, D-type

For details, see following talk “Asteroid spectroscopy and taxonomy” by Dr. Lin Hsing-Wen.

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Spectral Types

Type Albedo Metorite Analog Location E > 23 Enstatite mainbelt (inner edge) S 7-23 ordinary chondrite mainbelt (inner to central) M 7-20 stony-iron, iron mainbelt (central) V 38 basaltic achondrite mainbelt A 25 pallasite, olivine rare in mainbelt ∼ C 2-7 carbonaceous mainbelt (outer) chondrite P 2-7 cabonaceous (?) mainbelt (outer most) D 2-7 kerogen, mainbelt (outer most), carbonaceous Trojan Z 4-10 organic material Centaurs and KBOs

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Spatial Distribution of Asteroid Classes

Igneous bodies at < 2.7 AU Primitive bodies at > 3.4 AU

Physics and chemistry of protosolar nebula dynamical processes spatial mixing → Space-weathering change of surface →

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 How to measure the size of an asteroid?

radiometry simultaneous measurements of reﬂected sunlight in visible, Fvisible ∝ Ag S and thermal radiation in infrared. Fthermal ∝ (1 − AB )S + thermal models (balance of incoming and outgoing energies) projected cross-section + albedo → Asteroid thermal models STM (Standard Thermal Model) ILM (Isothermal Latitude Model) or FRM (Fast Rotating Model) NEATM (Near-Earth Asteroid Thermal Model)

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Thermal Models

STM (Standard Thermal Model) extreme case for slow rotation and small thermal inertia giving optimistic thermal ﬂux for given size and albedo 1 4 (1−Ab )Ss T (θ, φ)= ηǫσR2 sin(θ)cos(φ) n H o ILM (Isothermal Latitude Model) extreme case for fast rotation and large thermal inertia giving pessimistic thermal ﬂux for given size and albedo 1 4 (1−Ab )Ss T (θ, φ)= πǫσR2 sin(θ) n H o

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Predicted Thermal Flux of (28978) Ixion

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 How to measure the mass of an asteroid?

a binary system + Kepler’s 3rd law mass determination → 4π2 a3 M + M = 1 2 G P2 size + mass density estimate → range of measured asteroid densities: from 0.5 to 4.0 g/cc ∼ ∼ range of chondrite densities: from 2.0 to 4.0 g/cc structure of low density asteroids: rubble-pile (high porosity)

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Asteroid Families

20 Non-family member asteroids Flora family Themis family Eos family Eunomia family Nysa family Vesta family 15 Koronis family Hygiea family Ceres family Polana_nysa family Maria family Dora family Adeona family Massalia family 10 Lydia family Erigone family proper inclination [deg]

0 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 proper semimajor axis [AU]

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Asteroid Families

an asteroid family a group of asteroids with similar orbital elements formation of an asteroid family a collision of asteroids colliding bodies are completely shattered fragments become independent asteroids with similar orbits

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 A collision really produces an asteroid family?

Escape velocity of asteroids 1 GMm 2GM mv 2 = 0 v = 2 − R → r R Escape velocity of Ceres-sized body M 1021 kg, R 500 km v 0.5 km/sec ∼ ∼ → ∼ Random asteroid velocity typical collision velocity ∼ several km/sec ∼ random velocity escape velocity ≫ most collisions are disruptive → super-catastrophic collisions asteroid families → Ex. Eos, Koronis, Themis families Fragmentation by collisions and re-accumulation rubble-pile → structure Impacts on rapidly rotating bodies binary systems → 27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Size Frequency Distribution

Power-law distribution 3 Diﬀerential form −ζ R 2 N(R)dR = N0 dR R0 N(R)dR: number of 1 asteroids with radii between R and R + dR 0

Cumulative form∞ ′ ′ -1 N(> R)= N(R )dR ZR ζ 3 4 for larger asteroids, -2 ∼ − and ζ 2 3 for smaller asteroids ∼ − -3 Collisional cascade ζ = 3.5 100 10 1 0.1 →

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids from SDSS website / 50 Break-up of Asteroids

break-up mechanisms collision supercatastrophic collision cf. craters on asteroids tidal force tidal effects during close encounters with planets Rotational fission YORP effect solar radiation can accelerate (or decelerate) the rotation of small asteroids. some evidences for asteroid break-up asteroid family fragments travel on separate heliocentric orbits. rubble-pile structure re-accumulation of fragments power-law index of size distribution 3.5 (collisional cascade) ∼ 27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Polarimetry

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Polarization by reﬂection

Reﬂection of light results in some degree of polarization. asteroid polarimetry study of asteroid surface →

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Asteroid surface

Polarimetric observations can be used to infer the condition of asteroid surface. polarimetry surface texture, optical properties of surface material → asteroid surface: pulverized rocks + coarser grained material

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50 Asteroid surface

(25143) Itokawa observed by spacecraft Hayabusa

For details of results from recent space missions, see the talk by Cheng Yu-Chi on Friday.

27 June 2016 Astronomy Summer Camp 2016 Kinoshita Daisuke (Institute of Astronomy, NationalGeneral Central Introduction University, Taiwan) to Asteroids / 50