The YORP Effect On the Mechanical Power of

The YORP Effect On the Mechanical Power of Sun Light David Polishook Dept. of Planetary Sciences Tel-Aviv University YORP

YORP effect • A thermal torque on an asymmetric atmosphere-less body

YORP effect • affects on: – – – Asteroid spin-up or spin-down Axis orientation alignment Axis precession Shape modification Binary creation Orbital drift Ostro et al. 2001

YORP effect • Depends on: – – – – Asteroid size (1/D 2) Heliocentric distance (1/a 2) Asymmetric elliptic shape Spin period & direction Albedo Thermal inertia Structure & composition effects on heat conductivity

YORP effect: time scale • • Main Belt Asteroid: a = 2. 5 AU. D = 1, 000 m. τ ~ 10 Myr. • • Near Earth-Asteroid a = 0. 8 AU. D = 100 m. τ ~ 10 Kyr. • How to model the other parameters in K? ? ? • How does change with time?

The problem: size or orbit? Pravec & Harris 2000 • What is the spin distribution of small MBAs? • Maxwellian -> collisions rule. • Flat -> YORP rule. • Spin distribution of large asteroids (D>40 km) match a Maxwellian distribution Problem: more than half are NEAs… • Spin distribution of small asteroids (10>D>0. 15 km) mismatch a Maxwellian distribution

Spin Distributions: Size • Smaller MBAs do not spin in Maxwellian Distribution. • YORP is most effective under the size range of 10 km. Polishook & Brosch 2009, Icarus 199

Spin Distributions: Orbit • NEAs & MBAs have a similar spin distribution. • More data is needed to find significant differences between the histograms. • Conclusions: • YORP is the dominant mechanism for asteroids spin evolution. • Size matters!!!

YORP Detection • (54509) 2000 PH 5 • NEA. D=114 m. P=12. 17 min. • Photometry search for spin change (Lowry et al. 2007). • Radio search for phase change (Taylor et al. 2007). • Td ~ 550, 000 y.

YORP Detection • • (1862) Apollo NEA. D=1, 400 m. P=3. 1 h. Has a small satellite. Photometry search for spin change (Kaasalainen et al. 2007). • Td ~ 2. 6 My. • Can the satellite be explained by YORP? Miko Kaasalainen

YORP Detection • • (1620) Geographos NEA. D=5 x 2 km. P=5. 25 h. Very elongated. Photometry search for spin change (Durech et al. 2008).

YORP Not detected yet… • Why does Itokawa not show a YORP influence? • Where are the spin-down YORP asteroids?

The rubble pile spin barrier hours 2. 2 hours ~150 m Pravec et al. 2006 Shoemaker-Levy 9 (July 1994)

Binary Formation mpg. Binary. Formation-Walsh

YORP cycles • YORP accelerates -> • Spin higher then the rubble pile barrier -> • Asteroid loose mass -> • Spin reduce + binary formation -> • YORP accelerates again • = YORP cycle

Asteroids systems? • 2001 SN 263. • Small NEA triple asteroid. Nolan et al. 2008

Are all binaries stable? • Part/Some/Most mass is flying away… • Can we see this at real time? • “Pairs” of asteroids – separate asteroids with similar orbital parameters - are these YORP unstable binaries? • The study continue…

The YORP Effect. Copyrights belong to David Polishook. The images in the presentations are protected by copyrights laws and belong to their owners. The presentation should be used for private and/or educational purposes only. Do not edit the presentation and do not use it commercially. Many thanks David