Asteroids

Planetary positions had long been "known" to follow Bode's Law (1772), where the semi-major axis of the planets are given by

a=(4+0)/10
a=(4+3)/10
a=(4+6)/10
a=(4+12)/10
a=(4+24)/10
etc...

No physical basis -- just numerology.
Originally, only the first six planets were known -- Uranus's discovery (1781) "confirmed" the law.
Neptune and Pluto shed doubt on Bode's law. It is now thought by most astronomers to be a coincidence.
But a good one, nonetheless! So where is the planet between Mars and Jupiter?
Search led to the discovery of small rocky bodies known as asteroids!

Size distribution follows a power-law:

Early theory: exploded planet? Won't work:

Better idea: Leftovers from early solar system

Orbit classification:

Surface Features

For a long time we had very little idea what the surface of asteroids looked like -- too small, too far away.

On the way to Jupiter, the Galileo probe photographed the surface of a two asteroids - Gaspra and Ida:

Gaspra	Ida

Ida has a moon! (called Dactyl) (Ida: 58x23 km, Dactyl: 1.6x1.2 km)

Other asteroids do, too. 10-30% of asteroids may have sizable moons (even bigger than Dactyl).

Calculation of tidal evolution of asteroid satellites shows rapid evolution.

Moons orbiting past synchronous rotation will move outwards, eventually being lost from the parent asteroid.
Moons orbiting withing synchronous rotation will move inwards, and collide with the asteroid.
Timescale: a few hundred thousand years.
Question 1: Why so fast?
Question 2: Are moons of asteroids primordial?

Asteroid shapes

Many asteroids have dumbbell shapes. Look again at Ida rotating.

Look also at the tumbling asteroid Toutatis, in this radar imaging movie.

The shapes suggests that many asteroids are loosely bound clumps of a few large fragments.

How does this relate to what we discussed about asteroid moons?