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 simply a curious (and flawed) coincidence.
So is there a 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, unable to coalesce into a planet because of Jupiter's gravity.

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).

Signs of impacts abound: