infrared and visible brightness tells us proportions of incoming sunlight that asteroid
reflects and absorbs.
−Asteroid’s (1) Brightness and (2) Distance tell us its size.
−Sometimes we can determine shapes of asteroids by monitoring brightness variations as
asteroid rotates: Nonspherical asteroid with uniformly bright surface reflects more light
when it presents its larger side toward the Sun and our telescopes.
−Asteroids made mostly of metal and rock because they condensed within the frost line
in the solar nebula (S.N.).
−Those near outskirts of asteroid belt (A.B.) contain larger portions of dark, carbon-rich
−A few asteroids appear to be made mostly of metals (ex. iron) suggesting they may be
fragments of metal cores of shattered worlds.
−Only direct way to measure distant object’s mass is to observe gravitational effect on
−For asteroids with moons (ex. Ida) we can determine mass of central asteroid by applying
Newton’s version of Kepler’s third law to orbital characteristics of moon.
−Density can offer insight into asteroid’s origin and makeup Ex. Density of Mathilde is
1.5g/cm3 Too low for it to be a solid chunk of rock (w/ a density of 2.4g/cm3.)
−All asteroids orbit sun in same direction as planets.
−Average distance between asteroids in asteroid belt is millions of kilometres.
−Orbital Resonance: Occurs whenever two objects periodically line up with each other.
Because gravity tugs at the objects in the same direction at each alignment, the effects
can build up over time. Objects will periodically line up – and hence have an orbital
resonance – whenever one object’s orbital period is simple ratio of another object’s
period, such as ½, ¼, 2/5.
−Trojan Asteroids: Two sets of asteroids which share Jupiter’s 12-year orbit around Sun.
One clump always stays 60 degrees ahead and other clump stays 60 degrees behind.
−When the S.S. was forming, the asteroid belt contained enough rocky material to form
another planet as large as Earth or Mars, but the orbital resonance with Jupiter disrupted