Class 6 the nature of modern impactors


Class 6: Formation of the Moon and the nature of modern impactors

Finish off talking about the Moon

Recap possible formation theories.

Digression into scientific method.

The success of the giant impact theory.

The nature of modern impactors

Comets and asteroids.

First, let’s finish the material from last class on formation of the Moon…

III : Formation of the Moon

Possible theories…

Simultaneous formation theory

Predicts that composition should be same.


Fission theory

Not possible dynamically…

Capture theory

Capture requires simultaneous interaction of 3 objects (Earth, Moon and something else)… too unlikely.

The giant impact theory



The giant impact theory, cont’d

Mars sized object hits early Earth.

Ejects mantle material into space… forms a disk/ring orbiting around Earth

Material in ring then forms Moon.

Probably takes only a year or so!

Moon initially formed very hot, and gravity is not large.

Thus volatile elements and compounds are lost into space.

Explains age, composition, lack of iron; and is very plausible given conditions in early solar system.

The nature of “modern” impactors…

I : Asteroids


(20 km)

Eros (33 km)

Asteroid properties

We have already mentioned Earthcrossing asteroids (ECAs)…

Over 1000 known.

Over 300 are “potentially dangerous”… more than 150 m diameter and on orbits that bring them “close”.

Largest is 15 km.

Typically, they would have…

Density of about 2000-3000 kg/m 3 (rock).

Speed of about 20-30 km/s near Earth.

Asteroids, cont’d

Using these numbers…

150 m diameter asteroid hits with energy of

600 1-Mton H-bombs (or up to 22,000,000

Mtons for a 5 km sized asteroid).

Later we will discuss the likelihood of such events, and what we might be able to do to prevent them.

Do you think it makes a difference if asteroids are solid or made or rubble

(with the same bulk density)?

II : Comets


Halley (16 km)

Comet properties

A “typical” comet has…

Diameter = few km.

Speed = 40-50 km/s near Earth.

Density = 500 kg/m 3 (dirty snow).

Let’s take a very small comet (150 m)…

Energy of collision is 300 Mton. More typical is

5 km comet… 10,000,000 Mton!

So, both asteroids and comets can be very dangerous!

Questions to think about…

How much energy would it take to…

Deflect an object from its path?

 What factors would it depend on?

Blow it up?

 What’s keeping it together?