Loading module…

What you will learn in Comets, Asteroids and Small Bodies

1. The asteroid belt
The main belt runs from 2.1 to 3.3 au and is a thick ring, not a thin one. Ceres alone holds about 39% of its mass, and the space between its members is so empty that New Horizons crossed it in 95 days without a glance.
2. The Kirkwood gaps
Count asteroids by the size of their orbits and sharp gaps appear exactly where an orbit would fit a whole number of times into Jupiter's 11.86 years. The same resonances that empty some orbits fill others.
3. Anatomy of a comet
A comet's tails point away from the Sun, not back along its path. Sixty days before perihelion Halley's ion tail lies 141 degrees from its direction of travel; sixty days after, 46 degrees — the tail goes first.
4. Where comets come from
Halley returns every 75 years from beyond Neptune; Hale-Bopp every 2,500 years from 371 au. Short-period comets come from beyond Neptune and keep close to the planets' plane; long-period ones fall in from every direction, out of a cloud no one has ever seen.
5. Meteor showers
A shooting star is a grain of comet dust, usually smaller than a sand grain, burning up 80 to 120 km overhead. A shower's meteors seem to come from one point because their paths are parallel, and parallel lines meet by perspective.
6. Impacts
An impact's energy is half its mass times its speed squared, and the air decides what happens next. A 19 m stone over Chelyabinsk carried about 450 kilotons and burst in the air; a 40 m iron dug a 1.2 km crater; a 10 km rock carried 75 million megatons.

Loading module…

1. The asteroid belt
The main belt runs from 2.1 to 3.3 au and is a thick ring, not a thin one. Ceres alone holds about 39% of its mass, and the space between its members is so empty that New Horizons crossed it in 95 days without a glance.
2. The Kirkwood gaps
Count asteroids by the size of their orbits and sharp gaps appear exactly where an orbit would fit a whole number of times into Jupiter's 11.86 years. The same resonances that empty some orbits fill others.
3. Anatomy of a comet
A comet's tails point away from the Sun, not back along its path. Sixty days before perihelion Halley's ion tail lies 141 degrees from its direction of travel; sixty days after, 46 degrees — the tail goes first.
4. Where comets come from
Halley returns every 75 years from beyond Neptune; Hale-Bopp every 2,500 years from 371 au. Short-period comets come from beyond Neptune and keep close to the planets' plane; long-period ones fall in from every direction, out of a cloud no one has ever seen.
5. Meteor showers
A shooting star is a grain of comet dust, usually smaller than a sand grain, burning up 80 to 120 km overhead. A shower's meteors seem to come from one point because their paths are parallel, and parallel lines meet by perspective.
6. Impacts
An impact's energy is half its mass times its speed squared, and the air decides what happens next. A 19 m stone over Chelyabinsk carried about 450 kilotons and burst in the air; a 40 m iron dug a 1.2 km crater; a 10 km rock carried 75 million megatons.