Asteroids, Comets, and Impacts

1. Definitions of meteoroids, meteors, meteorites, asteroids, and comets

Meteoroids are small rocky objects (less than a few meters) traveling through space.
Meteors are meteoroids that burn up and light the sky as they pass through Earth’s atmosphere.
Meteorites are pieces that survive the atmosphere and reach Earth’s surface.
Asteroids are rocky bodies ranging from meters to hundreds of kilometers, while comets are icy, dusty bodies.

2. What regolith is

Regolith is a layer of loose dust and small particles on the surface of airless bodies.
It forms from many impacts over time.
Asteroids and the Moon have regolith because they lack erosion.

3. Where asteroids are found, whether the belt is crowded, and total mass

Most asteroids are found in the main asteroid belt between Mars and Jupiter.
The belt is not crowded; 1-km asteroids are spaced more than 500,000 km apart.
The total mass of the asteroid belt is only 4% of the Moon’s mass, with Ceres accounting for one-third.

4. Why comets are called dirty ice balls

Comets are mixtures of ice, rock, and dust.
They consist of a rocky core covered by ice and dust.
This composition gives rise to the term “dirty ice balls.”

5. What causes comet tails and the direction of the gas tail

As comets approach the Sun, solar radiation vaporizes ice, releasing gas and dust.
The gas tail is pushed directly away from the Sun by the solar wind.
The gas tail shows the Sun’s direction, not the comet’s motion.

6. Where meteor showers come from

Meteor showers occur when Earth passes through debris left behind by a broken-up comet.
The debris continues along the comet’s original orbit.
Many meteors entering at once produce a meteor shower.

7. Why Halley’s Comet is the most famous

Halley’s Comet is a short-period comet with an orbit of about 75 years.
It is visible to the naked eye and appears about once in a human lifetime.
This makes it the most well-known comet.

8. How we know the Oort Cloud is a sphere

Long-period comets come from all directions in space.
This distribution indicates a spherical source, not a flat disk.
That source is the Oort Cloud.

9. Relationship between asteroid size and crater size

Impact craters are typically 15–30 times larger than the impacting object.
This is because impacts occur at extremely high speeds and behave like explosions.

10. Why impact craters are always round

Asteroids strike Earth at very high speeds.
The energy is released equally in all directions, like an explosion.
This makes impact craters almost always circular.

11. Why large lunar basins are black and smooth

Large lunar basins were later filled with lava, forming dark plains called mare.
The lack of craters on these surfaces shows they are younger than surrounding terrain.

12. Why the Moon has more craters than Earth

The Moon has no erosion, water, or plate tectonics.
Old craters are not erased over time.
Earth’s surface constantly removes impact evidence.

13. Evidence that an impact killed the dinosaurs

A global clay layer contains iridium, shocked quartz, and soot.
A 180-km Chicxulub crater dated to 65 million years ago was found under the Yucatán Peninsula.
The event coincides with the K–Pg extinction, which killed 75% of species.

14. Strategies for changing the orbit of dangerous asteroids or comets

Strategies focus on deflection, not destruction.
Methods include kinetic impactors, sustained spacecraft thrust, focused solar heating, solar sails, and nuclear devices.
The goal is to slightly alter the object’s trajectory.