What falls from the sky
Every day Earth sweeps up tonnes of material from space, almost all of it as dust. This lesson sets out the vocabulary and the scale of what arrives.
- Distinguish meteoroid, meteor, fireball and meteorite.
- Describe the range of sizes of material that reaches Earth.
- Explain why most meteorites are never found.
Links to: Foundations: vocabulary and scale.
The vocabulary
A meteoroid is a small natural body in space, from dust size up to about a metre; larger bodies are asteroids. When a meteoroid enters the atmosphere it produces a meteor, a streak of light. An exceptionally bright meteor, brighter than the planet Venus, is a fireball, and one that explodes is often called a bolide. Any fragment that survives to the ground is a meteorite.
The smallest particles, below about 2 mm, are micrometeorites. They arrive in enormous numbers and are found everywhere, including on city roofs.
How much arrives
Estimates of the total mass reaching Earth each year range from several thousand to tens of thousands of tonnes, almost all of it dust and micrometeorites. Rocks big enough to produce meteorites weighing more than a few grams land far less often, but still many thousands of times a year worldwide.
Most land in the ocean, which covers about 71% of the surface, or in forests, mountains and farmland where they are hard to distinguish from Earth rocks. Only a handful of falls each year are seen and recovered. The rest wait to be found, or weather away.
Falls and finds
A fall is a meteorite seen to arrive, or recorded by cameras, and recovered soon after. A find is one discovered later with no record of its arrival. Falls are scientifically precious because they are fresh and their orbits can sometimes be calculated. Finds make up the great majority of the tens of thousands of known meteorites, mostly from Antarctica and hot deserts.
Where do they land?
If 10,000 meteorites heavier than a few grams land in a year, spread at random over the globe, roughly how many land on the United Kingdom, which covers about 0.05% of Earth's surface?
- 0.05% = 0.05 ÷ 100 = 0.0005.
- 0.0005 × 10,000 = 5.
Answer: Roughly five a year, illustrating why recovered UK falls are rare: most land unseen in fields, woods and the sea. The 10,000 figure is an illustration; real estimates are uncertain.
Read about one recent fireball on this site's fireball pages. Note the time, where it was seen and how long it lasted. Then think about what you would do if you saw one: where you were, which direction you faced, and how long it lasted are the most useful details for scientists.
Quiz
5 questions. Choose an answer to see whether you are right and why. Finish the quiz to complete the lesson.
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Question 1What is a fireball?
A fireball is a meteor brighter than about the planet Venus.
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Question 2What is a micrometeorite?
Micrometeorites are tiny particles that survive entry and are found all over Earth.
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Question 3What makes up almost all the material from space reaching Earth?
By mass, the incoming material is dominated by small particles.
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Question 4Why are most meteorites never found?
Oceans cover about 71% of Earth, and on land meteorites are easily mistaken for ordinary rocks.
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Question 5What is the difference between a fall and a find?
Falls are recovered soon after an observed arrival; finds have no record of when they landed.