Chunks of Space Debris

How much arrives on Earth? About 50,000 tons of material per year, most in very small pieces (every night you're likely to find one dust particle from space on every surface the size of the hood of your car).
From where? Most come from asteroids and comets, a few from the Moon or Mars (or from the Earth!), some are original pieces of leftover solar system material, and some are interstellar intruders (about 1% of meteoroids!).

  Shooting Stars

  • Meteors or "shooting stars": Small grains, at most pea-sized (not stars at all!) that crash into our atmosphere at 160,000 mph [compared to 2,230 mph for a rifle bullet]; Millions burn up every day several tens of miles up in our atmosphere.
  • Meteor showers: They occur at fixed dates, when the Earth goes through the trail of a comet; For example, the Perseids [mid-Aug], the Leonids [mid-Nov, from comet Tempel-Tuttle], or the Geminids [mid-Dec, from asteroid 3200 Phaethon, probably an "ex-comet"]; Halley's comet also produces a meteor shower, but a small one [the Eta Aquarids, early May].
  • Fireballs: Larger objects that burn as they fall in the atmosphere; Several seen every year, but many are not found on the ground.

Our Interplanetary Roulette Game

  • Danger to humans: Nobody in recorded history is known to have been killed by a meteorite; but a dog has, and they have caused damage to buildings. [Did 3000-2000 BC comet impacts contribute to the downfall of bronze age civilizations?]
  • Statistics: Every year, thousands of football sized meteorites and some larger ones* hit the ground, depositing hundreds of tons of material; Most go unnoticed.
  • Large ones: The 1908 Tunguska event in Siberia was probably caused by a 60-m wide low-density asteroid, the 50,000-yr old Barringer Meteor Crater in Arizona by a 40-m iron one; Impacts of that size can be very destructive but happen only every 100 years or so; At least 120 old craters of this type are known.
  • Old ones: The oldest known is the 40-km, 2 Gyr old Vredefort Dome in South Africa, the largest a 300-km, 250-Myr old one below the Antarctic ice. But the Moon and Mars have larger ones...

  The Most Destructive Impacts

  • Rates, effects: Asteroids/comets bigger than 1 mile come once every 100,000 years or more, and are catastrophic (it appears that about 25% are comets and 75% asteroids, but it's hard to tell which one caused a specific impact); They probably do not cause tsunamis as in some movies ("Armageddon" and "Deep Impact"), but they can displace and partially liquefy a volume larger than that of Mt Everest.
  • Cycles: There seems to be a 28-32 Myr cycle in large comet impacts (from extinctions), and speculation that it may be related to our 30-35 Myr period of up-down motion across the plane of the Milky Way.
  • Past impacts: The biggest ones have caused mass extinctions, like the 5-6 mile Yucatán impact (K/T boundary Chicxulub site) which probably led or contributed to the extinction of dinosaurs 65 Myr ago; Debris was more abundant in the past, the inner solar system was bombarded by asteroids, and Earth and other planets resurfaced, 3.9 Gyr ago.

On the Ground: Meteorites

  • Where are they? Everywhere, but they are preserved better, and easier to spot and collect in arid places, like icy Antarctica and Greenland or deserts like in Africa or AZ/NM.
  • Origin: Varies; Most come from asteroids, others come from the Moon (30 known) or Mars (20-30 known; we get on average one every month, but most are never found).
  • How do you recognize one? From the composition (Fe and Ni content, they tend to be magnetic), surface appearance and crystalline structure.
  • Interest: We occasionally get billion-year old ones on Earth that we can analyze for clues on water, the early solar system, and their possible role in the origin of life on Earth.
  • Value: Some meteorites are sold to collectors; Lunar or martian ones are more expensive than gold [they can go for $3,000/g!], and belong to the owner of the spot where they land.

Interesting Questions

  • Meteorites from Earth: Do other bodies, for example the Moon, have meteorites that came from here? What if they do?

* The largest meteorite ever found on Earth weighs more than 66 tons, and was found 20 km west Grootfontein, Namibia, on a farm named Hoba-West. It crashed to earth tens of thousands of years ago.

page by luca bombelli <bombelli at>, modified 29 sep 2012