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?