Revision summary
Asteroids are rocky, metal-rich small bodies; near-Earth objects are the ones that can meet our orbit. An extinction-level impact is geologically real (Chicxulub) and very unlikely on a human calendar given current catalogues. Smaller city-threatening objects are less fully known and are the practical hazard. Planetary defence: survey, characterise, then deflect, chiefly by kinetic impact. NASA’s DART mission in 2022 proved a spacecraft can change an asteroid moon’s orbit; civil defence is the short-warning backup.
Model answer
Copper italics in this answer — like this — are the key facts. Each one is unpacked in the Facts & figures rail.
Introduction
Asteroids are rocky leftovers of the early solar system, orbiting mainly between Mars and Jupiter, with some thrown onto paths that cross Earth’s. A large enough impact can end a world of species; that is not a film line, it is the K–Pg story. The daily threat to a city is much smaller. Strategy now is to find, track and, if needed, nudge — not to panic.
Body
What asteroids are
Asteroids are small Solar System bodies of rock and metal, larger than meteoroids and without a comet’s routine coma. The main belt sits between Mars and Jupiter. Near-Earth objects (NEOs) are those whose orbits come close to ours. A meteor is the streak; a meteorite is what lands. Composition ranges from rubble piles to nickel-iron. They matter because their kinetic energy at closing speeds of many kilometres a second is enormous even for a modest diameter.
- Peg: Main-belt asteroids are the population; NEOs are the ones planetary defence cares about.
- Peg: Chelyabinsk (2013) was a small airburst that still blew out windows; it was not an extinction object.
How real is extinction?
Earth has taken extinction-scale impacts. The Chicxulub event is the standard link to the end-Cretaceous mass dying. That class of object is very rare on human timescales. Surveys of NEOs have already catalogued a large share of the kilometre-class bodies that could cause a global catastrophe; none on the public lists is on an imminent extinction path. Smaller objects that could destroy a city are more numerous and less fully catalogued. So the honest answer is: extinction is real as geology, remote as this century’s calendar; regional disaster is the more practical risk. Complacency is wrong. Apocalypse as next year’s policy is also wrong.
- Peg: Extinction-class impacts are low-probability, high-consequence; city-killers are the gap in the catalogue.
- Peg: NASA and other agencies publish impact-probability tables; a non-zero Palermo or Torino scale number is a watch, not a prophecy.
Prevention strategies
- The first strategy is survey: optical and infrared telescopes (ground and space) to find NEOs and compute orbits decades ahead. The second is characterisation: radar and fly-bys to know if the body is a rubble pile. The third is deflection. NASA’s DART mission (2022) hit Dimorphos, moon of Didymos, and shortened its orbital period, proving kinetic impact. ESA’s Hera follows up on that system. Other paper strategies are a gravity tractor (a spacecraft that hovers and tugs) and, in extreme theoretical cases, nuclear standoff blast — politically and legally fraught. Civil defence (evacuate, shelter) is the last layer if warning is short, as with Chelyabinsk’s incoming from the sunward sky. International coordination sits in UNCOPUOS and planetary-defence groups. The workable doctrine is early find, small nudge, years before a predicted hit.
- Peg: DART is the first real deflection test, not a science-fiction sketch.
- Peg: Time is the weapon: a millimetre-per-second change years out is a miss at Earth.
Flow diagram
Conclusion
Asteroids are rocky Solar System bodies; some are near-Earth. Extinction impacts have happened and are unlikely this century; city-scale objects are the live risk. Strategy is catalogue, track and kinetic nudge, as DART showed. Early astronomy is the defence.
Quick related
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Next question on this syllabus topic (2024 · Q20). View answer →
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Should we nuclear-bomb asteroids as first choice?
No. Kinetic nudge with long warning is the tested, cleaner idea. Nuclear options are last-resort paper, with legal and debris problems.
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Is a dinosaur-killer due soon?
No known kilometre-class NEO is on an imminent extinction path. The scientific job is to keep the catalogue complete.
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