what are asteroids? How real is the threat of them causing extinction of life? What strategies have been developed to prevent such a catastrophe?

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.