I

n July 1994, Comet Shoemaker-Levy 9 spectacularly

riddled Jupiter with immense chunks of its erstwhile

self — a never-before-seen pummeling that grabbed the

attention of astronomers and the public worldwide. By

that time, most scientists had bought into the notion,

first advanced in 1980, that an impact from another huge

space object had wiped out the dinosaurs 65 million years

ago. But the 1994 comet crash was a stark reminder that

massive collisions still happen in our Solar System today.

Invaders

How real is the threat that a giant asteroid

or comet could strike Earth and wipe us out?

After first asking NASA to assess the threat, in 1996

Congress tasked the space agency with finding, within a

decade, 90% of the estimated 1,000 near-Earth asteroids

at least 1 kilometre in diameter. NEAs are those asteroids

with a perihelion of less than 1.3 astronomical units... that

is, an orbit whose closest point to the Sun is under about

195 million km. And 1 km across is the minimum size that,

if the object struck us, could potentially trigger a global

catastrophe — unleashing a years-long winter, causing

22 AUSTRALIAN SKY & TELESCOPE July 2018