Sky and Telescope - June 2018 - 12
How grave is the threat
that a giant asteroid or
comet could strike Earth
and do us serious harm?
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 comet crash was a stark reminder that massive collisions still happen in our solar system today.
J U N E 2 018 * SK Y & TELESCOPE
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 km (0.6 mile) 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, caus-