Australian Sky & Telescope - June 2018

http://www.skyandtelescope.com.au 25

X
BATTLE SCARS
Researchers have
found 190 confirmed
impact structures on
Earth, from little Carancas
(13.5 m from rim to rim) in
Peru to 160-km-wide Vredefort
in South Africa. Explore the
catalogue for yourself at
passc.net/EarthImpactDatabase.

“The foundational principle of
planetary defense is FIND THEM EARLY,”
says Lindley Johnson, NASA’s
Planetary Defense Officer. Depending
on the scenario and warning time,
we have at least three methods we
could consider to redirect or destroy
a hazardous object heading our way.
With significant lead time, we
might use a gravity tractor. This
is a spacecraft that would use the
gravitational force created by its own
mass to nudge an object off its Earth-
targeting path. This method has not
been tested, however, and we would
need decades to build, launch and

undertake this type of mitigation.

A kinetic impactor would smash

into the interloper at a high speed,

transferring its momentum to the

object. Ideally, this would change the

object’s velocity, causing its course

to deviate enough for it to miss us.

This one would require enough time

for the craft to be built, launched and

travel to its target — a few years at

minimum.

If the threat of impact was

imminent or the rock was too large

for these two methods to be viable, a

nuclear explosive device might be the

only option. We might have to deal

Short-period comets originate in the
Kuiper Belt out beyond Neptune’s orbit
and usually take 200 years or less to
orbit the Sun. Long-period comets lie
in the Oort Cloud, an extremely distant
region hosting billions of comets. A
single trip around our star could take a
long-period comet 30 million years.

Estimated diameter (metres)

(NOT TO SCALE)E)

es)

0–30 30–100 100–300 300–1,000 1,000+

with the fragments, but they’d have

more localised effects.

SHOBA METEORITE Due to its enormous

mass (about 60 tonnes), the largest

meteorite found on Earth, the Hoba iron

meteorite in Namibia, still sits where a

farmer discovered it in 1920. Scientists think

it fell to Earth about 80,000 years ago.