Sky and Telescope - June 2018 - 15

"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 on its way to a direct hit.
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 under-

take 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

with the fragments, but they'd have
more localized effects.
p HOBA METEORITE Due to its enormous
weight (about 60 tons), 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..

Estimated Diameter (m)
(NOT TO SCALE)
E)

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.

0-30

30-100

100-300

300-1,000

1,000+

uBATTLE SCARS
Researchers have found
190 conﬁrmed impact
structures on Earth, from
little Carancas (13.5 m from
rim to rim) in Peru to 160-kmwide Vredefort in South Africa.
Explore the catalog for yourself at
passc.net/EarthImpactDatabase.
sk yandtelescope.com * J U N E 2 018

15

http://www.passc.net/EarthImpactDatabase http://www.skyandtelescope.com

Sky and Telescope - June 2018

Table of Contents for the Digital Edition of Sky and Telescope - June 2018

Contents
Sky and Telescope - June 2018 - Cover1
Sky and Telescope - June 2018 - Cover2
Sky and Telescope - June 2018 - 1
Sky and Telescope - June 2018 - Contents
Sky and Telescope - June 2018 - 3
Sky and Telescope - June 2018 - 4
Sky and Telescope - June 2018 - 5
Sky and Telescope - June 2018 - 6
Sky and Telescope - June 2018 - 7
Sky and Telescope - June 2018 - 8
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Sky and Telescope - June 2018 - Cover3
Sky and Telescope - June 2018 - Cover4
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