Sky & Telescope - September 2022 - 16

Planetary Defense
The Hazard by the Numbers
Asteroid Size
Impact
Frequency
Effects
Est. Number
of Objects
% Discovered
4 meters
25 meters
~1 per year
Bright flash, no
ground damage,
possible
meteorites
~500 million
<0.1%
~1 per 100 years
Airburst explosion,
could cause widespread
injuries if over
populated area
~5 million
0.4%
140 meters
~1 per 20,000
years
Crater 1 to 2 km wide,
deadly over metro or
state-size areas,
mass casualties
~25,000
39%
1 kilometer
~1 per 500,000
years
10-km-wide crater,
global devastation,
possible civilization
collapse
~900
>95%
10 kilometers
~1 per 100-200
million years
100-km-wide crater,
global devastation,
mass extinctions
4
100%
NEAR-EARTH OBJECTS Scientists have found nearly all of the nearby asteroids that could cause global devastation, but they have a far more
incomplete tally of smaller, regionally dangerous objects. A given object's effect depends on many factors, including composition, speed, and incoming
angle, not just its size.
orbits of potentially hazardous objects.
Up to now, results from CNEOS have been comforting.
There's virtually no chance a known hazardous object will
impact Earth in the next 100 years. Even 101955 Bennu, one
of the most likely asteroids to hit Earth, only has a 1-in-1,800
chance of a future collision over the next three centuries.
But this doesn't mean we should sit back and relax. There
are still unknown objects flying through the solar system
that could suddenly pose a threat to Earth at any given
moment. At the time of writing, observers had discovered
28,539 near-Earth asteroids. Of these, 10,030 appear to be
at least 140 meters (460 ft) wide, and 881 are thought to be
more than 1 kilometer wide. Objects of these sizes would
have impacts ranging from significant regional damage to
worldwide extinction if they hit Earth. (For a gauge of what
" significant regional damage " means, think the devastation
of an entire U.S. state.)
According to NASA, only about 40% of asteroids larger
than 140 meters have been found. And to add to the uncertainty,
only about 2,000 of these objects have been properly
characterized by the only instrument suitable for the task:
NEOWISE, an old, partially broken down, and repurposed
space-based infrared telescope.
Amy Mainzer (University of Arizona) is principal investigator
on both NEOWISE and its purpose-built successor,
NEO Surveyor, scheduled to launch in 2026. With Surveyor,
she says, our knowledge of near-Earth objects (NEOs) will
expand dramatically; over its lifespan, the space telescope
could add a few hundred thousand objects to our roster.
Of these, there is no telling if any will be racing towards
16 SEPTEMBER 2022 * SKY & TELESCOPE
Earth in the near future. But if one does, Surveyor's infrared
data should enable scientists to accurately estimate its size
and probable shape and, when combined with visible-light
observations, the object's albedo and hints of its composition.
Having that knowledge well in advance of any potential asteroid
rendezvous with Earth is essential to planetary defense.
" We really want to try to give people time, because that's the
best weapon in this business, " says Mainzer.
But time is not the only weapon humanity would need.
There's little point in finding and characterizing threats if
we have no way of removing them. This is why DART is on a
collision course with Dimorphos now - so that scientists can
test mitigation measures, just in case they have to use them.
DARTing into Didymos
Since its launch on November 24, 2021, DART has raced
through space to reach the Didymos system. Didymos and
Dimorphos are approximately 780 meters and 160 meters
wide, respectively. They take 770 days to orbit the Sun at a
distance that varies from 1.0 to 2.3 astronomical units (a.u.),
and they orbit each other at a distance of about 1 kilometer.
They will make their " close " approach to Earth on October 4,
2022, at 10.7 million km - nearly 30 times farther away than
the Moon.
Sometime between September 26th and October 1st, just
before the asteroid's perigee, the 550-kilogram spacecraft
will zoom towards Dimorphos at 6.7 kilometers per second
(15,000 mph). As the moonlet comes around from the backside
of Didymos on its 11.92-hour orbit, DART will collide
with it head-on.
TERRI DUBÉ / S&T, SOURCE: NASA

Sky & Telescope - September 2022

Table of Contents for the Digital Edition of Sky & Telescope - September 2022