Sky and Telescope - June 2018 - 13
THE IMPACT HAZARD
The Missing 5%
Why does NASA feel conﬁdent that the total population
of 1 km and larger NEAs is likely less than 950? Harris
likens estimating their total numbers to keeping track of
ducks in wildlife management. "You tag 'em and turn 'em
loose, then you come back next year and you grab a bunch
of them and see what fraction of them has tags," he says.
"You know how many you tagged, so you just multiply back
up to what you now estimate to be the whole population."
Within the past year, three separate population estimates of NEAs, one of them by Harris, all wound up with
roughly similar numbers at all size ranges. "The three of
us all agree within a factor of two or so all the way up and
down the line," Harris says, including within a few percent at the 1-km-diameter size. "That gives me conﬁdence
that we're on the right track, because three different
groups with three different sets of data all come up with
about the same answer."
The "missing" 5%, the roughly 50 objects still lurking
in the shadows, are thought to lie in resonant orbits with
Earth. Imagine one on the far side of the Sun in an orbit
that takes just as long as we do to go around our star - a
1:1 resonance. We'd never see it. "It simply would never get
close enough to Earth to get into our detection window,"
says Paul Chodas (Center for Near Earth Object Studies,
Jet Propulsion Laboratory). "But that's a narrow niche, and
over time, if even just slightly outside that niche, it will
eventually come by our planet and we will see it."
We might know the size of the enemy's army, but uncertainties remain in understanding the combatants themselves. One is how to translate between the inherent
brightness of an NEA (which usually is a mere speck in
survey images) and the object's actual diameter and density. Planetary scientists start with the absolute magnitude
H, the brightness a fully illuminated body would have if
it were 1 astronomical unit from both the Sun and Earth.
Then they assume a particular surface reﬂectivity (typically 14%) in order to convert H to a physical size. But not
all asteroids are equally reﬂective.
Another ambiguity is the degree of impact damage by
girth. As noted, scientists use 1 km as the lower size limit
on what could provoke global climatic damage. "But we
don't really know that for sure - maybe it's 2 km," says
Harris. If that's the case, our chances are even better:
There's maybe a tenth as many 2-km NEAs as 1-km ones,
with a corresponding drop in the frequency that one might
collide with us. Moreover, almost all of the remaining
undiscovered NEAs lie in that 1-to-2-km size range. "That
means our risk of global catastrophe is uncertain mostly by
not knowing where that lower threshold is," Harris says.
E A R TH: N ASA ; ASTE ROIDS: THECR IM SO N M O N K E Y / G E T T Y IM AG ES
ing major crop failures, and conceivably ending civilization
as we know it. (By comparison, the "dino killer" at the end
of the Cretaceous was an estimated 10 km across.)
By 2011 asteroid-hunting surveys had met the legislative
mandate, and today the census is closer to 95% complete.
NASA's ofﬁcial catalog of NEAs 1 km or larger stands at 887
of an expected 934 (plus or minus 10 or so). Fortunately, not
a single one is on a collision course with our planet.
In fact, the risk of civilization-ending catastrophe "has
been largely put aside by discovery," says retired senior
research scientist Alan Harris (formerly Jet Propulsion
Laboratory), one of the leaders in this ﬁeld. It's one of the
great scientiﬁc achievements of our era.
by Peter Tyson
sk yandtelescope.com * J U N E 2 018