major crop failures, and conceivably ending civilisation 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 catalogue 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 civilisation-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.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 lurkingin the shadows, are thought to lie in resonant orbits with
Earth. Imagine oneonthefarsideoftheSuninanorbit
that takes just as long as we do to go around our star — a
1:1 resonance. We’dneverseeit.“Itsimplywouldneverget
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.”Outstanding questions
We might know thesizeoftheenemy’sarmy,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)andtheobject’sactualdiameterand
density. Planetary scientists start with the absolute
magnitude H, thebrightnessafullyilluminatedbody
would have if it were1astronomicalunitfromboththe
Sun and Earth. Then they assume a particular surface
reflectivity (typically 14%) in order to convertHto a
physical size. But not all asteroids are equally reflective.
Another ambiguityisthedegreeofimpactdamageby
girth. As noted, scientists use 1 km as the lower size limit
on what could provokeglobalclimaticdamage.“Butwe
don’t really know thatforsure—maybeit’s2km,”says
Harris. If that’s the case, our chances are even better:
There’s maybe a tenthasmany2-kmNEAsas1-kmones,
with a correspondingdropinthefrequencythatonemight
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 wherethatlowerthresholdis,”Harrissays.EARTH: NASA; ASTEROIDS: THECRIMSONMONKEY / GETTY IMAGES
http://www.skyandtelescope.com.au 23by Peter Tyson THE IMPACT HAZARD