Sky and Telescope - June 2018 - 17
Near-Earth Asteroid Discoveries by Survey (as of March 1, 2018)
~1km and larger NEAs
~140m and larger NEAs
p TALLY UP Observers have found roughly 95% of near-Earth asteroids that are 1 km or larger, with most of the discoveries made in the early 2000s
(left). Now their focus is on finding objects 140 m wide and larger (right). Note the different y-axis scales.
30." Our homegrown severe earthquakes, hurricanes, ﬂoods,
and tsunamis are all more likely hazards, he says. "It comes
down to just a very minor threat. How much of society's
resources do you want to pile into that?"
G REGG DINDER M A N / S&T, SOURCE: A L A N CH A MBERLIN (CNEOS / JPL- CA LTECH / N ASA)
The Comet Factor
Researchers' focus to date has been on the risk from asteroids
(and to a lesser extent from short-period comets, those that
come from the Kuiper Belt). This is wise: Asteroids that might
pose an impact threat far outnumber comets.
But long-period comets, those that originate in the Oort
Cloud, are typically huge - a km or more across - and arrive
at much greater speeds relative to Earth than asteroids do.
They also appear in our environs with little advance notice.
"They're simply undetectable until they approach within
the orbit of Jupiter or Saturn, so it's really not possible to see
them with decades of warning," says Chodas.
It doesn't leave time, as we might have with known
asteroids whose orbits we can calculate decades ahead, of
using deﬂective or destructive methods to remove or lessen
the impact (see sidebar page 15). "You will only know if an
impact is going to happen a matter of months in advance,
and the best you can do is evacuate and things like that,"
Harris says. "You know, Bruce Willis just can't save us."
The good news, again, is such icy rogues are very few and
far between. Long-period comets pass close to Earth only 1%
as often as NEAs do. "There are a lot fewer of them coming
into the inner solar system, and, frankly, space is a big place,"
says Johnson. "I'm not saying that's not a hazard we have
to deal with, but let's take care of the asteroids ﬁrst, then
hopefully future technologies will provide us better capability
against the less-probable threat."
It's not inconceivable, for example, that an out-of-the-blue
explosion in the sky like that over Chelyabinsk could spur
acts of aggression or even war by governments that mistake
them for attacks. For that reason, Johnson's ofﬁce works
closely with the U.S. Department of Defense to get details
about atmospheric impacts out quickly.
The NASA community also strives to keep the public
informed of the actual nature of the threat. This can backﬁre,
particularly when talking about "run-of-the-mill" asteroids
of the 1- to 10-m variety that regularly pass between Earth
and the Moon. "Most of the close approaches we report on
our website are astronomically close, but in human terms still
very far away," Chodas says (see https://cneos.jpl.nasa.gov).
"Yet the images that [news editors] post on their web stories
often depict giant asteroids passing extremely close."
These mini ones, including the thousands of tiny objects
that burn up harmlessly in the atmosphere every day, are just
not the focus of the big NASA search efforts. "It's not that
we don't need to pay attention to these impacts," says Linda
Billings (National Institute of Aerospace), who is a consultant
to Johnson's Planetary Defense Coordination Ofﬁce. "We are
paying attention. But these events, and NEO close approaches
to Earth, are happening all the time." We just didn't know
about them before we had robust systems in place to pick
them up, as we do now.
Johnson concurs. "I do worry a little bit that we will have
cried wolf one too many times, so to speak, and we will lose
the ear of the public when one comes around that we really
need to tell them about." It's true that we don't have to worry
about these smaller objects, he says. "But we do need to keep
an eye out and ﬁnd what's out there, because one of these
days there will be a bigger one that's going to impact us, and
we just don't know when that is."
Telling It Like It Is
Beyond ﬁnding, cataloging, and even visiting comets and
asteroids (see page 22), there's one more important piece:
improving general awareness.
¢ Among space rocks that strike Earth, S&T Editor in Chief
PETER TYSON favors those he can hold in his hand, such as
the Campo del Cielo meteorite he recently acquired.
sk yandtelescope.com * J U N E 2 018