http://www.skyandtelescope.com.au 27GREGG DINDERMAN /
S&T,
SOURCE: ALAN CHAMBERLIN (CNEOS / JPL-CALTECH / NASA)
onein10willhitapopulatedarea,ormaybeonein20or30.”
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?”Don’t forget the comets
Researchers’ focus to date has been on the risk from asteroids
(andtoalesserextentfromshort-periodcomets,thosethat
comefromtheKuiperBelt).Thisiswise:Asteroidsthatmight
pose an impact threat far outnumber comets.
But long-period comets, those that originate in the Oort
Cloud, are typically huge — a kilometre or more across — and
arriveatmuchgreaterspeedsrelativetoEarththanasteroids
do. They also appear in our environs with little advance
notice.“They’resimplyundetectableuntiltheyapproach
within the orbit of Jupiter or Saturn, so it’s really not possible
toseethemwithdecadesofwarning,”saysChodas.
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. “You will only know if an impact is going to
happenamatterofmonthsinadvance,andthebestyoucan
doisevacuateandthingslikethat,”Harrissays.“Youknow,
Bruce Willis just can’t save us.”
Thegoodnews,again,issuchicyroguesareveryfewand
far between. Long-period comets pass close to Earth only 1%
asoftenasNEAsdo.“Therearealotfewerofthemcoming
intotheinnerSolarSystem,and,frankly,spaceisabigplace,”
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.”Don’t panic
Beyond finding, cataloguing, and even visiting comets and
asteroids, there’s one more important piece: improving
general awareness.Discoverydate~140m and larger NEAsNumber discovered199560040020001003005002000 2005 2010 2015
DiscoverydateNumber discovered1995 2000 2005 2010 2015100757575~1km and larger NEAsNear-Earth Asteroid Discoveries bySurvey(as of March 1, 2018)
STALLY 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 inding objects 140 metres wide and larger (right). Note the different y-axis scales.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 US 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.” Among space rocks that strike Earth, PETER TYSON favours
those he can hold in his hand, such as the Campo del Cielo
meteorite he recently acquired.