Sky and Telescope - June 2018 - 14

The Impact Hazard

Size Matters
They say no good deed goes unpunished, and in 2002, when
it became clear that observing teams were well on the way to
ﬁnding 90% of kilometer-size NEAs, NASA chartered a Science Deﬁnition Team (SDT) to look at what it would take to
bring that 1-km threshold down even lower. Should we also
ferret out those NEAs that, while not global threats, might
still cause signiﬁcant regional devastation and thus still pose
a substantial risk to Earth's population?
The team's report, released in 2003, recommended discovering 90% of all NEAs larger than 140 meters (460 feet)
in diameter - the size at which widespread regional damage could result. Last fall, a reconvened SDT recommitted
to that same size threshold (see the full report at https://
is.gd/2017NASA_SDT).
As it had in the 1990s, Congress went with the recommendation, setting the end of 2020 for the survey's completion.
The decree did not come, however, with a substantial boost
in funding, which the NASA community would have needed
to meet that time constraint. Astronomers already know they
won't reach the Congressional deadline.
For one thing, pinpointing the smaller objects is a sigImpact energy (megatons)

Chelyabinsk

Cumulative population

105

100

108

Fireball measurements

108

Discovered

Tunguska
106

Survey-based population

100

Combined asteroid
surveys estimate

10
10

106

Impact interval (years)

1010

0.10

Chicxulub

100

108
1

0.01

0.1

Estimated diameter (kilometers)
p WHAT'S MISSING After decades of searching, astronomers have
found nearly all of the largest objects with diameters of 1 km or greater
- and they're relatively rare. Supergiants like the Chicxulub impactor that struck 65 million years ago are rarer still. But a large gap (gray
region) remains between what we've detected and what we expect exists
for the smaller but still potentially hazardous bodies.

J U N E 2 018 * SK Y & TELESCOPE

niﬁcantly harder task. Not only is it easier to ﬁnd the bigger
ones, but as you go down in size the number of asteroids also
increases exponentially. (It's different with comet nuclei,
which, once they approach the Sun, don't seem to hold
together for long if they're smaller than about 1 km.) Experts
put the number of NEAs with a size of 140 m or greater at
more than 24,000.
"It's one thing to suspect that size population, and it's
another thing to actually go ﬁnd them," says Lindley Johnson, who heads NASA's Planetary Defense Coordination
Ofﬁce. "We've got a ways to go."
Currently, we've found about 8,100 objects with an H of
22 or brighter, which corresponds to scientists' best estimate
of the absolute magnitude of objects 140 m and larger. So
they're about one-third complete. New wide-ﬁeld surveys

soon to come online, including the Large Synoptic Survey
Telescope in Chile, will help locate many of the NEAs in this
expanded census. But all ground-based systems, both professional and amateur, have limitations. First, objects this small
can only be spotted during the week or so when they pass
closest to Earth. Yet a given telescope on Earth's surface can
only search the half of the sky roughly opposite from the Sun.
The time it can be online is bounded, and weather and Moon
interference can also affect survey performance.
A telescope ﬂown in space wouldn't have such constraints.
"It is clear that if we want to get this catalog of NEAs completed in anything under several decades, we need to go to
space-based capabilities," Johnson says.

Seeing in the Dark
One of the most promising proposals is NEOCam. (NEO
stands for near-Earth objects, a term that also encompasses
comets.) This space telescope would be "parked" at Lagrangian point L1, a point of equilibrium between the respective
gravitational tugs of the Sun and Earth that sits between the
two bodies. "At L1, we have a wide view of the volume of space
surrounding the Earth's orbit, which is where NEOs that are
the most likely to be hazardous spend much of their time,"
says Amy Mainzer (Jet Propulsion Laboratory). Mainzer is
principal investigator both of NEOWISE, a highly successful asteroid-hunting mission now winding down, and of the
proposed NEOCam.
Equipped with a 0.5-m telescope, NEOCam would scan
the celestial sphere in the infrared, speciﬁcally the mid-

GR APH: GREGG DINDERMAN / S&T, SOURCE: A L A N H A R RIS; HOBA ME TEORITE: RUL OR NEL AS / FLICK R / CC BY
2.0; ASTEROIDS: THECRIMSO NM O NK E Y / G E T T Y IM AG ES, SOURCE: A L A N CH A MBER LIN (CNEOS / JPL- CA LTECH /
N ASA); IMPACT M A P: GREGG DINDER M A N / S&T, SOURCE: E A RTH IMPACT DATA BASE

Where a monster space rock comes down is also critical. The Chicxulub impactor, which triggered the extinction
of the dinosaurs (and much else) after smacking into the
Caribbean off what is today the Yucatán Peninsula, struck
thick sedimentary deposits rich in hydrocarbons and sulfur.
Spewed into the atmosphere, the resulting soot and aerosols
likely had a much more devastating environmental effect
than if the intruder had hit, say, a mountain of granite in
northern Canada.

https://www.is.gd/2017NASA_SDT https://www.is.gd/2017NASA_SDT

Sky and Telescope - June 2018

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