24 AUSTRALIAN SKY & TELESCOPE July 2018
GRAPH: GREGG DINDERMAN /S&T, SOURCE: ALAN HARRIS; HOBA METEORITE: RUL ORNELAS / FLICKR / CC BY 2.0; ASTEROIDS: THECRIMSONMONKEY /GETTY IMAGES, SOURCE: ALAN CHAMBERLIN (CNEOS / JPL-CALTECH /NASA); IMPACT MAP: GREGG DINDERMAN /S&T, SOURCE: EARTH IMPACT DATABASEWhere a monster space rock comes down also is 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.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 kilometre-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 metres in
diameter — the size at which widespread regional damage
could result. Last year, 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
significantly 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 estimate1110010410610810010410610810100.10 100 105 108Cumulative population Impact interval (years)Impact energy(megatons)0.01 0.1 1 10
Estimated diameter (kilometers)ChicxulubFireballmeasurements
Discovered
Survey-based population
Combined asteroid
surveys estimateTunguskaChelyabinskSWHAT’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 (grey
region) remains between what we’ve detected and what we expect
exists for the smaller but still potentially hazardous bodies.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 catalogue 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 L 1 , a point of equilibrium between the respective
gravitational tugs of the Sun and Earth that sits between the
two bodies. “At L 1 , 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.THE IMPACT HAZARD