Sky and Telescope - June 2018 - 16
The Impact Hazard
t DON'T PANIC The positions of known
infrared wavelengths around 10
asteroids in the inner solar system are plotmicrons. While we humans can't
ted for May 1, 2018. The green dots are all
perceive such wavelengths, asternumbered asteroids that do not approach
oids - especially dark ones - are
Earth. The yellow ones represent those
naturally brightest in this part of
that approach our planet but don't cross
its orbit. The red dots mark asteroids that
the spectrum, reradiating most of
cross Earth's orbit but don't necessarily
the sunlight they absorb in the midclosely approach our planet itself. Although
infrared, Mainzer says. NEOCam
the plot makes our neighborhood look
would detect NEAs at greater disclaustrophobically crowded, remember that
tances than ground-based telescopes
the space represented by this diagram is
can and at sizes smaller than 140 m.
It would also help astronomers get
a better handle on each NEA's size,
more than 1,600 people and caused
orbit, spin rate, and other factors.
at least $30 million in damage. The
This is useful not just for hazard
so-called Tunguska event in 1908
management but also for more
involved an object more than twice
generally understanding asteroids,
as large, at around 50 m. Fortunately
which serve as time capsules of solar system history and,
it exploded in the lower atmosphere over a sparsely populated
someday, could be spacefaring resources.
area of Siberia, but the multi-megaton blast still ﬂattened
NASA is currently assessing NEOCam's viability, but in
2,000 square km of forest.
the highly competitive environment of federally funded space
Both incidents are symptomatic of a small impact: While a
missions, there's no guarantee it will ﬂy. (NEOCam would
large asteroid would punch right through the atmosphere and
cost the American taxpayer about $600 million to build and
hit the ground intact, smaller rocks detonate high up. When
launch, Johnson told me.) "We are waiting to see what the
they do so, they unleash damaging shock waves that can
future holds," Mainzer says.
reach the surface. (Large asteroids would also generate shock
waves.) If a Tunguska-size event happened over a big city, millions could die.
Getting Even Smaller
Fortunately, the chance of something like a Tunguska
Earth's close encounters in the last century or so are a
ﬁreball exploding over a major city is slim, Harris says. "They
reminder that even modest-size objects can be dangerous.
only hit the Earth about once in a thousand years, and only
Thought to have been roughly 20 m in diameter, the meteorone in 10 will hit a populated area, or maybe one in 20 or
oid that exploded over Chelyabinsk, Russia, in 2013 injured
p BOMB-GRADE FIREBALLS Between 1988 and 2018, U.S. sensors have picked up 735 bright fireballs, a subset for which we have geographic
coordinates pinned down (shown). The large red dot marks the Chelyabinsk event.
J U N E 2 018 * SK Y & TELESCOPE
ASTEROID POSITIONS: CNEOS; FIREBA LL M A P: A L A N CH A MBERLIN (CNEOS / JPL- CA LTECH / N ASA)
Fireballs Reported by U.S. Government Sensors (April 1988 to February 2018)