Australian Sky & Telescope - June 2018

26 AUSTRALIAN SKY & TELESCOPE July 2018

THE IMPACT HAZARD

ASTEROID POSITIONS:

CNEOS; FIREBALL MAP: ALAN CHAMBERLIN (CNEOS / JPL-CALTECH / NASA)

Equipped with a 0.5-m telescope,

NEOCam would scan the celestial sphere

in the infrared, specifically the mid-

infrared wavelengths around 10 microns.

While we humans can’t perceive such

wavelengths, asteroids — especially dark

ones — are naturally brightest in this part

of the spectrum, re-radiating most of the

sunlight they absorb in the mid-infrared,

Mainzer says. NEOCam would detect NEAs

at greater distances than ground-based

telescopes can and at sizes smaller than

140 m. It would also help astronomers get

a better handle on each NEA’s size, orbit,

spin rate and other factors. This is useful

not just for hazard management but also for more generally

understanding asteroids, which serve as time capsules of Solar

System history and, someday, could be spacefaring resources.

NASA is currently assessing NEOCam’s viability, but in

the highly competitive environment of federally funded space

missions, there’s no guarantee it will fly. (NEOCam would

cost the American taxpayer about $600 million to build and

launch, Johnson told me.) “We are waiting to see what the

future holds,” Mainzer says.

Getting even smaller

Earth’s close encounters in the last century or so are a

reminder that even modest-size objects can be dangerous.

Thought to have been roughly 20 m in diameter, the

meteoroid that exploded over Chelyabinsk, Russia, in 2013

injured more than 1,600 people and caused at least $30

million in damage. The so-called Tunguska event in 1908

involved an object more than twice as large, at around 50

m. Fortunately it exploded in the lower atmosphere over a

sparsely populated area of Siberia, but the multi-megaton

blast still flattened 2,000 square km of forest.

Both incidents are symptomatic of a small impact: While

a large asteroid would punch right through the atmosphere

and hit the ground intact, smaller rocks detonate high up.

When they do so, they unleash damaging shock waves that

can reach the surface. (Large asteroids would also generate

shock waves.) If a Tunguska-size event happened over a big

city, millions could die.

Fortunately, the chance of something like a Tunguska

fireball exploding over a major city is slim, Harris says. “They

only hit the Earth about once in a thousand years, and only

Mercury

Venus

Earth

Mars

WDON’T PANIC The positions of

known asteroids in the inner Solar

System are plotted for May 1, 2018.

The green dots are all numbered

asteroids that do not approach

Earth. The yellow ones represent

those that approach our planet but

don’t cross its orbit. The red dots

mark asteroids that cross Earth’s

orbit but don’t necessarily closely

approach our planet itself. Although

the plot makes our neighbourhood

look claustrophobically crowded,

remember that the space

represented by this diagram is

predominantly empty.

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Energy

log (kilotons)

SBOMB-GRADE FIREBALLS Between 1988 and 2018, US government sensors have picked up 735 bright ireballs, a subset for which we have

geographic coordinates pinned down (shown). The large red dot marks the 2013 Chelyabinsk event.

Fireballs over Earth (April 1988 to February 2018)