asteroid was porous, causing it to burst
when it entered the atmosphere, which
functions as a brake. Known as an air burst,
this type of impact produces a shock wave,
which can be more destructive than when
an asteroid strikes Earth’s surface.
In January 2017, Earth only just avoided a
collision with the 2017 AG13 asteroid, which
passed by us at a speed of about 58,000
km/h at a distance of less than half the
distance between Earth and the Moon. The
asteroid, which is estimated to have been up
to 35 m long, was spotted by telescopes only
two days before passing close by our world.
The threat came from the Asteroid Belt
between Mars and Jupiter. The about
180-million-km-wide belt – i.e. slightly more
than the distance from the Sun to Earth –
consists of hundreds of thousands of known
rocks of all sizes. The actual number is
estimated to be much higher, however. Just
like planets, the rocks are orbiting the Sun,
but the gravitational pull of planets’ elliptical
orbits around the Sun sometimes push some
of the rocks into paths which bring them close
to Earth – known as Near-Earth Asteroids.
The comets, which mainly consist of ice,
come from a place even further away. Some
of them derive from the Kuiper Belt, which
also includes the dwarf planet of Pluto,
whereas others come from the Oort Cloud- a huge cloud of ice lumps, which is
believed to surround all planets. Seen from
our own perspective, comets and asteroids
become potentially hazardous objects
(PHOs), when they are about 7.5 million km
from Earth’s orbit.
SMALL ASTEROIDS ARE VERY HARMFUL
In 2005, the US authorities ordered NASA to
identify 90 % of all Near-Earth Asteroids
measuring 140+ m before 2020. Asteroids of
this size are easier to detect and keep an eye
on than the smaller ones and have such an
impact that they could destroy entire cities and
regions. But in 2017, the size of the objects will
probably be adjusted – not least because the
only 20-m-long asteroid, which struck
Chelyabinsk, demonstrated that much smaller
objects can also cause severe damage.
So far, astronomers have only identified
28 % of the asteroids and comets of 140+ m,
which probably exist in the Solar System, but
NASA aims to launch a new infrared space(^2)
COMBAT THE
ASTEROID
NASA has several
weapons in the
pipeline, which can
prevent a
threatening asteroid from
striking Earth. The ideas for anti-
asteroid weapons vary from
altering the path of the space
rock by means of a probe’s weak
gravity and combat it with lasers
to destroying it by means of a
nuclear warhead. The choice of
weapon depends on how much
time we have to react to the
threat from space.
LASER WEAPON
ALTERS DIRECTION
HEATING Small space probes can
bend an asteroid's path by means of
solar-powered laser beams. The lasers
are focused at a point on the asteroid,
causing a cloud of hot rocks and metals
to burn loose from the surface. This
becomes a stream of propellant, shifting
the asteroid in its course. This method,
which requires the construction of
huge lasers, was invented by the
University of Strathclyde and the
University of Glasgow.
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As it strikes the asteroid, the
nuclear warhead DETONATES,
destroying it. To avoid that
fragments hit Earth, the bomb can be
exploded at a distance from the
asteroid, so it will only change direction.
A computer simulation shows that the
detonation will leave fragments measuring
up to about 50 m.
telescope, the Near-Earth Object Camera
(NEOCam), which is going to find more of the
hidden killers orbiting the Sun from 2021.
It may seem as if Earth is under constant
bombardment, but luckily, major impacts
causing severe damage are relatively rare.
The largest recorded impact in recent history
was in 1908, when an asteroid of an
estimated length of 40 m exploded in the
atmosphere above the Tunguska River in
Siberia, destroying a forest the size of Tokyo.
This type of impact statistically only
occurs once every millennium, but Earth’s
surface is ripe with scars caused by much
more extensive prehistoric impacts. 190
craters have been recorded in the Earth
Impact Database. The most destructive
impacts, such as the one which may have
caused the dinosaurs to go extinct, take place
at intervals of averagely 100 million years.
COLLISIONS WITHOUT WARNING
Asteroids and comets strike with very
different warnings. Some collisions can in
principle be predicted centuries before they
occur, whereas others happen at a few days’
or no notice, and consequently, an important
element of NASA’s strategy is to develop and
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BECOME BETTER AT
PREDICTING IMPACTS
Data collected by
asteroid missions is
to make NASA better
at predicting their
paths. By feeding computers more
detailed information about an
asteroid’s speed, size, and shape,
any impact site can be calculated
more accurately. When
determining its path, it is also
important to know as much as
possible about the rock’s make-up.
ASTEROID PATH
ESA & SHUTTERSTOCK
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