Near Earth Objects – Dangers from space
Bernhard Hofer
From times immemorial, Earth has been in the line
of cosmic bullets; and even in our time, the danger
for our planet isn’t averted in any way. Asteroids
and comets are still numerous and can impact any
time. Additionally, too little observation facilities
have been established yet and there are few models
of what would happen in case of emergency. The
pivotal questions are now: How devastating would
be the effects of an impact of a really big object?
How likely is this to happen? And are there any
mitigation strategies? In this article we show the
danger from space through cosmic bullets and we
discuss the possible effects of an impact.
Frighteningly, we had to discover, that it seems that
politicians and population are underestimating the
danger through asteroids because of the rareness of
bigger impacts. But a view into the past and our
cosmic neighborhood prove that the threat is real as
well, as it has the potential of a global and
existence-threatening cataclysm. Our research has
shown that existing warning systems don’t suffice
by far to detect possible upcoming impacts early
enough and additionally there are no realistic
practical chances to prevent a future impact.
Furthermore, our investigation shows clearly that
many research, development and information need
to be done. It illustrates that it is necessary to set up
international cooperation to ensure a sufficing lead
time and adequate reactions to a future
threatening.
Our solar system is full of small objects, so-called
asteroids. While most of them are on stable orbits far
away from Earth, some of them come very close or
even cross the Earth orbit. These are called Near
Earth Object (NEOs). Comets, which are a
conglomerate of stone, dust and ice come from
depths of space and are far less predictable in their
orbit. Both sorts of NEOs can be so called Potentially
Hazardous Asteroids (PHAs), Objects that can
potentially impact on Earth. We will show which
classes of NEOs we know and were most of them are
located in our solar system. Astronomers try to find
as much NEOs as possible and calculate their orbits,
so they make sure none of them impacts on Earth
unpredicted. But because most of these cosmic
objects are very small and dark, they are very hard to
detect, so actually very big telescopes would be
needed to find almost all of them. Unfortunately,
politicians don’t give them the funding to do so.
Therefore it remains the questions how many NEOs
there are in our solar system and how likely they will
impact on Earth. Additionally, it remains unclear,
what the effects of an impact would really be, so we
tried to make some predictions what might happen,
depending on our current knowledge. Finally, we
asked the question if we can actually effectively react
if we become the target of a PHA. We validated the
practicability of large-scaled evacuations and even
mitigation strategies. What can we do depending on
the scale of the object and in which way do the
properties of the asteroid affect our possibilities? Is
an atomic bomb really capable of saving us from an
asteroid? In order to figure this points out we
investigated previous impacts in history of Earth.
Additionally we looked at the results of missions to
investigate asteroids, like the “Deep Impact” mission
of NASA in 2005. Our article will be structured in 4
Sections: Sec. (1) will present the different classes of
NEOs and were they are located. In Sec. (2) we will
give an overview over the existing observation
facilities and the methods of finding NEOs. Sec. (3)
will focus on the effects of an impact and the
likelihood they will happen. Finally in Sec. (4) we will
show what we need to know to avoid an impact and
how convertible mitigation strategies can be.
Sec. (1): NEOs
There are 2 main types of NEOs:
Asteroids are small objects orbiting the Sun. The
larger ones are called planetoids. Asteroids are too
small to assume a spherical form; their diameters
vary from a few meters to more than 100 km. In
comparison to comets they are not outgasing near
Sun. The biggest accumulations of Asteroids are the
Main belt between Mars and Jupiter and the Kuiper
belt beyond Neptune.
Comets are small icy objects of average 10-20 km
diameter that heat up when they come close to Sun,
begin to outgas and displaying a visible atmosphere
called “coma” or even a tail. Their main contents are
rock, dust, water ice, and frozen gases such as
methane, ammoniac, cyan and other hydrocarbons [1].
Therefore, American astronomer Fred Whipple called
them “dirty snowballs”. Most comets orbit sun in
widely elongated elliptic, almost parabolic, orbits.
Because of this and the fact that they lose material
every time they come close to Sun, they have to
come from far away from the center of the solar
system, so astronomer Jan Hendrik Oort assumed the
hypothetic Oort cloud in about 1,6 Light-years
distance as their origin [2].
Sec. (2): Detection and tracking
In astronomy, images of the starry sky are most of
the times made with CCD-cameras (Charged Coupled
Devices). That’s a checkered semiconductor-oxide
interface converting incoming photons into electron
charges. The advantage of this method is that no dark
room is needed, as the image is directly processed by
the computer, so astronomers can save a lot of
money. With long exposure time, imaging the same
spot of the sky, asteroids can be detected, being
noticed as stripes on the image, while the fixed stars
remain points. With further observation, the orbit of
the Objects can be calculated then. But
unfortunately, telescopes can’t observe the whole
sky, as looking in the direction of the Sun makes it
impossible to detect any objects there, because they
are outshined by the star. Comets are very tricky to
find, because they have such elongated orbits that
they seem to come from the depths of space and are
very dark before coming near Sun and therefore they
often remain undetected before they are very close.
If they even come from the direction of the sun, they
are nearly undetectable for astronomers.
In 1998, US-congress charged the NASA to detect
90% of all NEOs with more than 1 km diameter until
2008 [3]. For this purpose, several telescopes have
been installed like the CSS, LINEAR and Spacewatch.
Still, these were not enough to fulfill this goal and
additionally, smaller NEOs are extremely dangerous
as well and even harder to detect. While not only for
detecting NEOs, in 2009, the Wide-Field Infrared
Survey Explorer (WISE) was send to space. NASA
funded an additional mission to help ground based
telescopes in detecting NEOs and measuring their
diameter. Because WISE detection methods are
infrared based, it is better for finding low reflecting
objects than ground based telescopes. Currently the
NASA is searching for NEOs with 140m diameter or
more until 2020 [3], but larger and unfortunately more
expensive telescopes would be needed to detect
most of them.
Sec. (3): Danger through NEOs
Thankfully there wasn’t an apocalyptic impact in
human history yet, therefore it is not easy to suggest
what exactly would happen. So we tried to figure it
out, looking at previous impacts in Earth history. The
most famous impact was the asteroid/comet that is
said to have erased the dinosaurs. Because of the
tracks in geological layers and the according 200km
diameter Chicxulub impact crater that was found in
Mexico, Yucatán, experts can estimate the effects of
the impact [3]. The asteroid was estimated to ~ 10 km
in diameter and the impact energy would have been
as much as millions of atomic bombs. Earthquakes
with magnitude of around 10 raged near the impact
region and because the impact was near to the
ocean, mountain-high tsunamis flooded all coasts of
the planet [4]. Even worse, the raised dust darkened
Earth for many years and poisoned gases in the
atmosphere lead to sour rain. As a result, there was a
mass extinction of plants which lead to a mass
extinction of animals as well. These frightening
effects are the result of the big mass of asteroids and
the high average velocity of 42 km/s. That leads to a
huge momentum and energy. Experts are not sure
how big an asteroid has to be to lead to global
climatically effects, but most of them estimate a size
of around 1 km. It is estimated that such an impact
occurs once in around 100.000 years. An impact like
in Yucatán 65 million years ago is expected once in
100 million years.
But even smaller object can lead to devastating
impacts. 1908 in Tunguska, an asteroid of just a few
ten meters diameter exploded a few kilometers over
the Siberian tundra. The stenght of the explosion was
between 10-15 megatons of TNT, in comparison: the
Hiroshima bomb of 1945 had only 0,015 megatons of
TNT [5]. An asteroid like that can easily wipe out a big
city like New York and is estimated to impact on
Earth once in about 1000 years. The good news is
that most of the times these objects will impact
somewhere in wilderness or the ocean.
Fig. 1: Danger-model by Falko Langenhorst (2002) [6]
Sec. (4): Mitigation strategies [3]
If an asteroid aiming towards Earth is detected, it is
very important to know of which material they are
made of, what their shape is and how dense they are
to react in an effective way. For this propose,
astronomers can use radio telescopes to make 3
dimensional models of the object. But to find out
density and material they are made of, space
missions to the asteroid must be done. Therefore, the
NASA launched the “Deep Impact” mission [7]: In
2005, a space probe impacted on comet temple 1
with a velocity of 10,2 km/s while observed by a
second probe. About 15.000 tons material was
ejected from the comet and an impact crater with
100m diameter was left on the surface. Therefore,
the outer layers of the comet are made out of very
loose material. A similar mission of the ESA called
“Don Quijote” is planned in order to test if the impact
of a space probe on an asteroid can change its orbit.
Assuming we got all data needed from the NEO, what
mitigation strategies have been developed yet and
how effective can they be? If the asteroid isn’t too big
and if a warning time of a few decades is provided,
“slow push-pull methods” are a good choice. For
example it would be possible to send a spacecraft to
the asteroid, pulling the object with his gravitation a
little bit over a long time and changing its orbit just
enough not to hit Earth. The advantage of this
method would be that no contact to the asteroid is
needed and it would be very precise, but
unfortunately it can only work with small asteroids
and need a lot of time, which is not really realistic. A
more realistic scenario would be a warning time of a
few years or even months. Depending of the size and
composition of the asteroid, launching several small
probes to impact on the asteroid can be an option or
even an atomic bomb. To the latter need to be said,
that an atomic bomb hasn’t the same effect in space
as it has on earth, because there is no atmosphere, so
only the radiation can transfer momentum to the
asteroid. Additionally, if the asteroid is very dense,
material can be ejected or it can burst entirely. Then
a swarm of small asteroids would hit Earth which
would have an effect similar of a shotgun and be
even worse than one big bullet. If the asteroid is too
less dense, it would possibly consume too less
momentum and still hit Earth. That’s why it is so
important to know the composition of the NEO. Still,
an atomic bomb would be the only possibly effective
way to mitigate a bigger asteroid in short time, even
though it has only been tested on computer
simulations yet, so there remains uncertainty
regarding practicability.
In any case we came to the conclusion that humanity
only has a chance, if effective observation can
provide enough warning time and most important, all
nations work together. If an emergency occurs, there
will be no time for disputing who is going to lead and
who is going to pay, so these questions should be
answered before. We believe that a permanent
international committee should be established that
takes responsibility when an asteroid is detected and
decides what to do. Furthermore all information
about NEOs has to be shared to all astronomers in
the world and politicians should consider a higher
budget to provide better observations facilities,
maybe an international “NEO-observation budget” by
the UN could solve the financial problems so no
nations has to pay more than the others. Cause we
shouldn’t forget the following: We all live on the
same planet!
1. Keller, H.-U., Kompendium der Astronomie
(Franckh – Kosmos, Stuttgart, 2008)
2. Glover, L. K. et. al. Die große National Geographic
Enzyklopädie Weltall (National Geographic
Deutschland, Am Baumwall 11
20459 Hamburg , 2005)
3. Committee to Review Near-Earth-Object Surveys
and Hazard Mitigation Strategies Space Studies
Board et. al., Defending Planet Earth (THE
NATIONAL ACADEMIES PRESS, Washington D.C.,
2010)
4. Lesch H., Müller J., Sternstunden des Universums
(C. Bertelsmann Verlag, Müchen, 2011)
5. Steel D., Nature Vol453, 1157-1159 (2008).
6. Langenhorst F., Sterne und Weltraum 6/2002, 3444 (2002). (Text has been translated to English by
Bernhard Hofer)
7. Russel C.T., Deep Impact Mission (Springer, 101
Phillip Drive, Norwell, 2005)