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dealing with various aspects of the problem, from astronomical
research into the physical properties of NEOs to the develop-
ment of spaceflight technologies, such as guidance, navigation
and control systems, needed for a deflection mission. But there
is also on-going laboratory research into the material properties
of the minerals that make up an asteroid, model calculations of
the asteroids’ interior structure, as well as identification of NEOs
suitable as target objects for deflection test missions. The
NEOShield partners are also researching a general strategy for
asteroid defence – from identifying a threatening object and the
sequence of decisions that must be made, through to deter-
mining the observations and missions required and deciding
when and what kind of defence mission should be launched.
Three methods of asteroid deflection are being looked
into at present. Firstly, the so-called kinetic impactor – a large
space probe hits an asteroid at very high relative velocity
creating an impact sufficient to change its orbit slightly. This
method might be feasible using today’s technology for objects
with a diameter of up to one kilometre or so. But there are still
many unanswered questions. What effects will the interior
structure and porosity of the asteroid, its rotation and other
physical factors have on the outcome of the mission? How must
the impactor be steered to reach its target reliably and at the
correct angle and speed? In this regard, consideration must also
be given, for example, to the effect that movements of the fuel
in the spacecraft have in the critical navigation phase just prior
to impact.
A second method is to use a ‘gravity tractor’. This method,
which uses the weak attractive force between the asteroid and
the spacecraft, may require years or decades to provide a suffi-
cient change in the asteroid’s orbit. So its usefulness would
depend on having adequate warning time before a potential
impact. If a spacecraft is steered into the direct proximity of a
dangerous NEO, the small but significant attraction between
the spacecraft and the asteroid could work like a tow rope. The
probe might be able to use a solar electric propulsion system,
for example, to accelerate itself and the asteroid until a suffi-
cient change in the speed of the system, and the orbit of the
asteroid, has been achieved. With adequate forewarning,
changes of just a few centimetres per second or less might
be enough to avoid a catastrophic impact on Earth.
preserved today as a result of its location in a dry desert.
Depending on their geographic location, impact craters can
be rendered unrecognisable reasonably quickly due to erosion,
vegetation and geological activity. As metallic meteorite frag-
ments have been found in the crater surroundings, we can
assume that the 1200-metre-wide, 180-metre-deep crater is the
result of the impact of a massive metallic object. The diameter
of the impactor has been estimated at just 30 to 50 metres.
It should also be remembered that, statistically speaking, two
thirds of all impacts occur in the oceans, where they can trigger
destructive tsunamis.
Potentially hazardous objects
What happened back then in Arizona is impressive
enough. According to statistical estimates, an object with a
diameter of 30 metres is expected to impact Earth every couple
of centuries. As today’s NEO search programmes are only
finding a comparatively low number of these relatively small
objects, a devastating impact could theoretically occur without
warning. But there are currently several very sensitive NEO
search programmes underway, so we can hope that more of
these menacing small NEOs will be discovered years or even
decades before a potential impact.
Two NEOs that will come dangerously close to Earth in the
near future, relatively speaking, are Apophis and 2007 VK184.
With a diameter of around 300 metres, Apophis will skim past
Earth at a speed of 6 kilometres per second and at a distance
of just 30,000 kilometres in April 2029. And that is not all:
because its orbit is similar to Earth’s, Apophis will remain an
Earth-endangering object for the foreseeable future. There is
a one in 2000 chance that 2007 VK184, which has a diameter
of about 130 metres, might impact Earth at a relative speed of
68,000 kilometres per hour in 2048.
Warning time is crucial
Can we defend ourselves against an asteroid impact?
Current space technology offers several promising methods –
assuming that the search programmes guarantee a prediction
time of 10 to 20 years, and that the threatening object’s diam-
eter is no larger than one kilometre. NEOShield partners are
Although this method is relatively slow in having the desired effect, it has one
big advantage: the surface of the NEO remains undisturbed and no information of any
kind about the interior structure and physical properties of the surface of the object is
required. The best solution could be a combination of the two methods described, in
which, for example, a gravity tractor is deployed after a kinetic impactor has struck,
allowing small corrections to be made to the NEO’s new orbit. In this way, it may also
be possible to prevent future approaches of the NEO to Earth. NEOShield partners are
investigating how realistic this idea is and under which circumstances a gravity tractor
might be deployed.
The third option might be a nuclear explosion. In particular, if time is pressing or
the object is unexpectedly large, the methods described above might not be sufficient.
The largest force that could be deployed to deflect an asteroid would be one or more
nuclear explosions. Although this option is regarded as highly controversial, a nuclear
payload might be our last hope. But what effect would an explosion in the direct
vicinity of an asteroid or on its surface have in the vacuum of space? In the NEOShield
project this method is being studied in detail to provide information on its effectiveness
and the circumstances that might make its deployment necessary. However, the investi-
gations will be carried out using computer simulations; test missions with nuclear
devices will not be proposed!
Asteroid defence is a global task
Because a NEO can impact anywhere in the world – there are no regions on
Earth less likely to be hit than others – as many nations as possible should contribute to
researching defence methods and strategies. Six countries are collaborating in the
NEOShield Project: Germany, Great Britain, France, Spain, Russia and the United States.
Besides the goals set out above, NEOShield is forging links to related international
projects and initiatives to encourage international coordination in this field. In this
regard, the NEOShield Project has already established close contacts with the Near-
Earth Object section of ESA’s Space Situational Awareness programme and the United
Nations’ Committee on the Peaceful Uses of Outer Space (COPUOS) Action Team 14
on NEOs. There are also collaborations with national space organisations.
Although much greater investment and resources will be needed to launch
missions to test NEO deflection techniques – not to mention an international agree-
ment on carrying out defensive action – the EU-financed NEOShield project is an
important step. It provides an excellent opportunity to investigate how asteroids can be
successfully deflected in the future. As they used to say in the Asterix and Obelix comic
strips: “May the sky not fall on our heads!” •
Asteroid Defence | DLR maGazıne 136 · 137 | 19
A NEO with a diameter of just 30 to 50 metres created the Barringer Crater in Ari-
zona 50,000 years ago
Artist’s impression of an asteroid impact on Earth
Image: Don Davis/Southwest Research InstituteImage: Stefan Seip Image: Dan Durda/B621 Foundation and IAAA Image: ESA-AOES MedialabImage: NASA/JPL-Caltech/UMD
Artist’s impression of a gravity tractor with an asteroid in
tow
Artist’s impression of a kinetic impactor
mission. The impact is observed by a
reconnaissance spacecraft
A nuclear explosion is considered to be a last option to prevent
a collision, in the absence of feasible alternative methods.
About the author:
Alan Harris is a senior scientist at the DLR
Institute of Planetary Research in Berlin-
Adlershof and Honorary Professor of Astro-
physics at the School of Mathematics and
Physics at Queen’s University Belfast, UK. He
has overall responsibility for the NEOShield
project.
Partners in the
NEOShield Consortium
- German Aerospace Center (DLR)
- Observatoire de Paris, France
- Centre National de la Recherche
Scientifique, France
- The Open University, United Kingdom
- Fraunhofer Institute for High-Speed Dynamics,
Germany
- Queen‘s University Belfast,
United Kingdom
- Astrium GmbH, Germany
- Astrium Limited, United Kingdom
- Astrium S.A.S., France
- Deimos Space, Spain
- SETI Institute Corporation
Carl Sagan Center, United States
- TsNIIMash, Russia
- University of Surrey, United Kingdom
More information:
www.DLR.de/PF/en
www.NEOShield.net
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