FEATURES

Risks to the Earth from impacts

of asteroids and comets
Dr Harry Atkinson, chairman offormer UK government task force on the subject

hirty years ago few took seriously the risk to mankind of al context. (This followed the prompting of a British member of
T impacts on the Earth of asteroids and comets, or "near Earth
objects" (NEOs) - apart from a handful of dedicated astronomers.
parliament, Lembit Opik, whose grandfather had been an expert
on comets and a distinguished director ofthe Armagh Observato-
There seemed to be little evidence for such a risk: the craters on ry in Northern Ireland; and campaigning by Duncan Steel and Jay
the Earth and the moon were generally thought to be of volcanic Tate).
origin, not made by impacts; and while. since prehistoric times, The task force, which comprised Sir Cnspin TickeR, Professor
comets must always have aroused interest, or even dread, their David Williams and myself (as chairman), reported in September
true danger was not understood. As for the main risk, asteroids, that the risk was indeed real and comparable with other low proba"
they were so small and dark that the first (and biggest) was not bility but very high consequence risks taken seriously by
discovered until 1802. The first systematic survey of asteroids did governments. The threat from NEOs raises major issues, among
not begin until 1970. them the inadequacy ofcurrentknowledge, confirmation ofa hazard
Two things brought home the potential dangers: first, a sugges- after initial observation, disaster management, methods of mitiga-
tion in 1980 by Alvarez (father and son) et al that the dinosaurs tion and deflection, and reliable communication with the public.
had been extinguished as the result of a large object hitting the The recommendations of the task force covered both science
Earth 65 million years ago; and second, in July 1994, the collisions and organisation: for science, that a dedicated international pro-
of a succession ofpieces of a large comet. Shoemaker Levy-9, with gramme of advanced astronomical observations should be set up,
the giant planet Jupiter, each piece causing an explosion about the particularly to increase knowledge of NEOs of smaller size (down
size of the Earth (Figure 1). This triggered the production of two to 300m or less) than those systematically covered by the US sur-
films, Armageddon (with Bruce Willis) and Deep Impact, which vey; these smaller objects can cause great regional or continental
made the idea of NEO impacts familiar to a much wider public - damage. For organisation, that steps be taken at government level
but may have registered more as science fiction, with a strong dose to set in place appropriate bodies - international. European and in
of"giggle factor". Arthur C Clark had already pointed to the dan- the UK - where all these issues could be discussed and decisions
ger of asteroids in his novel Rendezvous with Rama in 1973, and taken. In our view the UK and Europe generally are well placed to
had coined the term "Spaceguard" subsequently used for surveys make a significant contribution to what should be a global effort.
and by concerned organisations. On 24 February; the British government gave its formal
By the early 1990s, however, the US Government had become response, welcoming the report and setting out an action-plan
,
convinced that NEO impacts were sci-
ence fact, not science fiction, and
Congress initiated expert studies of
both the detection and mitigation of
NEOs. As a result, NASA was given the
task of identifying, over a ten-year
period. 90% of all asteroids of diame-
ter greater than 1 kilometre.
Observations for this "Spaceguard"
survey began in 1998 using dedicated
wide-angle US Air Force surveillance
telescopes, of aperture 1 metre, each
equipped with a large CCD detector
array. About 500 of these really big
NEOs have already been discovered,
about half the estimated total number.
The Earth is now seen as orbiting in a
sea of near Earth asteroids, as graphi- Fig. 1: Impact of comet Shoemaker-Levy 9 on Jupiter, July 1994. Before impact the comet broke
cally illustrated in Figure 2. into a number of fragments each hitting the planet in a different place, as shown by the belt of
It is remarkable that no other gov- bright spots near the bottom of the picture. The impacts created fireballs as big as the Earth.
ernment took the threat seriously - The very bright spot at the top right is the Jovian moon 10. Photographed at infrared
that is until the British minister for wavelengths from Hawaii. [acknowledgement: NASA)
research, Lord Sainsbury, set up a task Fig. 2: Orbits of the 800 or so near Earth asteroids of all sizes known at the beginning of the year
force in January 2000 to advise the 2000, with the Sun at the centre.The asteroids which cross the Earth's orbit are in yellow.They are
government on the nature and risk of potentially dangerous.The others, coloured red, approach the Earth but cannot strike it. The picture
NEO impacts and on what the United shows that the Earth is hemmed in by a sea of asteroids. [acknowledgements: Scott Manley
Kingdom should do in an internation- (Armagh Observatory) and Duncan Steel (University of Salford))
126 europhysics news JULY!AUGUST 2001
Article available at http://www.europhysicsnews.org or http://dx.doi.org/10.1051/epn:2001403
 FEATURES

largely based on our recommendations. For interna-

Tsunami Travel lime in Hours
tional action, the OECD was suggested as a possible .....-----_--.:.._---=:---"'!!
coordinating body.

Nature of hazard
Asteroids and comets are primordial' material left over
from the initial process of forming the solar system.
Both types of object, in their millions and billions, nor-
mally orbit the Sun far away from the Earth. The
asteroids are in a belt between Mars and Jupiter (2 to 4
Astronomical Units from the Sun, one AU being the
Earth to Sun distance). The comets are much further
away, either in the Edgeworth-Kuiper belt, 30 to 1000
AU from the Sun, or in the Oort cloud, a spherical shell
of comets at the cold outer parts of the solar system at
40 to 100 thousand AU, nearly a quarter of the way to
the nearest star.
Very occasionally, individual asteroids or comets are
deflected by collisions or by gravitational forces into
paths coming close to the Earth. The near Earth aster- Fig, 3: Calculated progress of tsunami following impact (marked with a star)
oids usually have orbits rather similar to that of the of a large asteroid, Eltanin, in the SE corner of the Pacific about 2.15 million
Earth, with periods of the order of a year; they are often years ago. Assuming a 4 km asteroid diameter, the initial "crater" in the ocean
stony (perhaps as groups ofrocks held together only by must have been 60km wide and 5 km deep; after 5 hours the resulting wave
their own weak gravitational forces), but can be car- would have travelled 3000km and be 70m high.The evidence for the impact
bonaceous or metallic. The near Earth comets, comes from the ocean floor which shows damage over hundreds of square
essentially "dirty snowballs", are in highly elliptical kilometres.
. [acknowledgement: Steven Ward/Eric Asphaug, Ucal Santa Cruz]
orbits with long periods ranging from scores of years
(for example Halley's comet at 75 years) to periods so
long that they are essentially "one-offs", like Hale-
Bopp. These long period comets are totally
unpredictable, and can be seen approaching no more
than a year before possible collision, making them par- Numbers and effects of impactors of different size (diameter); few of those
ticularly dangerous. Fortunately, long.period comets with sizes in the shaded area survive down to the Earth's surface.
are only a fraction of all comets; and comets in general
are less numerous than asteroids: but comets travel Size Number Average Energy Effect
faster and therefore have ml1ch more energy. interval (TNT equivalent
between Kilo- or Mega-
The Table shows that while global effects result only Impacts tonnes)
from the relatively small numbers of objects of diame-
3m billions weeks 2KT usually explode harmlessly in upper atmosphere
ter 1 km and above, the smaller ones are also extremely without r..aching the Earth's surface. Observed
dangerous - and vastly more numerous. Even those of by US defen"" satellnes. However, metallic
asteroids ean reach the ground; one such object
diameter between 30 and lOOm, which do not normal- exploded over the Yukon in January 2000.
tripping the main eleetrieny network over a wide
ly reach the Earth's surface, can cause great damage area.
through blast; an example is the sOm Tunguska object,
of energy approaching that of the Bikini hydrogen 10m 150 decade 65 KT ditto
million (6 Hiroshima A
bomb, which would have devastated a major city if dif- bombs)
ferently placed. Two thirds of NEOs hit the sea: the
serious effects of the resulting tsunamis cannot be 30m 4 million <100 yrs 2MT explode in upper almosphere without reaching
surface, but blast waves eause serious ground
over-emphasised, for example the Eltanin impact damage (eg Tunguska impact of 50m object in
Siberia in 1908, which flattened 2000sq km of
shown in Figure 3 (Ward and Asphaug, 2000). forest)
Taking all sizes and impact frequencies into account,
the risk of an individual's dying from NEO impacts 100m 100,000 3,000 yrs 65MT penetrate atmosphere. Serious demage on land;
(5 Bikini ocean impacts give tsunamis. On average, 5,000
over his or her lifetime is estimated at about 1 in Hydrogen deaths per impact
20,000. This is roughly the same as the risk of an aver- devices)

age American dying in an aircraft accident. (Chapman

300m e.oOO 40,oooyrs 2,000 MT major sUb-global effects including big tsunamis;
and Morrison, 1994) half a million deaths probable
Nevertheless, the chance ofimpact of a 1 km NEO is
seen from the table to be very small, on average only 1 km 1,000 200,000 yrs 65,000 MT global effects similar to 'nuclear wintar'
(1,000 Bikini) calculated for all-<lut nuclear war. Local effects
once every 100,000 years or so. It might be thought that devastating; huge tsunamis if ocean hit. 1.5
this timescale is so long that the risk could in practice billion deaths (quarter of wortd's population)

be dismissed. However, in other areas, such low proba- 10km few 100 million 65 million MT extinction of species (for example of the
bility but high consequence risks are taken very yrs (1 million BikinQ dinosaurs at the Cretaceous-Tertiary, KIT,
boundary 65 million years ago). Most of world's
seriously indeed by bodies such as the British Health human popUlation would die.
and Safety Executive. For example, the Sizewell B
europhysics news JULY/AUGUST 2001 127
FEATURES

nuclear power station in the UK was originally designed so that Fig. 4: Asteroid Eros,
the risk of"melt-down" was less than once in 100,000 years. How- shaped like a potato, is
ever, that risk was subsequently thought to be unacceptably high, about 33 kilometres long,
and hundreds of millions of dollars have recently been spent to 13 kilometres wide and 13
reduce it. It is interesting to note that if the 1 km object (with its kilometres thick. The
risk of similar timescale) were "owned" by a company, that com- crater at the top is 5.3 km
pany would be prosecuted for not reducing the risk. in diameter. Most known
near Earth asteroids are
Mitigation less than 1 kilometre
across, much smaller than
My assumption, so far, is that nothing has been done to mitigate
Eros. Picture taken in
the risk. However, if mitigation were possible, the whole picture
February 2000 by NASA's
would change totally, moving from statistical estimates of risk Shoemaker-NEAR
towards calculated certainties. Studies show that countermeasures spacecraft orbiting Eros at
may well be possible, the most effective method being the deflec- 200 km above its surface.
tion of the NEO so that it misses the Earth entirely. Of other The numerous impact craters show that even asteroids are hit by
possibilities, moving people from the target area could help, for a other asteroids many times in their history. [acknowledgement:
small asteroid, but uncontrolled breaking-up ofthe object in orbit NEAR/NASA)
might only make things worse. Deflection requires the ability to
change the object's momentum in orbit. Many ways for doing this
have been considered, from solar-sails using the Sun's radiation its orbit and its composition. That is why the task force gave top
pressure to high-powered laser beams. At present the only practi- priority to a comprehensive survey of objects smaller than those
cal approach seems to be to use nuclear explosives. Unfortunately being observed by NASA, going down to diameters of 300m or
chemical explosives are far too heavy to deliver the punch less; only a tiny proportion of such objects have so far been
required. Some tonnes of nuclear explosive would be required to observed. This needs, on the ground, at least one dedicated wide-
deflect a large asteroid. Current large rockets are capable of angle 3m-class telescope for discovery (preferably through
launching such a charge in a suitable spacecraft. European cooperation), and conventional telescopes for accurate
Although suitable nuclear charges, designed rather differently orbit determination and spectroscopic observation of the NEO's
from nuclear weapons, have not been made or tested, most of the composition. Space missions are also most. important. We have
other technologies required have already been used, for example pointed out the potential value of the ESA missions BeppiColum-
in the recent Shoemaker-NEAR mission to Eros (Figure 4) in bo and GAIA for the discovery ofNEOs, and have recommended
which, for about a year, the spacecraft tracked the asteroid in its the use of relatively cheap "micro-satellites" to rendezvous with
orbit around the Sun, much of the time slowly orbiting Eros often different types ofasteroid and comet and gather detailed informa-
only a few tens of kilometres from its surface; finally, early this tion at first hand. These would greatly extend the work done by, or
year, the spacecraft landed safely on the asteroid - after transmit- planned for, the major rendezvous missions of NASA or ESA.
ting an unprecedented amount ofinformation about the nature of Finally, we recommended multi-disciplinary studies to learn more
the object. Going even further, NASXs Deep Impact spacecraft about the consequences of impacts. The studies would involve
will launch a 1/2 tonne copper projectile at a comet (Figure 5), astronomers, geophysicists, oceanographers, climatologists, econ-
ejecting material to form a crater more than a hundred metres omists and sociologists, and also universities, national research
across and "seven stories deep". The objective is to learn about the councils and the European Science Foundation.
inner structure of the comet - but the impact will, incidentally, The above paragraph summarises the task force's first eight rec-
deflect the comet slightly. ommendations for an enhanced international observational and
In deflecting an object, it is most important to know its compo-
sition and gross structure. As already said, many asteroids are
essentially piles of stones: these will simply fly apart unless rela-
tively gentle forces are applied (with acce1erations less than 1
metre per second). For this reason the asteroid may need a succes-
sion of nudges over a period of time from a succession of nuclear
charges. Each charge would be detonated within a radius or two of
the object; the x-rays and neutrons from the explosion will eject
material from the asteroid's surface, causing it to move in the
opposite direction.
While deflection is thus theoretically possible, the use or even
testing of nuclear explosives in space would raise serious political
problems. Indeed, the use of such means might only be contem-
plated if a major impact were otherwise inevitable.
It may be worth noting that for no other major natural hazard -
for example volcanic action, earthquakes or tsunamis from land-
slips - may it be possible to act so as to obviate the hazard Fig. 5: NASA's Deep Impact mission will project a 500 kg solid
mass into a comet (artist's impression) in 2004. Af1yby spacecraft
completely.
will take images and make measurements. The impactor will also
take images of the comet's surface prior to impact. The mission
Recommendations: more science; international organisation
aims to increase understanding of the composition and structure
Essential prerequisites to mitigation are the discovery of the NEO of comets. [acknowledgement: Ball Aerospace & Technologies
well in advance of possible impact, the accurate determination of Corp)
128 europhysics news JULY/AUGUST 2001
scientific programme. Regarding mitigation, we recommended
that the UK government, with other governments having the nec-
essary technology, should set in hand studies to look into the
practical possibilities of countermeasures, both mitigation of
impacts and deflection of incoming objects.
Finally, we made the following recommendations regarding
organisation (of which the first two are given in full):

that the government urgently seek with other governments and

international bodies. (in particular the International Astro-
nomical Union) to establish a forum for open discussion of the
scientific aspects of NEOs, and a forum for international
action. Preferably these should be brought together in an inter-
national body. It might have some analogy with the
intergovernmental Panel on Climate Change, thereby covering
science, impacts, and mitigation (including countermeasures).
(Recommendation 10)

that the government discuss with like-minded European gov-

ernments how Europe could best contribute to international
efforts to cope with NEOs, coordinate activities in Europe, and
work towards becoming a partner with the United States, with
complementary roles in ~pecific areas. We recommend that the
European Space Agency and the European Southern Observa-
tory, with the European Union and the European Science
Foundation, work out a strategy for this purpose in time for
discussion at the ministerial meeting of the European Space
Agency in [November] 2001. (Recommendation 11)

Regarding organisation in the UK, the task force recommended

that overall responsibility be assigned to a single government
department; and, most importantly, that a British national centre
be created to provide independent scientific advice to the public,
parliament, and the government (Recommendations 10 to 14).
The British government has taken a major step forward in its
response to the report of the task force. As said in the response,
.negotiations with and between international institutions, and
analysis of complex scientific proposals, take time. It is welcome
news that the government has therefore undertaken to provide a
further report later this year on its progress in implementing its
plans. There is still much to be done and I await further progress
in this vital area.

Further reading
Atkinson HH, Tickell C and Williams DA (2000), Report ofTask Force on
Potentially Hazardous Near Earth Objects, British National Space Centre,
London SW1W 9SS. (An electronic version of the report and of the Gov-
ernment's response, together with links to other useful web sites, are on:
wviw.nearearthobjects.co.uk
Chapman and Morrison (1994), Impacts on the Earth by asteroids and
comets: assessing the hazard, Nature, 367, 33
Gehrels T (Ed), Hazards due to Comets and Asteroids, University ofAri-
zona Press (1994), ISBN 0-8165-1505-0 (covers comprehensively all
aspects of NEOs)
Steel, Duncan, Target Earth (2000), Time Life Books, London ISBN 0-
7054-3365-X
Ward SN. and E. Asphaug E (2000), Asteroid Impact Tsunami: A proba-
bilistic hazard assessment, Icarus, 145, 64-78 (and other papers)

europhysics news JULY/AUGUST 2001