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Threats from space: 20 years
of progress
John L. Remo and Hans J. Haubold
Abstract
It has been 20 years since planning began for the 1995 United Nations International Conference on Near-Earth
Objects. The conference proceedings established the scientific basis for an international organizational framework
to support research and collective actions to mitigate a potential near-Earth object (NEO) threat to the
planet. Since that time, researchers have conducted telescope surveys that should, within the coming decade,
answer many questions about the size, number, and Earth impact probability of these objects. Space explorations
to asteroids and comets have been successfully carried out, including sample recovery. Laboratory
experiments and computer simulations at Sandia National Laboratories have analyzed the effects of highenergy-density
soft x-ray radiation on meteoritesÑwhich might help researchers develop a way to redirect
an incoming asteroid by vaporizing a thin layer of its surface. An Action Team on NEOs, established in 2001 in
response to recommendations of the Third UnitedNations Conference on the Exploration and Peaceful Uses of
Outer Space, identified the primary components of NEO mitigation and emphasized the value of finding
potentially hazardous NEOs as soon as possible. Recommendations from the action team are meant to
ensure that all nations are aware of the NEO danger, and to coordinate mitigation activities among nations
that could be affected by an impact, as well as those that might play an active role in any eventual deflection or
disruption campaign.
Keywords
asteroid, Chelyabinsk, comet, near-Earth object, NEO, United Nations
O
nthemorning ofFebruary15, 2013,
a meteor nearly 30 times brighter
than the sun streaked into EarthÕs
atmosphere at a speed of about 20 kilometers
per secondÑalmost 60 times the
speed of soundÑand exploded in the air
near the Russian city of Chelyabinsk. A
shock wave hit the ground, followed by
meteorite fragments, causing many injuries
and substantial damage but, fortunately,
no fatalities (Popova et al., 2013).
The object responsible for this sudden
and dramatic event was an asteroid with
an explosive energy equivalent to about a
half-million tons of TNT. The Chelyabinsk
impact and explosion is thought to
be the most energetic since the poorly
observed Tunguska explosion of 1908,
which had an estimated energy of 3 to 15
megatons of TNT (Boslough and Crawford,
1997; Svetsov and Shuvalov, 2008).
A global survey of airbursts with
energy equivalents of at least one kiloton
of TNT has found dozens of these
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2014, Vol. 70(4) 85–93
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explosions since 2000, all thought to be
caused by asteroid fragments entering
EarthÕs atmosphere (Brown et al., 2013).
A worry is that such an unanticipated
explosion over a conflicted region
might be misinterpreted as an act of
war, with serious consequences.
Geological and paleontological records
reveal that near-Earth objects
(NEOs) of even larger size have struck
Earth in the past, and astronomers say
that it is practically inevitable that an
object from outer space will threaten
regional or even global civilization at
some point in the future. The positive
news is that, given sufficient warning
time, the people of Earth have a good
chance of mitigating this danger if they
act collectively and decisively.
The organization best suited to
coordinate NEO mitigation is the
United Nations. That is why one of us
called for and began organizing the
United Nations International Conference
on Near-Earth Objects in 1994
(Remo, 1996). It is now time to look
back on what has been achieved, politically
and scientifically, in the past 20
yearsÑand to consider what can be
expected in the future. Even after two
decades of progress, there is still an
urgent need to find, characterize, and
develop plans for responding to potentially
hazardous asteroids and
cometsÑthose that are large enough,
and pass close enough to EarthÕs orbit,
that they Òmight conceivably yield a
globally-damaging impact in the foreseeable
futureÓ (Marsden, 1997).
An international problem
Held at UN headquarters in New York in
1995, the United Nations conference
established a scientific basis and an
international organizational framework
within which the NEO hazard to the
inhabitants of Earth could be discussed
and possibly mitigated. The publication
of the conference proceedings outlined
critical scientific and engineering topics
that must be addressed to successfully
meet the challenges of effective NEO
mitigationÑthat is, the deflection or
destruction of potentially hazardous
asteroids and comets, with deflection
as the preferred option because it is
less risky.
During the past 20 years, there has
been much to report on UN activities
and research focused on near-Earth
objects. Scientists have expanded NEO
population surveys, launched space missions
to asteroids and comets, done basic
laboratory analytical research aimed at
learning more about the composition of
these objects, created computer models
to study the potential for intercepting
and diverting Earth-threatening objects,
and made observations of atmospheric
impact events. We cannot comprehensively
describe, or even list, all of this
work here. However, we can highlight
some of the most important discoveries,
political activities, and new challenges
that have arisen since the publication of
the conference proceedings (Remo,
1997) and lay the groundwork for future
UN activities.
Finding them before they find us
The first step in dealing with the NEO
specter is to conduct space surveys
aimed at identifying the most dangerous
objects. NASA-sponsored surveys using
ground-based telescopes dominate the
field and are thought to have found most
of the asteroids that are hundreds of
meters or more in diameter, big enough
86 Bulletin of the Atomic Scientists 70(4)
towipeoutentirecivilizations(Yeomans,
2013). Asteroids typically approach closer
to Earth than comets, and are more
numerous, so they are the focus of most
observationsÑalthough a comet impact
would be catastrophic to life on Earth.
Ground-based search programs
supported by NASA include MITÕs Lincoln
Near-Earth Asteroid Research
(LINEAR) telescope in New Mexico,
the Catalina Sky Survey telescopes in
Arizona and Australia, the Spacewatch
project that uses telescopes on ArizonaÕs
Kitt Peak, and the Panoramic Survey
Telescope and Rapid Response System
(Pan-STARRS) in Hawaii. Using the
space-based Wide-field Infrared Survey
Explorer (WISE) telescope launched in
December 2009, the NEOWISE project
hunted for asteroids from September
2010 to February 2011 and was reactivated
in December 2013 for a three-year mission
focused on near-Earth objects.
NEOWISE is the most accurate survey
to date of the size distribution of
NEOs, but there is still a huge range
of uncertainty.
Despite the information provided
by surveys, NASAÕs 2005 congressional
directive to identify the orbits of 90 percent
of the extremely dangerous NEOs
Ñthose that are at least 140 meters in
diameterÑby 2020 is behind schedule,
with only about 10 percent found as of
July 2013 (Irion, 2013). Furthermore,
only 1 percent of the estimated million
near-Earth asteroids have been identified.
As of May 6, NASA had found
11,023 near-Earth objects, 862 of which
are at least a kilometer wide, and 1,469
of which have been classified as potentially
hazardous asteroids (NASA, 2014).
On the positive side, NASAÕs program is
discovering asteroids at a rapidly
increasing rate.
Clearly, deploying new orbital telescopes
would help to increase the discovery
rate of NEOs even further. Infrared
telescopes orbiting the sun near Venus
are critical for this task but can only alleviate,
not eliminate, the NEO riskÑespecially
from smaller asteroid fragments
like the one that exploded near Chelyabinsk
(see Figure 1). Other space telescopes,
orbiting the Earth, would help
extend the search for these smaller-diameter
objects (Mainzer et al., 2011).
Russia plans to launch a new detection
telescope in 2017 and also to extend the
International Scientific Optical Network,
which already includes 32 facilities in 13
countries (Russian Academy of Sciences,
2013). These new telescopes should provide
a better picture of the NEO populationÑsuch
as how the sizes of these
objects compare with their numbers and
their potential for Earth impactsÑwithin
the coming decade.
Exploring outer space
Visits to asteroids and comets can tell
astronomers even more about these
objects than telescopic observation. Outstanding
among several successful
explorations was JapanÕs mission to the
small near-Earth asteroid Itokawa. The
unmanned Hayabusa spacecraft rendezvoused
with Itokawa in 2005, studied
its characteristics, landed on it, and
returned to Earth in 2010 carrying samples
with a composition matching that of
LL chondrites, the most common
meteorite type on Earth. Chemical, mineralogical,
and isotopic analysis determined
that the samples from Itokawa
were altered by Òspace weatheringÓ
on the asteroidÕs surface over millions
of years in orbit. These results established
a relationship between telescope
Remo and Haubold 87
observations of asteroids and laboratory
examinations of meteorites found
on Earth.
There are different types of asteroids,
and they may have variations in structure.
For example, some may be more
like flying rubble piles than solid
masses; some may have massive cracking;
and some may be more porous than
others. Understanding these characteristics
is critically important for estimating
what might happen if Earthlings tried to
redirect a dangerous asteroid.
NASAÕs Dawn spacecraft may help
answer some questions about the composition
and origins of asteroids.
Launched in 2007, Dawn has already visited
the asteroid Vesta and is expected to
arrive at the asteroid Ceres in February
2015. These are the two most massive
asteroids known, butthey are very different:
Vesta is dry and rocky, while the icecovered
Ceres might have a liquid ocean
beneath its surface. Close inspections of
both will help astronomers understand
these differences.
Variations in composition and structure
are not the only challenge for an
asteroid-diverting mission. Some nearEarth
asteroids have very high rotation
rates, or even ring-like orbital fragments,
that could make it difficult to predict the
effects of a collision with an interceptor
sent from Earth. Heading off a comet
bound for Earth would be even more difficult,
because of uncertainties about
cometsÕ population distribution, size,
varying compositions and structures,
highly active evolution, and nongravitational
interactionsÑfor example, jetting
emissions can affect their orbit.
The important and impressive Deep
Impact mission, which arrived at the
comet Tempel 1 in 2005, probed the
structure and composition of the cometÕs
6-kilometer-wide nucleus by firing a
massive copper impactor at it (AÕHearn
et al., 2005). The collision excavated a
crater and ejected a complex array of
icy and dusty materials. The impact
appeared to change the cometÕs orbit by
only the order of 10 to 20 meters per year,
Figure 1. Near-Earth asteroid fragment explodes over Chelyabinsk, Russia, in February 2013
Photo credit: Cgozb keqemohq‘ l‘d Wej~ahlpimk
88 Bulletin of the Atomic Scientists 70(4)
whichwouldnotbeenoughtospareEarth
from an incoming comet. But this is no
surprise, and only underscores the need
for high-energy payloads that would have
a bigger impact (Remo et al., 2013). The
Deep Impact mission showed that it was
possible to rendezvous with a comet and
change its momentum. The Stardust
spacecraft revisited Tempel 1 in 2011 and
observed what is believed to be a shallow
crater carved out by the impactor; it is
about 50 meters wide and surrounded
by brighter material ejected during the
impact. The Rosetta spacecraft, launched
in 2004 by the European Space Agency,
is anticipated to carry out landing and
sampling on Comet Churyumov-Gerasimenko
in August 2014, providing further
insights into the composition of comets.
Developing a defense
The most menacing near-Earth objects
tend to be the biggest ones, and it will
take a huge amount of energy to divert
a massive object that is on a collision
course with Earth. Since the 1995
United Nations NEO conference, there
has been very little innovative computer
modeling of how such an energy transfer
might be accomplished. The possibilities
include high-speed mechanical projectiles
fired atthe NEO, conventional explosives
detonated at or below the surface of
the NEO, and x-ray or neutron radiation
such as that which might be produced
by a nuclear explosive device (Shafer
et al., 1997).
A high-speed projectile, for example,
could shift an NEOÕs orbit by mechanically
transferring momentum to the NEO,
and would be most useful for deflecting
relatively small NEOs with long warning
times. For shorter warning times and
larger NEOs, high-density radiation
could be used to vaporize a thin layer
of the NEOÕs surface, creating a jet
of material that imparts a momentum
change to the NEO. In either case,
repeated interceptions may be necessary
to move the potentially hazardous object
sufficiently out of harmÕs way.
A long-term experimental study using
the Z-pinch facilities at Sandia National
LaboratoriesÑalso used to test nuclear
fusion dynamicsÑsimulated the effects
of high-energy-density soft x-ray radiation
on meteorite targets, which were
used as surrogates for near-Earth asteroids
(Remo et al., 2013). This study
showed that radiation is a viable option
for the orbital deflection of large objects
with shorter warning times.
Simulation of high-energy mechanical
and radiation interactions with NEO
models is critical for designing a successful
mitigation mission. Current rocket
technology with minor modifications
Ñfor example, using an Ares I, Atlas V,
Athena, or Delta IV launch vehicleÑis
thought to be up to the task of delivering
a single high-energy payloadÑone mechanical
impactor, for exampleÑwithin a
time framewhen the NEO is most susceptible
to a hit that would change its course.
TodealwithmoremassiveNEOswithina
short warning time, the Ares V system is
capable of launching six independently
targeted payloads. The magnitude of the
hazard and the available time frame will
determine the type and size of the
required launch payload.
The 2013 Chelyabinsk explosion
demonstrated the potential effects of
near-Earth objects, as well as the difficulty
of detecting them. The impact
was well recorded because of the ubiquity
of consumer video devices.
Images of the event, along with timely
and well-managed damage assessments,
Remo and Haubold 89
provided quantitative estimates of key
parameters: a shallow entry angle of
about 18 degrees, a speed of about 20 kilometers
per second, an airburst altitude of
about 27 kilometers, and a debris path of
about 100 kilometers. Mineralogical analysis
of recovered meteorite fragments
indicate a strongly shocked LL chondrite
with a density of 3.3 grams per cubic
centimeter (similar to that of sand) and
an entry body about 20 meters in diameter
that weighed about 13,000 metric
tons (roughly the size of a bus).
That the Chelyabinsk impact occurred
without warning should come as no surprise,
according to work by Syuzo Isobe
and Makoto Yoshikawa (1997) presented
at the 1995 conference, which showed
that ground-based telescopes have a
blind spot surrounding the sun in which
an asteroid approaching the Earth
cannot be seen because of the bright
sky background. The number of NEOs
coming from this area to within less
than 0.01 astronomical units from the
Earth (that is, less than one-hundredth
the distance from the Earth to the Sun)
is more than 30 percent of the total. One
way to detect these NEOs is to deploy
infrared-sensitive survey telescopes
between the Sun and Venus, as discussed
above. Syuzo Isobe (Isobe and Hirayama,
1998) organized an International
Astronomical Union NEO conference, as
a follow-up to the UN conference, in
Kyoto, Japan, in 1997.
An international action team
Since the 1995 UN conference, there have
been numerous meetings, workshops,
and topical conferences sponsored by
member states of the UN Committee
on the Peaceful Uses of Outer Space
(COPUOS) to recommend, initiate, or
develop scientific initiatives and topics
laid out in 1995 (Isobe and Hirayama,
1998; Remo and Haubold, 2001; Wamsteker
et al., 2004).
In 2001, COPUOS established an
Action Team on Near-Earth Objects, in
response to recommendations of the
Third United Nations Conference on the
Exploration and Peaceful Uses of Outer
Space (Remo and Haubold, 2001; UNISPACE
III, 1999) held in Vienna in 1999,
and charged the team with reviewing the
content, structure, and organization of
ongoing efforts in the field of near-Earth
objects; identifying any gaps in the
ongoing work where additional coordination
is required and/or where other
countries or organizations could make
contributions; and proposing steps for
the improvement of international coordination
in collaboration with specialized
bodies. Since 2001, the action team has
considered annual reports submitted by
the member states active in NEO work,
as well as recommendations concerning
an international response made at workshops
and conferences conducted by the
international NEO community.
The action team identified three
primary components of mitigation:
discovering hazardous asteroids and
comets and identifying those objects
requiring action; planning a mitigation
campaign that includes deflection and/
or disruption actions and civil defense
activities; and implementing a campaign,
if the threat warrants. The action team
emphasized the value of finding hazardous
NEOs as soon as possible in order to
avoid unnecessary delays in planning
and launching mitigation missions. The
team called for establishing an international
asteroid warning network that
would link existing institutions and recommended
that space agencies establish
90 Bulletin of the Atomic Scientists 70(4)
a space mission planning advisory group
to consider options for planetary defense.
Recommendations by the action team are
meant to ensure that all nations are aware
of potential hazards, and to ensure the
design and coordination of mitigation
activities among nations that could
be affected by an impact and those
that might play an active role in any eventual
deflection or disruption campaign
(COPUOS, 2013).
A double-edged sword
At its 68th session on December 11, 2013,
the General Assembly of the United
Nations embraced the recommendations
for an international response to the nearEarth
object impact threat, which
were endorsed by the Scientific and
Technical Subcommittee of COPUOS
and by COPUOS (United Nations General
Assembly, 2013). As the COPUOS
action team noted in its recommendations,
ÒThe risk that an asteroid would
impact the Earth is extremely small,
but, depending on its size and impact
point, the consequences could be catastrophic.
However, perhaps uniquely
among natural hazards, there is the
potential to prevent near-Earth object
(NEO) impact events through timely
actionsÓ (COPUOS, 2013). The emphasis
in the coming decades should be on mitigating
the potential impacts of large
NEOs, rather than relatively small ones
that can be consumed by the EarthÕs
atmosphereÑsuch as the asteroid fragment
that exploded near Chelyabinsk.
There is also potential to prevent a
colossal misunderstanding. NEOs pose
a dual danger to civilization: not just
the possibility of a catastrophic collision
with a massive object, but also the
chance that a near-Earth object entering
the atmosphere might be mistaken for an
intentional attack. The effort to prevent a
celestial object from causing massive
extinction of life on Earth is a project
that can unite nations around the world
in the peaceful use of outer space. Given
the long lead time needed to develop
technology that can safely and reliably
divert a threatening object, now is the
time for an open discussion of means
and methods.
Funding
This research received no specific grant from any
funding agency in the public, commercial, or notfor-profit
sectors.
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Author biographies
Physicist John L. Remo is a research associate
at the Harvard-Smithsonian Center for Astrophysics
and the Departments of Astronomy
and Earth and Planetary Sciences at Harvard
University, USA. His expertise is in planetary
science, astrophysics, and laser physics. He
organized the 1995 United Nations International
Conference on Near-Earth Objects. He
used the Z facilities at Sandia National Laboratories
to simulate the effects of high-energydensity
soft x-radiation on planetary bodies; to
simulate the origin of solar system”like planets;
to explore the structure of exoplanets; and to
study the potential for deflecting near-Earth
objects in the future. He received the Nininger
Meteorite Award for his prediction and discovery
of the ductile-brittle transition in meteoritic
irons. Minor planet Remo 2114 T-2, also known
as asteroid 9137 Remo, is named after him in
recognition of his work in laser physics and
planetary science; it is not a potentially hazardous
object.
Astrophysicist Hans J. Haubold is a professor
of theoretical astrophysics at the United
Nations Office for Outer Space Affairs in
Vienna and the Centre for Mathematical
Sciences in Pala, India. His expertise is in solar
92 Bulletin of the Atomic Scientists 70(4)
physics and neutrino astrophysics. From 1991 to
2012, he organized the UN Basic Space Science
Initiative (Haubold, 2013) with support from the
European Space Agency, NASA, the Japan
Aerospace Exploration Agency, and member
states of the United Nations. The issue of
near-Earth objects was part of this initiative
and was introduced to the United Nations
through the UN International Conference
on NEOs.
Remo and Haubold 93