SIMON P. WORDEN, BRIGADIER GENERAL, USAF
Deputy Director for Operations
United States Space Command
Peterson AFB, CO
July 10, 2002
The opinions and concepts expressed are those of the author and do
not necessarily represent the position of the Department of Defense
or the United States Space Command
Introduction
A few weeks ago the world almost saw a nuclear war. Pakistan and
India were at full alert and poised for a large-scale war – which
both sides appeared ready to escalate into nuclear war. The
situation was defused – for now! Most of the world knew about this
situation and watched and worried. But few know of an event over the
Mediterranean in early June of this year that could have had a
serious bearing on that outcome. U.S. early warning satellites
detected a flash that indicated an energy release comparable to the
Hiroshima burst. We see about 30 such bursts per year, but this one
was one of the largest we’ve ever seen. The event was caused by the
impact of a small asteroid – probably about 5-10 meters in diameter
on the earth’s atmosphere. Had you been situated on a vessel
directly underneath the intensely bright flash would have been
followed by a shock wave that would have rattled the entire ship and
possibly caused minor damage.
The event of this June caused little or no notice as far as we can
tell. But had it occurred at the same latitude, but a few hours
earlier, the result on human affairs might have been much worse.
Imagine that the bright flash accompanied by a damaging shock wave
had occurred over Delhi, India or Islamabad, Pakistan? Neither of
those nations have the sophisticated sensors we do that can determine
the difference between a natural NEO impact and a nuclear detonation.
The resulting panic in the nuclear-armed and hair-trigger militaries
there could have been the spark that would have ignited the nuclear
horror we’d avoided for over a half-century. This situation alone
should be sufficient to get the world to take notice of the threat of
asteroid impact.
The Threat
I’ve just relayed the aspect of the near-earth objects (NEO) that
should worry us all. As more and more nations acquire nuclear
weapons – nations without the sophisticated controls and capabilities
build up by the United States over the 40 years of Cold War – we must
first and foremost ensure that the 30-odd impacts on the upper
atmosphere are well understood by all to be just what they are.
A few years ago those of us charged with protecting this nations
vital space system, such as the Global Positioning System (GPS)
became aware of another aspect of the NEO problem. This was the
Leonid meteor storm. This particular storm occurs every 33 years.
It is caused by the debris from a different type of NEO – a comet.
When the earth passes through the path of a comet, it can encounter
the dust thrown off by that comet through its progressive passes by
the Sun. This dust is visible on the Earth as a spectacular meteor
storm. But our satellites in space can experience the storm as a
series of intensely damaging micrometeorite strikes. We know about
many of these storms and we’ve figured out their parent comet
sources. But there are some storms arising from comets that are too
dim or spent for us to have seen that can produce “surprise” events.
One of these meteor storms has the potential of knocking out some or
even most of our earth-orbiting systems. If just one random
satellite failure in a pager communications satellite a few years ago
seriously disrupted our lives, imagine what losing dozens of
satellites could do!
Most people know of the Tunguska NEO strike in Siberia in 1908. An
object probably less than 100 meters in diameter struck over Siberia
releasing the equivalent energy of up to 10 megatons. It leveled a
forest 50 miles across. But most people don’t know that we have
evidence of two other strikes during last Century. One occurred over
the Amazon in the 1930s and another over central Asia in the 1940s.
Had any of these struck over a populated area, thousands and perhaps
hundreds of thousands might have perished. Experts now tell us that
an even worse catastrophe that a land impact of a Tunguska-size event
would be an ocean impact near a heavily populated shore. The
resulting tidal wave could inundate shorelines for hundreds of miles
and potentially kill millions. There are hundreds of thousands of
objects the size of the Tunguska NEO that come near the earth. We
know the orbits of but a handful.
Finally, just about everyone knows of the “dinosaur killer”
asteroids. These are those objects a few kilometers across that
strike on timescales of tens of millions of years. While the
prospect of such strikes grab people’s attention – and make great
catastrophe movies – too much focus on these events has in my opinion
been counterproductive. In my organization, the Department of
Defense, I have tried to raise our concern and interest in addressing
the very real threats outlined above. However I get the predictable
response. “General, if this threat only hits every 50 million years,
I think we can focus our attention of more immediate threats!” In
short the “giggle factor” in the professional scientific and national
security community has meant that we have gotten little done on this
problem.
What Should We Do?
First and foremost we must know when an objects strikes the earth
exactly what it is and where it hit. Fortunately our early warning
satellites already do a good job of this task. And our next
generation system, the Space-Based Infrared System (SBIRS) will be
even better. The primary difficulty here is that this data is also
used for vital early warning purposes and its detailed performance is
classified. However, in recent years the U.S. DoD has been working
to provide extracts of this data to nations potentially under missile
attack with cooperative programs known as “Shared Early Warning.”
Some data about asteroid strikes has also been released to the
scientific community. Unfortunately this data takes several weeks to
get released. Thus my first recommendation is that the United States
DoD make provision to assess and release this data a soon as possible
to all interested parties – exercising proper cautions of course to
ensure that sensitive performance data is safeguarded.
We have begun to scope what an NEO warning center might look like.
We believe adding a modest number of people, probably less than 10
all told, to current early warning centers and supporting staffs
within Cheyenne Mountain could accomplish this. A Natural Impact
Warning Clearinghouse has been scoped to do this job.
Perhaps the most urgent mid-term task has already been begun. This
is the systematic observation and cataloguing of close to all
potentially threatening NEOS. We are probably about halfway through
cataloging “large” NEOS (greater than a kilometer in diameter). It’s
interesting to note that the most effective sensor has been the MIT
Lincoln Lab LINEAR facility in New Mexico. This is a test bed for
the next generation of military ground-based space surveillance
sensors. But this ground-based system, however effective, can only
really address the “large”, highly unlikely threats. We find out
every few weeks about “modest” asteroids a few hundred meters in
diameter. These are often caught as they sail by the earth, often
closer than the Moon, unnoticed until they have nearly passed. Most
recently the object 2002MN had just this sort of near miss – this
time only a few tens of thousands of kilometers from the earth!
Moreover, ground-based systems such as LINEAR are unable to detect
one of the potentially most damaging classes of objects, those such
as comets that come at us from the direction of the sun. New
space-surveillance systems capable of scanning the entire sky every
few days are what’s needed.
New technologies for both space-based and ground based surveys of the
entire space near the earth are available. These technologies could
enable us to completely catalog and warn of objects as small as the
Tunguska meteor (less than 100 meters in diameter). The LINEAR
system is limited primarily by the size of its main optics – about 1
meter in diameter. By building a set of three-meter diameter
telescopes equipped with new large-format CCD-devices, the entire sky
could be scanned every few weeks. But more important the follow-up
observations necessary to accurately define orbits, particularly for
small objects could be done.
The most promising systems for wide-area survey – particularly to
observe close to the sun to see objects coming at up from that
direction – are space-based surveillance systems. Today the only
space-based space surveillance system is the DoD’s “MSX” Satellite.
This was a late 1990s missile defense test satellite and most of its
sensors have now failed. However one small package weighing about 20
kg and called the “SBV” sensor is able to search and track satellites
in Geosynchronous orbit using visible light. This has been a
phenomenally successful mission having lowered the number of “lost”
objects in GEO orbit by over a factor of two. MSX is not used for
imaging asteroids, but a similar sensor could be. The Canadian Space
Agency, in concert with the Canadian Department of National Defense
is considering a “microsatellite” experiment with the entire
satellite and payload weighing just kg. This Near-Earth Surveillance
System (NESS) would track satellites in GEO orbit, as MSX does today.
However, it would also be able to search the critical region near the
sun for NEOs that would be missed by conventional surveys.
The U.S. DoD is planning a constellation of somewhat larger
satellites to perform our basic satellite-tracking mission. Today
our ground-based radars and telescopes, and even MSX only track
objects that we already know about. These systems are not true
outer-space search instruments as the LINEAR system is. However, the
future military space surveillance system would be able to search the
entire sky. As an almost “free” byproduct it could also perform the
NEO search mission. Corresponding, larger aperture ground based
systems could then be used to follow up to get accurate orbits for
the NEOs discovered by the space-based search satellites. Again, I
believe there is considerable synergy between national security
requirements related to man-made satellites and global security
related to NEO impacts.
Regardless of how well we know NEO orbits and how well we can predict
their impacts the fact remains that today we have insufficient
information to contemplate mitigating an impact. We do not know the
internal structure of these objects. Indeed, we have reason to
believe that many, if not most are more in the nature of “rubble
piles” than coherent objects. This structure suggests that any
effort to “push” or divert a NEO might simply fragment it – and
perhaps turn a single dangerous asteroid into hundreds of objects
that could damage a much larger area.
What are needed are in-situ measurements across the many classes of
NEOs, including both asteroids and comets. This is particularly the
case of small (100meter) class objects of the type we would most
likely be called upon to divert. Until recently missions to gather
these data would have taken up to a decade to develop and launch and
cost 100s of millions of dollars. However, with the rise of
so-called “microsatellites” weighing between 50-200 kg and which are
launchable as almost “free” auxiliary payloads on large commercial
and other flights to GEO orbit, the situation looks much better.
These missions can be prepared in one-two years for about $5-10M and
launched for a few million dollars as an auxiliary payload. Such
auxiliary accommodation is a standard feature on the European Ariane
launched and should be, with proper attention, here in the United
States on our new EELV launcher systems.
With a capable microsatellite with several kilometers per second
“delta-V” (maneuver capacity) launched into a GEO transfer orbit (the
standard initial launch orbit for placing systems into GEO) the
satellite could easily reach some NEOs and perform in-situ research.
This could include sample return and direct impact to determine the
internal structure and potential to physically move a small object.
Indeed, NASA is planning several small satellite missions. The key
point here, however, is that with missions costing $10M each, we can
sample many types of objects in the next decade or so to gain a full
understanding of the type of objects we face.
There is an interesting concept to consider. If we can find the
right small object in the right orbit we might be able to nudge it
into an orbit “captured” by the earth. This would make a NEO a
second natural satellite of earth. Indeed, there is at least one NEO
that is close to being trapped by the Earth now, 2002 AA29. If such
an object were more permanently in earth orbit it could not only be
more closely studied but might form the basis for long-term
commercial exploitation of space. Moreover, a very interesting next
manned space flight mission after the Space Station would be to an
asteroid, maybe even one we put into earth’s gravity sphere.
The key of each of these proposed actions on developing the ability
to mitigate NEO impacts is that they are all items our national
security community and we in the United States are likely to do for
other reasons. If these efforts can be adapted to the NEO threat
problem, this would add minimal additional expense.
One of the most important aspects of NEO mitigation is often
overlooked. Most experts prefer to focus on the glamorous
“mitigation” technologies – diverting or destroying objects. In
fact, as the military well knows the much harder part is what we call
“command and control.” Who will determine if a threat exists? Who
will decide on the course of action? Who will direct the mission and
determine when mission changes are to be made? Who will determine if
the mission was successful? And there are hosts more.
These command and control issues are those that the military has
long struggled with. The NEO community has not faced this essential
issue. Indeed, the United States Space Command has just completed a
concept of operations for the first step in NEO mitigation – a
Natural Impact Warning Clearinghouse. This operation is a command
and control function. It would be able to catalog and provide
credible warning information on future NEO impact problems as well as
rapidly provide information on the nature of an impact.
International Issues
Much discussion has been expended suggesting that any NEO impact
mitigation should be an international operation. I would
respectfully disagree. International space programs such as the
International Space Station fill many functions. An NEO mitigation
program would have only one objective. In the latter case a single
responsible nation and organization would have the best chance of a
successful mission. Moreover, the nation responsible would not need
to worry about giving up national security sensitive information and
technology as it would build and control the entire mission itself.
For as pointed out the means to identify threats and mitigate them
overlap considerably with other national security objectives.
It does, however make considerable sense that the data gathered from
surveys and in-situ measurements be fully shared among all. This
will maximize the possibility that the nation best positioned to
perform a mitigation mission would come forward. One of the first
tasks of the Natural Impact Warning Clearinghouse noted above would
be to collect and provide a distribution point for such data.
Summary
NEO Mitigation is a topic whose time has come. Various aspects
related to NEO impacts, including the possibility than an impact
would be misidentified as a nuclear attack, are critical national and
international security issues. The focus of NEO mitigation efforts –
both in finding and tracking them and in exploring and moving some
should shift to smaller objects. Not only are the near-term threats
much more likely to come from these “small” objects (100 meters in
diameter or so), but we might also be able to divert such objects
without recourse to nuclear devices.
After a suitable class of NEOs are found, microsatellite missions to
fully explore and perhaps perform test divert operations should
commence. The technologies for low-cost NEO missions exist today.
The necessary command and control, sensor and space operations
technologies and equipment are all “dual use” to the military. We
have similar, and in some cases almost identical requirements. It
thus stands to reason that strong military involvement and even lead
in the decades ahead on NEO mitigation is in order. As the U.S.
Government considers how to proceed on this critical issue, the major
role that the military and the technologies it controls should be
carefully integrated into our overall national work.
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ON IMPACT FREQUENCY AND NEED FOR A WARNING CENTER
David Morrison
Impact Frequency
In his written statement (above) Pete Worden mentions three large
impacts during the 20th century, and in his oral testimony he called
all three of these 100-m class impacts. He wrote: “Most people know
of the Tunguska NEO strike in Siberia in 1908. An object probably
less than 100 meters in diameter struck over Siberia releasing the
equivalent energy of up to 10 megatons. It leveled a forest 50 miles
across. But most people don’t know that we have evidence of two other
strikes during last Century. One occurred over the Amazon in the
1930s and another over central Asia in the 1940s. Had any of these
struck over a populated area, thousands and perhaps hundreds of
thousands might have perished”. Others have made similar comments,
sometimes also including the dramatic Sikhote-Alin iron meteorite
fall of February 12, 1947.
Of these four events, the Tunguska impact (June 30, 1908) of an
asteroidal object nominally 60 m in diameter was by far the most
dangerous, producing an airburst releasing 5-15 megatons energy.
Sikhote-Alin was well observed and studied, and more than 40 tons of
iron were recovered from multiple craters, but the estimated diameter
of the projectile was no more than 3 meters. The Amazon impact in the
1930s has been discussed but is based on scattered human reports with
no supporting physical evidence, and most researchers suspect that
this impact is spurious. I have not heard anything about the
Kazakastan impact of the 1940s, and I suspect that is spurious also.
Thus by my count for the 20th century we have one confirmed 60-m
impactor (Tunguska) and no evidence of anything else approaching this
size (although of course we would miss most impacts since they would
occur in the ocean; absence of evidence in this case is not evidence
of absence). For comparison, the latest estimated frequency of impact
of 60-meter projectiles is only about once per millennium, rather
lower than the older estimates of once every couple of hundred years.
Call for an NEO Warning Center
Several participants in the NEO Roundtable called for establishing a
NEO coordination and warning center. In the summaries by the
panelists this was a nearly unanimous recommendation. Worden wrote
above that “We [USAF Space Command] have begun to scope what an NEO
warning center might look like. We believe adding a modest number of
people, probably less than 10 all told, to current early warning
centers and supporting staffs within Cheyenne Mountain could
accomplish this. A Natural Impact Warning Clearinghouse has been
scoped to do this job.”
It would be interesting to me to understand better what is meant by
such a warning center. I think everyone can share Worden’s concern
about misidientification of meteors that hit the atmosphere and
explode with kiloton-scale energies. I certainly support his proposal
that this information be disseminated more widely and quickly.
However, these are not what I call “warnings” — they are timely
reports on events that have already happened and been observed from
space.
The only warnings I know of would concern asteroids or comets
discovered to be on possible impact trajectores. Over the past 6
years there have been several short-lived “warnings” of possible
future impacts that were quickly withdrawn as new data and/or better
orbital calculations became available. Today with multiple
international centers for calculating orbits and improved data
sharing, it is likely that there will be fewer such public warnings.
In fact, the only legitimate warning (if you want to call it that) on
the books today is NEA 1950DA, with a nominal chance of 1 in 300 of
an impact in March 2880.
As the NEA surveys increase in power, there will almost certainly be
additional cases of newly-discovered NEAs that appear for a short
time to have a possibility of colliding with the Earth. These will
all be predictions for far in the future, probably at least several
decades. Some will be reported in the press, but most will be quietly
checked out and their orbits refined without the glare of publicity.
Astronomers in several countries today have this computational
capability. I therefore wonder what is the purpose of the proposed
warning center, and just what sort of warnings it anticipates issuing?
Perhaps it is worth repeating that none of the proposed surveys is
designed to look for any NEA on its final plunge to collision with
the Earth. Indeed, it would be very difficult and non-cost-effective
to try to design such a “last minute warning” system. The approach
first articulated a decade ago is to survey the sky, discover NEAs,
determine their orbits, and predict their future paths. Any potential
impactor should be picked up decades (or more) in advance. We can do
this because orbital dynamics is an exact science, and asteroids do
not change orbits capriciously. This approach will apply as well to
the smaller NEAs that are discovered in the future as it does to
those being found today. “Warning” is a word that conveys the wrong
impression: In my opinion, what we should be talking about are
long-term predictions, based on a comprehensive survey of NEAs.
