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Hearings on the threat of near-Earth asteroids (NEAs) before
the Subcommittee on Space and Aeronautics, House Committee
on Science, October 3, 2002. Testimony on the likelihood and
effects of asteroid impacts, focusing on the next steps to
be taken beyond the current Spaceguard Survey of NEAs with
diameters larger than 1 km. The following information is posted:
(1) Press Release describing the hearings; (2) Background
prepared by the Science Committee Staff; (3) Statement by
witness David Morrison; (4) Statement by witness Edward Weiler;
(5) Statement by witness Joseph Burns; (6) Statement by witness
Brian Marsden; and (7) Statement by witness Pete Worden.
Committee on Science
U.S. House of Representatives
Sherwood Boehlert, Chairman
Ralph M. Hall, Ranking Democrat
October 3, 2002
WASHINGTON, D.C. -- Scientists are making progress
in cataloguing and tracking large near-earth objects (NEOs),
but a serious threat still remains from smaller objects, an
expert panel told the Space and Aeronautics Subcommittee today.
These smaller asteroids (200-500 meters wide)
could potentially demolish a city with a direct hit or cause
a tsunami capable of wiping out entire coastal areas if they
land in the ocean. NASA has catalogued nearly 50 percent of
asteroids 1 kilometer wide and larger. Astronomers estimate
that between 900 and 1300 of the larger asteroids exist while
there could be as many as 50,000 in the smaller range.
Subcommittee Chairman Dana Rohrabacher (R-CA)
stated, "The threat posed by incoming asteroids and comets
is a serious, potentially life-threatening topic. Given the
number of near-earth objects in space, it is a matter of time
before we are faced with an event unparalleled in human history.
I hope that my legislation, H.R. 5303 [http://www.house.gov/science/press/107/107-286.htm],
passed by the House on Tuesday will strengthen existing government
capabilities for tracking natural space objects by encouraging
private citizens to observe asteroids and comets."
Subcommittee Ranking Member Bart Gordon (D-TN)
added, "NASA's Mission Statement says that part of its
mission is '... to protect our home planet.' I hope NASA will
heed the message of today's hearing and work with other agencies
of the U.S. government to craft a timely, cost-effective plan
to detect and catalog as many as possible of the Near-Earth
asteroids and comets that could potentially threaten our population.
We cannot afford to be complacent."
Dr. David Morrison, senior scientist at the
NASA Ames Research Center, discussed NASA's goals and accomplishments
in monitoring NEOs through the "Spaceguard" program.
Morrison noted that Spaceguard was halfway to its goal and
he expected that by 2008 NASA will have 90 percent of large,
kilometer-sized threatening asteroids catalogued. Morrison
added, "Our objective should be to find a large impactor
far in advance, and thus provide decision-makers with options
for dealing with the threat and defending our planet from
cosmic catastrophe."
NEOs also pose a serious concern for the military,
Brigadier General Simon P. Worden testified. Worden told of
an asteroid that entered the atmosphere and exploded above
the Mediterranean during last year's India-Pakistan conflict.
U.S. satellites detected an energy release and shockwave comparable
to the Hiroshima bomb, and Worden explained that had the event
taken place at the same latitude two hours earlier and mistaken
for a nuclear detonation it could have had devastating consequences.
Worden added, "I believe there is considerable synergy
between national security requirements related to man-made
satellites and global security requirements related to NEO
impacts."
Witnesses also debated the merits of continuing
the cataloging effort on smaller NEOs once the Spaceguard
program is completed. Dr. Brian Marsden, Director of the Minor
Planet Center of the Smithsonian Astrophysical Observatory,
testified that handling the large amount of data from surveys
of smaller NEOs would be a challenging, but feasible, task.
Dr. Joseph Burns, a member of the Solar System Exploration
Survey Committee of the National Research Council, testified
that NASA should partner with the National Science Foundation
to build and operate a large ground-based survey telescope
because of NSF's expertise in ground based astronomy and NASA's
traditional support of ground-based solar system observations
that support space missions.
Dr. Ed Weiler, NASA Associate Administrator
for Space Science, disagreed saying, "I feel that it
is premature to consider an extension of our current national
program to include a complete search for smaller-sized NEOs."
He also noted that NASA did not feel the agency "should
play a role in any follow-on search and cataloging effort
unless that effort needs to be specifically space-based in
nature."
Rep. Anthony Weiner (D-NY) said, "For too
long we've assumed that the worst asteroid risk would come
from Hollywood -- in the form of a sequel to flops like Deep
Impact or Armageddon. But the threat posed by Near Earth Objects
is real, and if we can plow $100 million into a summer flick,
we can certainly give NASA the means to make us safer from
real life blockbusters."
Witness testimony and an archived web cast of
the proceedings can be found
at http://www.house.gov/science/
The Threat of Near-Earth Asteroids
Thursday, October 3, 2002 10:00 a.m.
2318 Rayburn House Office Building
On Thursday, October 3, 2002, at 10:00 a.m.
in room 2318 of the Rayburn House Office Building, the Subcommittee
on Space and Aeronautics will hold a hearing on the threat
of Near-Earth Asteroids. The hearing will examine the status
of the current national survey of asteroids and comets known
as Near-Earth Objects ("NEOs"), the threat of a
NEO impact, future goals for the survey, and the national
policy regarding NEOs .
Asteroids and comets with orbital distances
from the sun similar to Earth's are designated as NEOs. While
many of these pose no threat of collision with the Earth,
a subset known as "Earth-crossing asteroids" (ECAs)
and "potentially hazardous asteroids" (PHAs) have
orbits with the potential for a close encounter or collision
with the Earth. The Earth is bombarded by small meteorites
every day, but most of these objects are less than 50 meters
in size and burn up in the atmosphere. Larger objects impact
the Earth less frequently but can cause enormous damage depending
on their size, as described in Figure 1. For example, scientists
now generally believe that the mass extinction at the end
of the Cretaceous period, which included dinosaur extinction,
was the result of climate and ecosystem disruption from a
massive asteroid impact off the Yucatan peninsula. The fossil
record includes a layer of extra-terrestrial material, churned
up and distributed by the impact around the globe, at exactly
this time-period. More recently, the asteroid impact of 1908
in Tunguska, Siberia flattened 2000 square kilometers of forest
with an impact energy 1,000 times that of the Hiroshima atomic
bomb. Thus the potential for disaster by an asteroid impact
has already been demonstrated in our planet's history.
The threat of hazardous Near-Earth Objects has
gained greater attention in the public and press recently,
in part as a response to several close encounters with asteroids
discovered by the current national survey for such objects.
Currently NASA is surveying large NEOs with a goal of finding
and cataloging 90 percent of objects larger than one kilometer
by 2008. Over 600 of these large objects have already been
found (Figure 2). In addition to examining the status and
results of this survey and the NEO threat, this hearing will
explore the question of next steps beyond this survey goal,
including the costs, benefits, and technical challenges of
extending the survey to include smaller, yet still potentially
very hazardous, objects. Agency roles, interagency cooperation,
and the possibilities for international contributions to the
NEO survey effort will be discussed.
In particular, the important role of amateur
astronomers in the NEO survey and tracking effort will be
highlighted. Amateur astronomers are responsible for much
of the important tracking of NEOs after they are discovered.
Earlier this year, Rep. Dana Rohrabacher (R-CA) introduced
the "Pete Conrad" bill, H.R. 5303. This bill would
establish awards for U.S. amateur astronomers who contribute
the most toward the discovery and tracking of Near-Earth Asteroids.
Status of the Current U.S. Survey for Near-Earth
Objects. At the request of Congress in 1994, NASA initiated
a plan to locate all NEOs larger than one kilometer in diameter.
The resulting strategy, known as the "Spaceguard"
goal, is to discover and catalog 90 percent of these large
objects by 2008. The Near-Earth Object Program Office at the
NASA Jet Propulsion Laboratory was established in 1998 to
coordinate NASA efforts to discover and track these potentially
hazardous NEOs. Congress recently provided $3.5 M in FY2001
and an additional $3.5 M in FY2002 for NASA's NEO survey activities.
The status of the survey and likelihood of reaching the Spaceguard
goal will be addressed in the hearing. Other related questions
include: What survey projects are currently funded by NASA?
What contributions do Air Force telescopes make to NEO survey
projects?
Amateur Astronomer Contributions. Amateur astronomers
play an important role in NEO monitoring. While their equipment
is generally not suitable for the discovery of many new objects,
these astronomers are often well suited for tracking objects
already discovered, which is crucial for predicting orbital
paths and detecting objects deviating from their predicted
orbit. Legislation introduced by Rep. Rohrabacher (the "Pete
Conrad" bill, H.R. 5303) will offer monetary awards through
NASA to reward U.S. amateur astronomers who contribute the
most toward the discovery and tracking of NEOs. The importance
of contributions from amateur astronomers in both current
and future NEO survey efforts will be highlighted in the hearing.
Future Direction of National NEO Survey and
Response Efforts. The question now is what to do next in the
survey of, and in planning for a response to, hazardous NEOs.
While the current survey is designed primarily for objects
larger than one kilometer in size, most NEOs are smaller than
one kilometer, and asteroids of only a few hundred meters
in size could potentially destroy an entire city or country.
Asteroids of this smaller size are far more likely to collide
with Earth within the next century than are the kilometer-sized
objects. What should be the future goal for NEO surveys? What
is the cost of extending the survey down to objects of a few
hundred meters in size? What is the threat of these objects
relative to the cost and technical challenge of finding and
monitoring them? What technologies are needed for future NEO
survey work? Which agencies are best suited for the NEO survey,
data management, and planning for a response to a threatening
NEO? What should be the role of NASA, the Department of Defense,
the National Science Foundation, and other relevant agencies
in developing and executing a unified set of recommendations
for protection from NEOs?
Data Management: Currently all asteroids and
comets discovered around the world are reported to the Minor
Planet Center (MPC) of the Smithsonian Astrophysical Observatory
at Harvard University. The MPC disseminates information on
new discoveries and orbit parameters internationally, making
for an efficient coordinated world-wide system. However, the
enormous magnitude of new data that would come from a survey
of smaller NEOs may require significant increases in computing
capabilities and personnel at the MPC for managing such data.
Questions include the following: What would be the increased
personnel, computational, and funding requirements for the
increased data rate that would result from extending the survey
to smaller objects? Would the MPC be able to handle the volume
of data from proposed NEO survey telescopes like the Large-Aperture
Synoptic Survey Telescope (NSF) and the "Pan-Starrs"
Panoramic Optical Imager (Air Force)?
Recent Impacts and Near-Misses: In early January of this year
(2002), an asteroid designated as 2001 YB5 passed the Earth
at a distance of 510,000 miles, less than twice the distance
of the Moon. It is estimated to be several hundred meters
in size, which is large enough to destroy an entire country
the size of England. (Asteroids of about a kilometer in size
could wipe out life on the entire planet.) The asteroid was
discovered only one month earlier by the NEAT (Near Earth
Asteroid Tracking) telescope at Mount Palomar. At present,
nothing could have been done to avert it if the asteroid had
been found to be on a collision course with the Earth. Another
asteroid, 2002 EM7, passed the Earth at roughly the distance
of the Moon on March 8th of this year, but was not detected
until March 12th after it moved out of the Sun's glare. More
recently, asteroid 2002 MN, a football-field sized object,
passed by Earth at only one-third the distance to the Moon.
Such discoveries are stark reminders of the possibility of
impacts, but they also signify the importance of performing
the NEO survey. It is expected that many of these discoveries
will occur after the object has passed by the Earth. The current
survey picks up some of these smaller objects, but a complete
survey of such objects will require an extension of the survey
goals, capabilities, and support. There are other impacts
of note within the last decade. In 1994, for example, Comet
Shoemaker-Levy 9 collided with Jupiter in a spectacular display.
Expert Recommendations for NEO Survey Strategies:
The critical issue is that there is no current unified, cohesive
federal vision and plan for future NEO surveys and responses.
As a result, multiple independent proposals involving different
telescopes, technologies, and agencies are under consideration
(see below). If all are pursued independently, these different
approaches may result in unnecessary duplication of effort.
A more integrated and coordinated program may result in a
more effective use of these assets. These differing ideas,
discussed below, will be discussed and debated at the hearing.
Astronomy/Solar System science: The recent National Research
Council decadal survey report on solar system exploration,
"New Frontiers in the Solar System: An Integrated Exploration
Strategy," includes extensive analyses and recommendations
regarding the survey and study of Near Earth Objects. Their
primary recommendation is for NASA and the National Science
Foundation to contribute equally to the construction and operation
of a new "Large-Aperture Synoptic Survey Telescope"
(LSST) to efficiently survey all NEOs down to a size of 300
meters. The LSST would be a very sensitive and efficient instrument
for surveying the entire sky quickly and regularly for both
small and large NEOs. The telescope would serve a dual-use
function as it would also serve as an instrument for other
astronomy surveys.
Military Community: Brigadier General Pete Worden,
Deputy Director for Space Operations of the U.S. Strategic
Command, has suggested that the U.S. military could play a
greater role in future NEO strategy. At present the U.S. Air
Force already contributes some search instruments to NASA-directed
survey projects such as the Lincoln Near Earth Asteroid Research
Project (LINEAR) at the White Sands Missile Range in New Mexico.
Worden proposes that future military surveillance systems
could make a valuable contribution to the NEO survey. The
Air Force is also developing the Panoramic Optical Imager
(Pan Starrs) telescope facility in Hawaii that could be operational
in four years and could potentially search the entire sky
every few days, detecting objects nearly 100 times fainter
than the best existing NEO search telescopes. However, as
discussed above, the science-based LSST is also proposed as
an efficient and sensitive instrument for full-sky asteroid
surveys. One emerging questions is whether both telescopes
(or other alternatives) are needed for NEO surveys. In either
case, data from the surveys would need to be quickly accessible
to the scientific community. In addition to supporting surveys,
the military could possibly develop mitigation strategies
should a threatening Near-Earth Object be detected. Clearly,
such plans would need to be made in advance of any such discovery
or close approach. General Worden also warns that NEOs that
explode in the Earth's atmosphere several times every year
could be mistaken for a nuclear detonation in times of international
tension, triggering an unwarranted response. Data from NEO
explosions detected by U.S. military surveillance systems
could potentially be quickly shared with affected nations
if an appropriate warning center is developed.
NASA: Current U.S. NEO survey efforts are funded
and coordinated through NASA. Such efforts include primarily
the LINEAR and Near-Earth Asteroid Tracking (NEAT) projects
using Air Force telescopes. The resulting survey data are
handled by the Minor Planet Center of the Smithsonian Astrophysical
Observatory (see below). Nearly 1800 Near-Earth Asteroids
have been discovered (Figure 2). If the NEO survey is extended
to comprehensively include objects smaller than one kilometer,
larger telescopes and augmented data management resources
will be needed. NASA would also be likely to take the lead
should it be determined that a satellite-based telescope is
best-suited for future NEO surveys. NASA is also best-suited
for detailed studies of the composition of threatening asteroids;
this is pertinent to plans for any type of mitigating response.
Dr. David Morrison, Senior Scientist, NASA Ames
Research Center, has been asked to address the following questions:
What are the hazards we face from Near-Earth Objects? How
does that threat depend upon the size of the objects, and
what is the likelihood of an impact that is dangerous for
life on Earth? What is the justification for the current Spaceguard
survey goal of finding 90 percent of objects larger than one
kilometer by 2008? What are the benefits and challenges of
extending the survey to comprehensively include smaller objects
of a few hundred meters in size?
Dr. Edward Weiler, NASA Associate Administrator
for Space Science, has been asked to address the following
questions: How is NASA currently carrying out their mandate
to conduct a comprehensive survey of Near-Earth Objects? What
is the status of meeting the "Spaceguard Goal" for
finding 90 percent of all NEOs larger than one kilometer by
2008? What roles can NASA best fill in future NEO activities
such as surveys, scientific studies, data management, and
planning for possible mitigation of a threat?
Dr. Joseph Burns, Irving Porter Church Professor
of Engineering and Astronomy, Cornell University, has been
asked to address the following questions: What are the recommendations
of the recent decadal survey reports from the National Academy
of Sciences regarding the future of NEO surveys? Why did the
Solar System Exploration decadal survey report recommend that
NASA and the National Science Foundation partner equally to
design, build, and operate a survey telescope such as the
Large-Aperture Synoptic Survey Telescope (LSST) for surveys
of NEOs? How do agency roles and cooperation impact the work
of astronomers conducting the NEO survey?
Dr. Brian Marsden, Director, Minor Planet Center,
Smithsonian Astrophysical Observatory, has been asked to address
the following questions: What role does the Minor Planet Center
play in the NEO survey? What is the role of amateur astronomers
in discovery and tracking of NEOs? How do awards such as those
offered in the "Pete Conrad" bill (H.R. 5303) encourage
amateur contributions toward NEO observations? What challenges
for data management would result from the large increase in
data if the NEO survey is extended to include smaller, more
numerous objects?
Brigadier General Simon "Pete" Worden,
U.S. Air Force, has been asked to address the following questions:
What is the current role of the U.S. Air Force in surveys
of Near-Earth Objects? What is your perspective on the threat
NEOs present to national security? What future military surveillance
systems could efficiently search the sky for NEOs? What issues,
such as restrictions on data release, would need to be addressed
if the U.S. Air Force were to conduct NEO surveys or to serve
as a clearinghouse for such data? What could the role of the
military be in planning mitigation efforts should a threatening
object be discovered? (Note: General Worden is representing
his own personal views as a military leader and an expert
on military surveillance and Near-Earth Objects. His views
are not necessarily those of the U.S. Air Force.)
Mr. Chairman and Members of the Subcommittee:
It is an honor to return to this committee almost
ten years after my first
appearance in 1993. At that time I presented the conclusions
of the NASA
workshop that proposed a Spaceguard Survey to search for potentially
threatening asteroids large enough to endanger civilization.
Ten years ago
there was very little recognition or support outside this
committee for
dealing with the asteroid impact hazard. I could not have
predicted then
that by 2002 we would already be past the halfway mark in
discovering these
large Earth approaching asteroids. Thanks to the Spaceguard
Survey, we can
now assert that we have reduced the risk from an unforeseen
catastrophic
impact by more than a factor of two. This is a notable achievement
in an
effort to protect humanity from the worst known class of natural
disasters.
The nature of this risk was stated well by this
Committee in 1991, when you
wrote: "The chances of the Earth being struck by a large
asteroid are
extremely small, but since the consequences of such a collision
are
extremely large, the Committee believes it is only prudent
to assess the
nature of the threat and prepare to deal with it. We have
the technology to
detect such asteroids and to prevent their collision with
the Earth."
It is only during the past decade that we have
come to appreciate that
impacts by asteroids and comets (often called Near Earth Objects,
or NEOs)
pose a significant hazard to life and property. Comet impacts
constitute
only about 10% of the risk, so the focus of my remarks is
on the more common
impacts by Near Earth Asteroids, or NEAs. The most catastrophic
of these are
the extinction level events that can create a severe global
environmental
disaster. The impact of an asteroid about 10 miles in diameter
(as large as
the Washington beltway) 65 million years ago not only ended
the existence of
the dinosaurs, it wiped out more than 99% of all life on Earth.
Fortunately
for us, such mass extinction events are extremely rare. We
can already state
with assurance that there are no asteroids this large with
orbits that could
pose a threat to us. We are safe (for the present) from such
impacts, but
not from the smaller NEAs that actually dominate the current
risk.
The greatest risk today is associated with NEAs
large enough to perturb the
Earth's climate on a global scale by injecting large quantities
of dust into
the stratosphere. These are not extinction level impacts,
but they are still
large enough to temporarily depress temperatures around the
globe, leading
to massive loss of food crops and possible breakdown of society.
Various
studies have suggested that the minimum mass impacting body
to produce such
global consequences is several tens of billions of tons, resulting
in a
ground burst explosion with energy in the vicinity of a million
megatons of
TNT -- many times greater than the sum off all the world's
nuclear
stockpiles. The corresponding threshold diameter for NEAs
is between 1 and 2
km, or roughly one mile in diameter. Current investigation,
including the
Spaceguard Survey, focuses on these global threats. It is
entirely
appropriate that we deal first with the worst danger, even
though the
probability of an impact in this class is exceedingly small.
After NEAs that are large enough to risk a global
catastrophe, we naturally
turn our attention to smaller impacts that never-the-less
would be capable
of destruction on a vast scale, killing tens of millions of
people. These
are impacts by NEAs less than 1 km in diameter, but still
large enough to
devastate a large region. Such sub-kilometer NEAs are most
dangerous, in
fact, if they strike in the oceans. The resulting tidal wave
or tsunami is
an effective way to carry the energy of the collision to large
distances
from the point of impact. The tsunami from the ocean impact
of a NEA 500 m
in diameter could inundate many coastal cities in a single
event. While not
posing as great a risk as the global scale impact from NEAs
more than 1 km
in diameter, the danger from such ocean impacts may eventually
be judged
great enough to warrant action.
At even smaller sizes, NEA impact can still
do a great deal of damage on a
local scale. We have witnessed one example of such a small
impact, which
took place in Siberia in 1908. The energy of the explosion
was about 15
megatons, and it destroyed more than 1000 square miles of
forest. However,
such impacts actually pose a much smaller risk than many other
natural
disasters, such as earthquakes and hurricanes.
It is fortunate for us that the greatest danger
is posed by the largest
NEAs, which are the easiest to discover. We are finding these
at a rate that
will allow us to retire that risk within a few more years
(unless we find
that one of these objects is on a collision course with Earth).
As discovery
techniques improve, we can shift our search toward smaller
NEAs. How far to
go depends on analysis of the costs and benefits of a particular
defense
scheme.
Based on our recent observations, astronomers
have concluded that there are
between 900 and 1300 NEAs larger than 1 km that could potentially
pose a
threat. We can estimate the risk we each run from these impacts,
which is
about 1 in a million per year. This is similar to the risk
of one round-trip
commercial air flight. The risk from smaller impacts is less,
roughly one in
ten million. These are all very low numbers. The asteroid
impact hazard is
an extreme example of a risk of very low probability but potentially
catastrophic consequences.
Much effort has gone into estimating the statistical
frequency of impacts
and evaluating their consequences. However, from a policy
perspective we do
not need precise estimates of either the frequency of impacts
or their
consequences. We recognize that the actual risk is not statistical;
if there
is any sizable asteroid on a collision course with the Earth,
it can be
found and the impact predicted decades (or more) in advance.
Our objective
should be to find any large impactor far in advance, and thus
provide
decision-makers with options for dealing with the threat and
defending our
planet from a cosmic catastrophe. That is the purpose of the
Spaceguard
Survey.
Half-a-dozen specially designed telescopes today
are contributing to the
Spaceguard Survey. As mandated by this Committee in 1995,
the objective of
the Spaceguard Survey is to find the NEAs larger than 1 km
in diameter --
that is, to find any with the potential for global catastrophe
if they
collided with Earth. Specifically the Spaceguard goal is to
find 90% of
these NEAs by the end of 2008. The philosophy of Spaceguard
is to monitor a
large volume of space around the Earth using automated wide-field
optical
telescopes with advanced detectors and computational capability.
Any
asteroid that could hit the Earth will repeatedly pass close
to our planet,
providing plenty of opportunity for discovery. Once the NEA
is discovered,
its orbit is computed and its position is predicted for many
decades in
advance. Such long-term predictions are possible because the
solar system is
actually a very well behaved place; asteroids do not alter
their orbits
capriciously. If there is the possibility of collision in
the future, we
expect to have decades or even centuries of advanced warning.
Note that Spaceguard is not a last-minute warning
system that attempts to
find incoming objects on their final plunge toward impact.
Such a system
would be more complex and expensive than the current approach,
and the few
days or hours of warning it might provide would be insufficient
to take
defensive action in any case. The Spaceguard approach of cataloging
all
potentially dangerous NEAs is cost-effective and will yield
the long lead
times needed to effectively mitigate any future impacts.
The current Spaceguard program is focused on
the NEAs that pose the greatest
risk. Today the Spaceguard telescopes are finding many NEAs
smaller than 1
km, but the level of completeness for such sub-kilometer asteroids
is rather
low. A logical next target might be NEAs in the range of 200-300
m diameter,
since these pose the greatest tsunami danger. (Below this
size, the total
risk is much smaller.) Approximately 50,000 NEAs exist larger
than 300 m in
diameter, so the technical challenge is substantial. However,
the exact
target size, if any, could be above or below this range, and
will need to be
the subject of broad discussion within and outside the science
community.
Data from the existing Spaceguard Survey, as well as numerical
simulations,
will provide us with the information we need to make informed
choices about
future search goals. Once the target size is known, search
strategies and
requirements for smaller asteroids would need to be subject
to trade studies
and external review to ensure that we are getting the most
effective survey
possible for our investment.
We are the first generation of humans that both
appreciates the long-term
threat of cosmic impacts and has the technological capability
to deal with
it. However, this is one of many natural hazards that we face,
and I believe
that the costs as well as the effectiveness of the surveys
need to be
considered in the allocation of resources to deal with this
hazard.
The search for NEAs is a little like taking
out fire insurance for your
home. You do not expect your home to burn. The great majority
of us will
never experience a fire. Yet we buy insurance to protect against
even such
an unlikely event, because our homes are too valuable to lose.
In a similar
way, we undertake the Spaceguard Survey, not because we expect
an impact
within our lifetimes, but because the consequences of an impact
would be too
horrendous to be acceptable.
Mr. Chairman and Members of the Subcommittee:
It is a privilege to be here
today and report to you on the progress of NASA's Near Earth
Object (NEO)
search effort. In addition to identifying NEOs, this program
is also focused
on determining the shapes, densities, internal structures
and compositions
of the NEOs and their parent population, the main-belt asteroids.
I will
also share with you my views on the future role of NASA with
respect to
exploration of these bodies.
NASA's NEO Program makes ground-based observations
with the goal of
identifying 90 percent of those NEOs that are 1 km or larger
and
characterizing a sample of them. This is a ten-year program,
which began in
1998 and should be completed in 2008. (It should be noted
that NASA had
begun searching for NEOs many years before this program officially
started.)
The threshold size for an asteroid striking
the Earth to produce a global
catastrophe is 1 km in diameter. NASA has an active program
to detect such
objects that could potentially strike the Earth and to identify
their
orbits. The best current estimates are that the total population
of NEOs
with diameters larger than 1 km is about 1000. The 1-km diameter
limit for
an NEO was set after extensive discussions within the scientific
community
to determine the size of an object that would likely threaten
civilization.
This community consensus is codified in the Spaceguard Report
and in the
Shoemaker Report. For comparison, the object that likely caused
the
extinction of the dinosaurs was in the 5-10 km range. The
current survey of
NEOs in that range is considered complete.
As of the end of September, NASA has detected
619 NEOs with diameters larger
than 1 km. We are currently discovering about 100 per year.
At the present
time, we have six groups which are funded by NASA's Near Earth
Objects
program to conduct this type of research. These groups, selected
though peer
review, have ten telescopes among them searching for NEOs.
One of these
groups just completed (and another one is about to complete)
major upgrades
to its facility; therefore, we expect this pace of discovery
to continue, if
not increase. In some cases, the search programs are not able
to obtain the
number of observations required to determine the orbit elements
of certain
objects to sufficient accuracy to fully characterize the orbital
parameters.
These objects require additional astrometric observations,
commonly called
"follow-up observations." We have also funded four
investigations to obtain
astrometric follow-up observations of those objects that cannot
be easily
followed by the primary search programs.
Now, how well are we doing? I am happy to report
that we are doing quite
well; in fact, we are even a bit ahead of schedule. The graph
below shows
the discovery of NEOs over time and also the upper and lower
boundaries of
the likely population of NEOs with diameters larger than 1
km.
There have been various reports to the effect
that NASA would not reach its
metric - 90 percent of all the NEOs with diameters larger
than 1 km - until
many years after the end of 2008. However, these analyses
have been based on
the performance of individual search efforts, and they have
tended not to
use the current performance of the NEO search effort as a
whole. As with
most things, experience increases proficiency; therefore,
we expect the rate
of detection to increase. Even if we were to stay at our current
rate,
however, we are more than halfway to our goal of 90 percent
by the end of
2008.
That does not mean we will grow complacent;
we intend to continue to
vigorously pursue detection of NEOs. In fact, we anticipate
even better
results due to technological developments such as better detector
arrays,
migration of existing search efforts to larger telescopes,
and additional
telescopes dedicated to the search program. In short, we are
working to
achieve both our goal and our metric and expect to be successful
at both.
One unanticipated result of the NEO search will be a list
of over 1,000
potential candidates for future space science missions.
Next I would like to turn to another question.
What should NASA's role be in
the future? NASA is a space agency. While we are proud of
our success in
implementing the Congress's direction to us with regard to
the search for
NEOs, we do not feel that we should play a role in any follow-on
search and
cataloging effort unless that effort needs to be specifically
space-based in
nature. There are other agencies with far more expertise in
ground-based
observations that would be more suitable candidates to lead
that portion of
a future NEO endeavor.
NASA does, however, continue to have a large
role to play in the scientific
space exploration of asteroids. The frequent access to space
for small
missions offered by NASA's Discovery Program has benefited
the study of
asteroids and comets as no other program to date. The first
in-depth study
of an NEO, Eros, was performed by the NEAR-Shoemaker mission.
The body of
data returned by NEAR-Shoemaker was so large, and the quality
of the data so
high, that NEAR's database will require years of analysis.
Just this year,
we initiated funding for the first 17 investigations of that
data.
NEAR-Shoemaker's exploration of Eros will be followed by detailed
exploration of two other asteroids, Vesta and Ceres, by the
upcoming DAWN
mission, currently scheduled to launch in 2006. There is no
reason to expect
that science-driven exploration of the asteroids, and of course
NEOs, will
not continue through the Discovery program. We believe that
the critical
measurements required for developing potential mitigation
efforts are
substantially the same as those required to achieve the pure
scientific
goals identified for these objects. We must be able to understand
and
characterize these objects before any mitigation efforts are
even
considered.
In addition to NEAR and DAWN, NASA has several
other missions dedicated to
studying comets and asteroids, such as Deep Impact and Stardust.
Our total
investment in understanding these bodies, both in the past
and in our
current FY 2003 budget run-out, is approximately $1.6 billion.
That does not
even take into account those spacecraft that have provided
"bonus"
information, such as Galileo, which found a moon orbiting
asteroid Ida, and
Deep Space 1, a technology demonstration mission that performed
a close-up
fly-by of comet Borelly. NASA deeply regrets not having the
potential
discoveries from the recently failed CONTOUR mission, which
was to have
studied Comets Encke and Schwassmann-Wachmann 3.
NASA's bold new technology initiatives, the
In-Space Propulsion (ISP)
Initiative and the Nuclear Systems Initiative (NSI), together
offer new
opportunities to enable capable new missions to NEOs early
in the next
decade. Improvements in solar-electric propulsion and development
of solar
sails are examples of new capabilities that might allow a
spacecraft like
NEAR-Shoemaker to visit many NEOs during a single mission
rather than just
one (and at the cost of a Discovery mission). If we are ever
faced with the
requirement to modify the motion of an NEO over time to ensure
that the
object will not come close to the Earth, nuclear propulsion
may very well be
the answer. The Nuclear Systems Initiative could address two
elements in
understanding the potential hazards of NEOs by: (1) providing
technologies
that could significantly increase our ability to identify
and track NEOs,
and (2) to possibly - in the future - provide sufficient power
to move an
Earth-intersecting object. The NSI could enable power and
propulsion for an
extended survey (in one mission) of multiple NEOs to determine
their
composition, which is a critical factor in understanding how
to mitigate the
risk of an Earth-intersecting object. In the future, the technologies
under
development by the NSI could provide us with the means to
redirect the path
of an Earth-intersecting asteroid, once we understand the
orbital mechanics
of these objects sufficiently to understand how to do this.
These programs
are being developed to serve a wide range of needs across
NASA, but they
will most certainly prove beneficial for space missions that
help us to
better understand and characterize NEOs.
I feel that it is premature to consider an extension
of our current national
program to include a complete search for smaller-sized NEOs.
There are
several reasons for this belief. The first is that we need
to have a better
understanding of the true size of the population down to at
least 100 m. How
will we get the improved data we need on this population?
We will obtain the
necessary data from the existing NASA search effort for NEOs.
The search
program now finds about two NEOs with diameters less than
1 km for every
large one (diameter greater than 1 km) that we find. In addition,
we are
supporting a search program which is optimized to detect smaller
NEOs. We
expect by the end of this decade to have a much better picture
of the true
size of the population, and hence, what will be required to
detect all of
them.
The second issue is how such a search could
be most efficiently and
cost-effectively implemented. Two groups that wish to build
large survey
systems have argued that the search goal should be extended
to 300 m. NASA
has at least two concerns with this proposition. First, we
do not possess a
non-advocate trade study to tell us how best to do such a
search. For
example, one issue to be addressed is whether it would be
better to build
one large 8-m class telescope or 2 4-m search telescopes.
At these sizes, is
a space-based system an option? Second, why 300m? The present
limiting
diameter of 1 km was the product of a broad public discussion.
When we have
another broad public discussion, the answer could be: "Leave
the present
limiting diameter as it stands." Or, perhaps the result
of broad national
debate on this issue would be: "Catalog the population
down to 100 m." We at
NASA don't know the answers to these questions, and we believe
that further
commitments to extend the search are simply premature at this
point.
Within the Office of Space Science, the Solar
System Exploration Division
Director has appointed a small Science Definition Team (SDT)
to consider the
technical issues related to extending the search for NEOs
to smaller sizes.
The goal of the SDT is to evaluate what is technologically
possible today.
The scope of the SDT does not include consideration of any
change to our
present NEO search goal.
NASA has made impressive strides in achieving
its goal of cataloging 90
percent of all Near-Earth Objects with diameters of more than
1 km and
characterizing a sample of them. We are currently ahead of
schedule with
respect to having this effort completed in the 2008 time frame.
While NASA
certainly agrees that because these objects pose a potential
threat to the
Earth, they should be studied and understood, we respectfully
defend our
position that any expansion of NASA's current NEO effort is
premature.
Before any further effort is undertaken, we would want input
from the
scientific community as to how this subject should be approached,
and if
indeed NASA is even the proper agency to lead this type of
an undertaking. I
will be happy to expand on any of these thoughts during the
question-and-answer period. Thank you, Mr. Chairman and Members
of the
Subcommittee.
Mr. Chairman, Ranking Minority Member, and members
of the subcommittee:
thank you for inviting me to testify on behalf of the National
Academies'
Solar System Exploration Survey. My name is Joseph Burns,
and I am Irving
Porter Church Professor of Engineering and Professor of Astronomy
at Cornell
University. I appear today in my capacity as a steering group
member of the
Solar System Exploration (SSE) Survey, and as a former chair
of the National
Research Council's Committee on Planetary and Lunar Exploration
(COMPLEX). I
was also a member of the Astronomy & Astrophysics Survey's
panel on
Ultraviolet and Infrared Astronomy from Space.
As you know, the Astronomy and Astrophysics
community has a long history of
creating, through the National Research Council (NRC), decadal
surveys of
their field. These surveys lay out the community's research
goals for the
next decade, identify key questions that need to be answered,
and propose
new facilities with which to conduct this fundamental research.
In April 2001, NASA Associate Administrator
for Space Science Edward Weiler
asked the NRC to conduct a similar survey for planetary exploration.
Our
report, New Frontiers in the Solar System, is the result of
that activity.
The Solar System Exploration Survey was conducted by an ad
hoc committee of
the Space Studies Board (SSB), overseen by COMPLEX. This committee
was
comprised of some 50 scientists, drawn from a diverse set
of institutions,
research areas, and backgrounds; it also received input from
more than 300
colleagues. The SSE Survey had four subpanels which focused
on issues
pertaining to different types of solar system bodies (Inner
Planets, Giant
Planets, Large Satellites, and Primitive Bodies) and received
direct input
from COMPLEX on Mars issues and from the Committee on the
Origins and
Evolution of Life on issues pertaining to Astrobiology.
New Frontiers in the Solar System (the Executive
Summary is appended to this
statement) recommends a scientific and exploration strategy
for NASA's
Office of Space Science that will both enable dramatic new
discoveries in
this decade and position the agency to continue to make such
discoveries
well into the future. Your invitation indicated that I should
focus on the
conclusions that the SSE Survey reached in the area of Near-Earth
Objects
(NEOs).
The SSE Survey's charge from NASA included a
request to summarize the extent
of our current understanding of the solar system. This task
was delegated
to the subpanels, which in the particular case of NEOs was
handled by the
Primitive Bodies Panel.
Scientifically, the history of impacts on the
Earth is vital for
understanding how the planet evolved and how life arose. For
example, it
has been suggested that a majority of the water on this planet
was delivered
by comet impacts. A better known example of the role of impacts
is the
Cretaceous-Tertiary event that led to global mass extinctions,
including
that of the dinosaurs. Another case is the 20 megaton (MT)
equivalent-energy
explosion that devastated 2000 square-kilometers of pine forest
in the
Siberian tundra in 1908. The SSE Survey identifies the exploration
of the
terrestrial space environment with regards to potential hazards
as a new
goal for the nation's solar system exploration enterprise.
Current surveys have identified an estimated
50 percent of NEOs that have a
diameter of 1 kilometer or greater and approximately 10-15
percent of
objects between 0.5 and 1 km. The vast majority of these latter
objects have
yet to be discovered, but a statistical analysis indicates
a 1% probability
of impact by a 300-m body in the next century. Such an object
would deliver
1000 MT of energy, cause regional devastation, and (assuming
an average of
10 people per square-kilometer on Earth) result in 100,000
fatalities. The
damage caused by an impact near a city or into a coastal ocean
would be
orders of magnitude higher. As of a year ago, 340 objects
larger than a
kilometer had been catalogued as Potentially Hazardous Asteroids.
In
addition, the number of undiscovered comets with impact potential
is large
and unknown.
"Important scientific goals are associated with the NEO
populations,
including their origin, fragmentation and dynamical histories,
and
compositions and differentiation. These and other scientific
issues are
also vital to the mitigation of the impact hazard (emphasis
added), as
methods of deflection of objects potentially on course for
an impact with
Earth are explored. Information especially relevant to hazard
mitigation
includes knowledge of the internal structures of near-Earth
asteroids and
comets, their degree of fracture and the presence of large
core pieces, the
fractal dimensions of their structures, and their degree of
cohesion or
friction."
While almost all of the SSE Survey's recommendations
involved NASA flight
missions, the Primitive Bodies subpanel recommended that ground-based
telescopes be used to do a majority of the study of NEOs,
supplemented by
airborne and orbital telescopes.
A survey for NEOs demands an exacting observational
strategy. To locate NEOs
as small as 300 m requires a survey down to 24th magnitude
(16 million times
fainter than the feeblest stars that are visible to the naked
eye). If
images are to be taken every 10 sec to allow the sky to be
studied often,
the necessary capability is almost 100 times better than that
of existing
survey telescopes. NEOs spend only a fraction of each orbit
in Earth's
neighborhood, where they are most easily seen. Repeated observations
over a
decade would be required to explore the full volume of space
populated by
these objects. Such a survey would identify several hundred
NEOs per night
and obtain astrometric (positional) measurements on the much
larger (and
growing) number of NEOs that it had already discovered. Precise
astrometry
is needed to determine the orbital parameters of the NEOs
and to assign a
hazard assessment to each object. Astrometry at monthly intervals
would
ensure against losing track of these fast-moving objects in
the months and
years after discovery.
In its most recent decadal survey, the Astronomy and Astrophysics
community
selected the proposed Large-aperture Synoptic Survey Telescope
(LSST) as
their third major ground-based priority. In addition, our
SSE Survey chose
LSST to be its top-ranked ground-based facility. Telescopes
like HST and
Keck peer at selected, very localized regions of the sky or
study individual
sources with high sensitivity. However, another type of telescope
is needed
to survey the entire sky relatively quickly, so that periodic
maps can be
constructed that will reveal not only the positions of target
sources, but
their time variability as well. The Large-aperture Synoptic
Survey
Telescope is a 6.5-m-effective-diameter, very wide field (~3
deg) telescope
that will produce a digital map of the visible sky every week.
For this type
of survey observation, the LSST will be a hundred times more
powerful than
the Keck telescopes, the world's largest at present. Not only
will LSST
carry out an optical survey of the sky far deeper than any
previous survey,
but also -just as importantly-- it will also add the new dimension
of time
and thereby open up a new realm of discovery. By surveying
the sky each
month for over a decade, LSST would revolutionize our understanding
of
various topics in astronomy concerning objects whose brightnesses
vary on
time scales of days to years. NEOs, which drift across a largely
unchanging
sky, are easily identified. The LSST could locate 90 percent
of all
near-Earth objects down to 300 m in size, enable computations
of their
orbits, and permit assessment of their threat to Earth. In
addition, this
facility could be used to discover and track objects in the
Kuiper Belt, a
largely unexplored, primordial component of our solar system.
It would
discover and monitor a wide variety of variable objects, such
as the optical
afterglows of gamma-ray bursts. In addition, it would find
approximately
100,000 supernovae per year, and be useful for many other
cosmological
observations.
The detectors of choice for the temporal monitoring
tasks would be thinned
charge-coupled devices (CCDs); the requisite extrapolation
from existing
systems should constitute only a small technological risk.
An infrared
capability of a comparably wide field would be considerably
more challenging
but could evolve as the second phase of the telescope's operation.
Instrumentation for LSST would be an ideal way to involve
independent
observatories with this basically public facility.
Historically, the National Science Foundation
(NSF) has built and operated
ground-based telescopes, whereas NASA has done the same for
space-based
observatories. Although the Astronomy and Astrophysics Survey
was
noncommittal on who should build the LSST, the SSE Survey
included a
recommendation that NASA share equally with NSF in the telescope's
construction and operations costs.
Such an arrangement has precedent. The SSE Survey
noted that "NASA continues to play a major role in supporting
the use of Earth-based optical telescopes for planetary studies.
It funds the complete operations of the IRTF (InfraRed Telescope
Facility), a 3-m diameter telescope located on Hawaii's Mauna
Kea. In return for access to 50 percent of the observing time
for non-solar-system observations, the NSF supports the development
of IRTF's instrumentation. This telescope has provided vital
data in support of flight missions and will continue to do
so. As another example, NASA currently buys one-sixth of the
observing time on the privately operated Keck 10-m telescopes.
This time was purchased to test interferometric techniques
in support of future spaceflight missions such as SIM (Space
Interferometry Mission) and TPF (Terrestrial Planet Finder)."
The solar system exploration community is concerned
that the NSF is often
unwilling to fund solar system research. This is particularly
unfortunate
given NSF's charter to support the best science and its leadership
role in
other aspects of ground-based astronomy.
The shared responsibility between NASA and the
NSF that we recommend is also
endorsed by the more general findings last year of the NRC's
Committee on
the Organization and Management of Research in Astronomy and
Astrophysics
(COMRAA), chaired by Norman Augustine. COMRAA's report recommended
that NASA continue to "support critical ground-based
facilities and
scientifically enabling precursor and follow-up observations
that are
essential to the success of space missions." COMRAA also
noted that in 1980
the NSF provided most of the research grants in astronomy
and astrophysics,
but today NASA is the major supporter of such research.
The roles of the agencies also affect the ability
of scientists to conduct a
census of Near-Earth Objects. The SSE Survey commented that:
"interestingly enough, NASA has no systematic
survey-capability to discover
the population distribution of the solar-system bodies. To
do this, NASA
relies on research grants to individual observers who must
gain access to
their own facilities. The large NEOs are being efficiently
discovered using
small telescopes for which NASA provides instrumentation funding,
but all
the other solar system populations-e.g., comets, Centaurs,
satellites of the
outer planets, and Kuiper Belt Objects-are being characterized
almost
entirely using non-NASA facilities. This is a major deficiency..."
The construction of the LSST would provide a
central, federally sponsored
location for such research.
LSST Costs and Survey Below 300 Meters
The costs of the LSST are projected by the 2001 Astronomy
and Astrophysics
Survey as being $83 million for capital construction and $42
million for
data processing and distribution for 5 years of operation,
for a total cost
of $125 million. Routine operating costs, including a technical
and support
staff of 20 people, are estimated at approximately $3 million
per year. The
LSST will be able to routinely discover and characterize NEOs
down to 300 m
in diameter. Increasing the sensitivity of the survey to 100
m would mean
increasing the sensitivity of the telescope by a factor of
ten. This may
represent a "beyond the state-of-the-art" challenge
to telescope builder,
and certainly a much larger telescope - 3 times the LSST and
probably 10 to
100 times the cost unless innovative designs are found. The
number of
discovered objects would correspondingly increase substantially;
this large
data set may challenge current capabilities.
Concluding Thoughts
By way of summary, let me place the LSST into the context
of a robust
scientific program. Systematically building an inventory of
the Near-Earth
Objects is crucial to an improved understanding of Earth's
environment,
especially to the prediction of future hazards posed to our
species. It is
also a necessary first step towards a rational program of
NASA's exploration
of these bodies with spacecraft: many of the most interesting
targets may
remain, as yet, undiscovered. The ability to create and play
a "motion
picture" of the night sky will also provide new insights
in a wide variety
of disciplines from cosmology to astrophysics to solar system
exploration.
A suitable analog might be the deepened knowledge that is
obtained from
dynamic movies of swirling clouds and weather patterns, as
compared to an
occasional static photo.
The immense volume of data from the LSST would
provide a reservoir of
information for numerous graduate students and researchers,
as well as
established scientists. Further, LSST will support flight
missions - for
example, identifying possible fly-by targets for a spacecraft
mission to
explore the Kuiper Belt. All in all, the SSE Survey committee
believes that
broad areas of planetary science, particularly NEO studies,
would benefit
very substantially from the construction of the LSST for a
relatively small
investment.
Thank you again, Mr. Chairman, for the opportunity
to appear before the
subcommittee today. I would be glad to answer any questions
that you or
your subcommittee members may have.
Accurate measurements of the positions of asteroids
and comets, including
known and candidate NEOs, are received by the Minor Planet
Center (located
at the Smithsonian Astrophysical Observatory in Cambridge,
Masachusetts)
many times a day in e-mail messages from up to perhaps 150
observatories
(both professional and amateur) around the world. Although
something like
half a million observations are received every month, it is
important to
appreciate that NEOs comprise only between 0.1 and 1 percent
of the
observations of asteroids as a whole, almost all of which
are confined at
quite safe distances from the earth in what is termed the
"main belt"
between the orbits of Mars and Jupiter. Particularly when
they are near the
earth, NEOs are usually recognizable by the fact that their
apparent motions
across the sky are greater than those of the main-belt asteroids,
although
when they are farther away (and, of course, fainter), the
sky motions of
NEOs and main-belt asteroids can be comparable and therefore
not easily
distinguishable.
The principal programs in the world for surveys
for new NEOs are the ones
bearing the acronyms LINEAR and NEAT (programs based in Massachusetts
and
California, respectively, that are largely funded by NASA
but use USAF
telescopes in New Mexico and Hawaii, the latter also in conjunction
with a
non-USAF telescope on Palomar Mountain in California), as
well as three
programs (also largely funded by NASA) using telescopes in
Arizona. Data
from these programs represent well over 80 percent of the
observations
received at the Minor Planet Center, where they generally
arrive during the
afternoon after the images were exposed. On its most productive
nights
LINEAR might record as many as 15,000 different objects, in
which case the
data may not reach the Minor Planet Center until evening.
With typically
from three to five observations of each object made over the
course of 30-60
minutes the objects with the more unusual apparent motions
can readily be
picked out (usually by the observers themselves), and calculations
are then
made at the Minor Planet Center, first to check whether these
objects are
already known, and if not known, to identify those that seem
most likely to
be NEOs. Within 15-30 minutes of the receipt of the data,
the Minor Planet
Center is then able to place predictions of the likely sky
positions (for
the next day or so ) of the best NEO candidates in the WWW
on what is known
as "The NEO Confirmation Page" .Observers around
the world regularly check
this webpage. Since afternoon in Massachusetts is already
evening in Europe,
it is sometimes then a matter of less than an hour before
the Minor Planet
Center receives confirmatory observations of the NEOs from
observatories in
Europe, at which point the orbit calculation can be refined
and an improved
prediction posted on the webpage well before it is night-time
in the U.S.
and further observations can be made from there. Those new
U.S. observations
will frequently include both further deliberate observations
of the
candidate NEOs and more accidental observations of the same
objects by the
survey programs that will come to light when the Minor Planet
Center
examines the next night's data from those programs.
With three separate groups of observations (
the discovery data from LINEAR
or NEAT, then ideally confirmatory data from Europe and follow-up
data from
North America the night after the discovery), it is usually
possible to
derive a moderately good estimate of the real orbit of an
NEO, and at this
point a unique designation is given to the object (the year,
two letters and
sometimes additional numerals), and all the relevant information
(including
appropriate credit to the observers) is collected and published
on an
official Minor Planet Electronic Circular, which is both distributed
by
e-mail and made accessible on the WWW. At the same time, the
prediction on
The NEO Confirmation Page is removed, in order to make way
for further
entries. At any given time, there might be as many as 20 or
30 NEO
candidates awaiting confirmation, but by pruning the list
there is more
chance that the follow-up observers will concentrate on the
objects most in
need of attention. Of course, further refinement of the NEO
orbits is still
very necessary using observations made during the weeks (and
also the years)
after discovery, and the Minor Planet Center routinely disseminates
this
additional information in a "Daily Orbit Update"
Electronic Circular that is
prepared automatically in the wee hours of the morning from
the data
received the previous day.
The current scientific staff of the Minor Planet
Center consists of one
Federal Employee (Smithsonian Institution), one person funded
from a
contract with NASA via the Jet Propulsion Laboratory and one
person paid
from subscriptions to the Minor Planet Center's publications.
Allowing for
absences, this is technically insufficient for the 16/7 operation
the Center
tries to maintain. There is clearly a need for at least two
more employees,
including a systems engineer who would be charged with maintaining
the
Center's cluster of computers, which are purchased from gifts
made to the
Smithsonian by a private foundation in California.
As a final step in the dissemination process,
it should be noted that
calculations are now regularly performed by other groups,
notably at NASA's
Jet Propulsion Laboratory, of any remote possibilities that
specific NEOs
could collide with the earth during the next century. Such
calculations are
fairly extensive but are quite automatic and entirely based
on the
observations organized and distributed by the Minor Planet
Center. They are
also routinely updated using the daily updates of NEO observations.
Of
course, it is almost always to be expected that, as further
data are
acquired, the impact possibilities completely disappear. That
is, they will
disappear unless the earth is actually going to experience
an impact - a
point the dinosaurs 65 million years ago were unable to appreciate.
Most of the deliberate confirmatory and deliberate
follow-up observations of
NEOs, particularly those obtained in the U.S., are made by
amateur
astronomers. There are perhaps ten U.S. amateur groups and
individuals
(notably in Arizona, California, Kansa, New Mexico, Oregon,
Tennessee and
Wisconsin) who can be depended upon to make such observations,
reliably and
systematically. Although amateurs do still regularly discover
main-belt
asteroids ( despite the dominance of the professional surveys),
it is really
quite rare for them to discover NEOs, but there have been
NEO discoveries by
amateurs in Arizona, and even Massachusetts, during the past
two or three
years. Amateurs tend to do better at discovering comets-some
of which are
technically NEOs-because these usually have a distinctive
appearance and can
often be found in the parts of the sky that are closer to
the sun than are
covered by the professional surveys. The Edgar Wilson A ward
for comet
discoveries has therefore actually been made to between two
and seven
amateur astronomers each year. While the part of Pete Conrad
A ward for NEO
discoveries will also be of some encouragement to recipients,
the part
awarded for follow-up observations should actually be more
so. Perhaps the
principal encouragement to amateurs nowadays is to make it
possible for them
to have ready access to the equipment they need to carry out
their work.
Government and private grants that have provided amateurs
with electronic
detectors during the past few years have been particularly
effective. Of
course, the Conrad and Wilson A wards could provide the same
end result, but
there is no guarantee. It should also be noted that there
are better
prospects for amateur discoveries in the southern hemisphere,
because of the
absence of professional surveys there.
For more than a half-century after it.<;
inception in 1947, the Minor Planet
Center functioned with just two scientific staff. By the time
a third member
was added in May 2000 the number of observations in its files
had grown to
4.5 million (effectively from zero) and the number of objects
with orbit
determinations to 80,000--0f which the 15,000 of guaranteed
quality (i.e.,
the asteroids that have been given sequential numbers, and
in some cases,
names) represented a tenfold increase over the situation in
1947. The number
of known NEOs in May 2000 was under 1000, with some 400 of
them more than 1
km across. Now there are more than 15 million observations
and very nearly
200,000 objects with orbit determinations-now almost 50,000
of these being
numbered asteroids. There are now more than 2000 NEOs, of
which some 640 are
larger than 1 km. The Minor Planet Center's staff has been
able to keep up
with the influx, but only because of its extreme dedication.
As already
noted, a modest further increase in the size of the staff
would be
desirable-and it will be essential if the Center is to keep
up with the
exponential increase in data for much longer. Computing capabilities
at the
Minor Planet Center are very good, with new machines added
from time to
time, and since one staff member is particularly involved
with upgrading the
software, some augmentation of the staff would also allow
that member to
concentrate more on this important task.
Although the official NA8A mandate is to concentrate
on NEOs that are 1 km
across or larger, there are already data on many smaller NEOs
in the files.
There are some 1800 NEOs down to 200-300 meters (this number
increasing by
around 400 annually), out of perhaps 40,000 that must exist.
Even with the
present observational and computational capabilities, the
inventory of known
objects could be a substantial fraction of the estimated total
after several
more decades (particularly if one also considers redefining
NEOs to include
only those objects that pass somewhat closer to the sun than
the present
limit of some 120 million miles, for asteroids at that minimum
distance
cannot possibly be a significant threat to the earth, at 91-95
million
miles, for millions of years into the future). Making use
of larger
telescopes could allow 200-meter NEOs to be sampled to a completeness
level
approacl1ing 90 percent after just a decade or two. (One worry
about some of
the proposed telescopes is that they are really designed for
surveys of
objects outside the solar system, and therefore only one image
of a
particular field would be obtained on a given night. As noted
at the
beginning of this testimony, the apparent motion of an object
over an hour
or so is essential for recognizing NEOs. It is also essential
for linking
data on a particular object from one night to another.) Given
the expected
increases in computing capabilities during that time, the
Minor Planet
Center could keep up with this (as it has clearly done before),
again
provided that there are sufficient staff members. It should
be remembered
that NEO observation, with the need for confirmation and follow-up,
is
necessarily an international activity, for the simple reason
that it is not
possible to observe the whole sky from the U .8., and it is
not possible to
observe the reachable sky at all times. The Minor Planet Center,
with its
international connections, is well-equipped to attend to this
point.
If it is decided that it would ultimately be
desirable to extend the NEO
searches down to a size limit of, say, 50 meters, with perhaps
a million
objects to find, the whole perspective does change quite significantly,
and
it would clearly then become efficient to make the searches
from space-based
telescopes. Data-management requirements would also become
much more
intensive, with a clear need for round-the-clock attention.
While this might
be the ultimate goal, the more obvious immediate step is to
go down to the
200-300-meter level, as was discussed in the comprehensive
Task Force Report
on NEOs to the U .K. government in 2000. This would be a logical
and
effective transition that could be accomplished quite rapidly,
and the
increased data-management requirements could be reasonably
addressed, as
discussed in the previous paragraph.
Chairman Rohrabacher, Congressman Gordon, and
members of the committee:
Interest in the threat caused by natural objects
("Near-Earth Objects" or
NEOs) impacting the earth or its atmosphere is growing. High-level
commissions have met to consider the problem in such places
as the United
Kingdom. In the United States, NASA has devoted a few million
dollars per
year to studying the phenomenon. But no concrete plan exists
to address the
overall NEO problem.
The U.S. Department of Defense (DoD) has not
perceived the NEO issue as
pressing. However, DoD is assisting NASA in studying the problem.
It has
been DoD-developed technology, particularly in the space surveillance
area,
which has obtained the bulk of data we currently have on NEOs.
I have been asked to address my perspectives
on the NEO threat and what
should be done about it. I make the following comments not
as a
representative of the U.S. DoD, but rather as a scientist
who has studied
NEOs, and as a space expert familiar with the technologies
that might be
applicable to the problem.
Two and a half months ago, 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 has defused-for now. Most of the
world knew about
this situation and watched and worried. But few know of an
event over the
Mediterranean on June 6th 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 have 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 received little or no
notice as far as we can tell.
However, if it had occurred at the same latitude just 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 India or
Pakistan. To our knowledge, neither of those nations have
the sophisticated
sensors that can determine the difference between a natural
NEO impact and a
nuclear detonation. The resulting panic in the nuclear-armed
and
hair-triggered opposing forces could have been the spark that
ignited a
nuclear horror we have avoided for over a half century.
I've just relayed one aspect of NEOs that should worry us
all. As more and
more nations acquire nuclear weapons-nations without the sophisticated
controls and capabilities built up by the United States over
the 40 years of
Cold War-we should ensure the 30-odd yearly 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 Nation's vital
space systems, such as the Global Positioning System, 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 have figured out their parent
comet
sources. But there are some storms arising from comets that
are too dim for
us to see 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 Siberia,
releasing
equivalent energy of up to 10 megatons. Many experts believe
there were two
other smaller events later in the century-one in Central Asia
in the 1940s
and one in the Amazon in the 1930s. In 1996, our satellite
sensors detected
a burst over Greenland of approximately 100 kiloton yield.
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 than
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 just a few.
Finally, just about everyone knows of the "dinosaur
killer" asteroids. These
are objects, a few kilometers across, that strike on time
scales of tens of
millions of years. While the prospect of such strikes grabs
people's
attention and make great catastrophe movies, too much focus
on these events
has, in my opinion, been counterproductive. Most leaders in
the United
States or elsewhere believe there are more pressing problems
than something
that may only happen every 50-100 million years. I advocate
we focus our
energies on the smaller, more immediate threats. This is not
to say we do
not worry about the large threats. However, I'm reasonably
confidant we will
find almost all large objects within a decade or less. If
we find any that
seem to be on a near-term collision course-which I believe
unlikely-we can
deal with the problem then.
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