I can show people evidence of real strikes inflicting local and regional damage less than a century ago. Even more compelling are the frequent kiloton-level detonations our early warning satellites see in the earth's atmosphere.
Within the United States space community there is a growing concern over "space situational awareness." We are beginning to understand that it is essential to identify and track virtually everything in earth orbit. Some of these objects, down to a few centimeters in size, present a potential threat to commercial and civil space operations such as the International Space Station.
What then should we do? What role should the US Government, and specifically the US DoD play in what everyone agrees is an international concern?
I'll begin my short CCNet-essay with a disclaimer. The US Department of Defense (DoD) has no official view on the Near-Earth Object (NEO) hazard. We have agreed to assist the overall United States effort led by NASA with technology and observational support. Official disclaimers out of the way, I'll provide my personal views in the remainder of the essay.
For those readers who don't know me, I'm a US Air Force officer with a background as a research astronomer. Although I began as a solar physicist my current research interests--in the few moments I have time to do research--are focused on NEOs and meteors. Most of my recent work for the Air Force has been in developing options to perform selected national security missions from and through space. In the past decade I was responsible for much of the US DoD work to develop small satellites, microsatellites and reusable satellite launchers. The 1994 Clementine mission to the moon (originally intended to include an asteroid flyby) was one of the my programs.
I will assume that most readers share in the view that NEOs have and will continue to play a central role in the evolution of life on this planet. I'll also assume that we more or less agree that we face a continuing threat from these objects. Most analyses focus on the big threats--objects which can threaten life globally and have the potential to destroy or seriously damage our species. I for one believe we should pay more attention to the "Tunguska-class" objects--100 meter or so objects which can strike up to several times per century with the destructiveness of a nuclear weapon.
NEO discussions in the United States have, as I believe they have everywhere, suffered from the fact that catastrophic NEO impacts are so rare and hence so unlikely to occur in our lifetimes. Whereas people may pay good money to see a movie thriller about asteroid strikes or read with great interest of the demise of the dinosaurs, a once- every-few- tens-of-millions-of- years possibility is not real to most people. Decision makers simply are unwilling to spend scarce resources on such an unlikely catastrophe -- however terrible it may be or even if it is inevitable.
Conversely, I can show people evidence of real strikes inflicting local and regional damage less than a century ago. Even more compelling are the frequent kiloton-level detonations our early warning satellites see in the earth's atmosphere. These are threats the public and its leaders will take seriously. These are threats we can understand. And these are even threats we could mitigate, if required, without recourse to nuclear technology.
Many of my colleagues in the US national security community have advocated a proactive role for our community. They would have us build and demonstrate a NEO defense system--perhaps based on nuclear weapons. This is premature. What we need now is a full characterization of first the phenomenon and then the threat which it might entail. We need to know how many objects there are, where they are and when any closely approach the Earth. And we need to know the composition and structure of all classes of NEOs. This is where the US national security establishment can play an important role.
Within the United States space community there is a growing concern over "space situational awareness." We are beginning to understand that it is essential to identify and track virtually everything in earth orbit. Some of these objects, down to a few centimeters in size, present a potential threat to commercial and civil space operations such as the International Space Station. Commercial space operations exceeded government space operations for the first time in the last few years. We have begun the era of "global utilities" such as the Global Positioning System (GPS) in which our ways of life are becoming critically dependent on space systems. All of these demand the ability to search essentially all of space near the earth about once every few hours and track up to 300,000 objects.
The US Department of Defense has a network of sensors that perform this function but in a limited capacity compared to what is desired. We plan on upgrading the system over the next decade or so--including space-based sensors--to provide comprehensive search, detection and tracking of space objects. The LINEAR system which has been so successful over the past year in detecting NEOs is a prototype of our next generation ground-based optical system. With relatively simple modifications to operations, our future space surveillance system could produce a comprehensive catalog of NEOs at little or no expense to the scientific community.
If the international community had to duplicate this network of sensors, it would cost 100s of millions of dollars, if not more. Simple economics argue that this is a portion of the NEO problem that should be urged upon and given to the US Military. For this reason I believe the growing community of experts on the NEO threat should not direct their efforts to building and funding more ground-based telescopes. Although I must caveat that not everyone in the DoD is as eager as am I to take on the NEO task!
Enthusiasm grows for the next generation satellite, the so-called "microsatellite." These are 100 kilogram-class satellites costing about $5-10M US to build and an equivalent amount to launch. Leading the world in the development of these microsatellites are a number of European groups. Their rapid progress has been enabled by the capacity of the Ariane IV and V launchers to carry as auxiliary payloads up to eight microsatellites into a GEO transfer orbit at a cost of about $1 million per satellite. NASA, US academic institutions and US aerospace companies have begun efforts to develop microsatellites for space science and planetary exploration missions.
Similarly, the DoD is beginning the development of similar small size microsatellites for servicing and re-fueling larger mission satellites. These microsatellites should allow for low-cost missions to a wide range of NEOs--including missions to fully characterize their structure and possibly bring back samples to earth. Microsatellite missions can also assist in the surveillance and cataloguing of NEOs as there is some doubt that sensors based on the earth or in earth orbit can detect all of the potentially threatening objects. Microsatellites internal to the earths orbit--perhaps in a Venus-type orbit--could provide a low-cost solution. This is potentially another task for low-cost microsatellites. The NEO research community community will have access to both low-cost spacecraft and low-cost launch. In addition to Ariane, the United States Air Force is putting a similar auxiliary microsatellite adapter on our new EELV (Evolved Expendable Launch Vehicle) launch systems.
What then should we do? What role should the US Government, and specifically the US DoD play in what everyone agrees is an international concern? I believe we in the US DoD can and should agree to modify our space surveillance systems to identify and track all potentially threatening NEOs--probably down to about the 100 meter class. In parallel, in situ studies of NEOs using low-cost microsatellite missions should begin immediately. These missions can and should involve NASA, ESA, other European space agencies as well as the US DoD. These missions can use new technology to rendezvous, inspect, sample, and even impact NEOs to study their composition and structure. With an estimated cost of about $10-20M per mission, including data reduction and launch, this is an affordable program. Here is where I would focus the growth of official interest in NEOs as evidenced by the recent UK decision to stand up a formal program.
And finally, I would propose focusing on the very small end of NEOs--100 meters diameter or less. At any given time there are probably tens of objects 10 meters or larger in cislunar space. These are easily accessible to the low-cost microsatellite mission.
Should we worry now about mitigating the NEO hazard? I would say no, until a bona fide threat emerges. This will avoid much of the political consternation that has arisen in the past from nuclear weapon experts advocating weapons retention and even testing in space. After all, we can't reliably divert an NEO until we know much more about its structure. This we'll get from a decade of dedicated microsatellite missions. Some of these missions may even have as a side experiment moving very small (10-50 meter class) NEOs by impacting them. This could give us much of the necessary experience should a true threat emerge in the near future.
Another benefit of a focused international NEO space mission suite is public awareness and enthusiasm. From a scientific standpoint, these are primordial objects--the stuff of which we were made. People throughout the world, as well as the entire scientific community, will truly embrace such an exciting endeavor. Moreover, space visionaries often look to the NEOs as the raw material of eventual space industrialization. We originally chose the title "Clementine" for the 1994 lunar and NEO probe launched by the DoD for this purpose. An old American song about a frontier miner's daughter, Clementine, was the origin of the mission's name. We hoped to evoke not only the spirit of the frontier but also to leverage the appeal that valuable lunar and asteroid mineral resources might have.
In summary, I believe we have an opportunity to harness public interest, government attention and existing expertise on the NEO problem. An objective program should have two complementary parts. First, to detect and to catalog virtually all threatening objects. This can be considerably easier and cheaper if the US DoD can be persuaded to adopt it as part of its current space surveillance mission. Second, we should mount a modest, low-cost program to fully characterize the composition and structure of all classes of NEOs. The latter can and should be an international effort involving space agencies around the world. When, and not until, we find a likely threat is the time to work hard on mitigation.
S. Pete Worden, Brigadier General (sel),
USAF Deputy Director for Command and Control Headquarters,
United States Air Force
The Pentagon, Washington, DC USA