Statement of John D. G. Rather, Ph.D.
Assistant Director for Space Technology (Program Development)
National Aeronautics and Space Administration

Before the Subcommittee on Space of the Committee on Science, Space, and Technology

U.S. House of Representatives, One Hundred Third Congress

March, 24, 1993

It is a great privilege to report to you the findings of NASA's Workshop on Near Earth Object Interception, for which I served as Chairman in January, 1992. The meeting was especially interesting because of its cross-cultural nature, involving national and international experts from diverse disciplines of civilian and defense science and engineering who do not often interact with each other. It is encouraging to see how they pulled together to tackle this difficult challenge and have continued to work together throughout the past year.

The summary findings of our workshop are contained in a document available from NASA Headquarters', and the companion technical proceedings can be obtained from Los Alamos National Laboratory. Today I will summarize some of the principal issues and results derived from the workshop and its aftermath.

In the past decade, there has been a major shift in the perception of potential hazards to human life from Earth-approaching cosmic objects. A vast increase in evidence linking large-scale extinctions of species to past impacts on the Earth has motivated this increased concern. Simultaneously, there has been a great increase in the rate of discovery of near-Earth objects (NEOs), including some which have made close passes by Earth. These new horizons of understanding led to your enlightened concerns and your mandate to NASA to

1. study ways to increase the detection rate of Earth-approaching objects and
2. investigate ways to alter the orbits of, or destroy, such objects if they should pose a danger to life on Earth. I report today on the findings of the workshop devoted to the latter issue.

Happily, the majority opinion of the Interception Workshop participants is that there now exist technically credible approaches for preventing most impact catastrophes, provided that

the efficiency and thoroughness of early warning capabilities are greatly increased,
an appropriate experimental program is eventually undertaken, and
the requisite interception technology capabilities are maintained and/or developed. Let me elaborate briefly on each of these three key areas:

Early Warning and Target Acquisition

Interception of Earth-approaching asteroids or comets cannot be decoupled from comprehensive observations. Early detection obviously gives a longer reaction time. More importantly, interceptions far from Earth -- made feasible by early warning -- are much more desirable and easier than interceptions near Earth because

1. small deflections far away will produce greater miss-distances at the Earth,
2. if fragmentation occurs, it is essential for all of the fragments to miss the Earth because we don't want to convert a cannon shell into a cluster bomb, and
3. remote interception permits a shoot-look-shoot strategy to optimize deflection success.

For cases in which exact orbits can be predicted decades to centuries in advance, it will be possible to send precursor missions to the object to prepare for a subsequent interception. Fortunately this is the most likely case for the majority of large NEO asteroids -- large enough potentially to cause global catastrophes -- because they are bright enough to be detected in a survey lasting two or three decades. It is estimated that there are about two thousand asteroids in this category.

For cases in which the warning time is only a few years, the orbit will be less certain and an immediate effort will be needed to refine the orbit by all possible means. Precursor missions would be less possible and the required launch energy may be much higher than for the first class of objects. This category contains hundreds of thousands of smaller asteroids and comets.

With less than a year's warning that an object (probably a large, long-period comet) is on a collision course with Earth, interception devices would have to be launched in short order and with a very high launch energy-perhaps 100 times higher than that required in the first case.

For the worst case, a large object discovered to be on a collision course with Earth in a matter of days, there is at present no response that has a high probability of success. Therefore, a program for early detection is essential. With the proper development of improved detection systems and the rest of the program to be recommended, this worst case need never arise.

The other NEO Detection Workshop has recommended the construction of a system of earth-based observatories that would greatly augment the present knowledge base on a time scale of twenty five years. While supporting the need for such capabilities, participants of our Interception Workshop who were cognizant of presently classified technical capabilities opined that important immediate progress can result from sharing existing defense search, tracking, and homing technologies with the civilian sector and from implementing protocols to transfer data on NEO discoveries from defense and intelligence assets to appropriate centers for determining precise orbits and other relevant data. This combination of efforts to expand the knowledge base should, clearly, have the highest near-term emphasis.

Needed Experiments

In order to greatly improve the required knowledge base for deflection, it is feasible to perform experimental missions to observe a representative sample of NEOs with small modifications to existing capabilities. The workshop devoted considerable effort to questions of how to intercept NEOs and what the purposes of preliminary research missions should be. Two types of research missions were studied:

1. precursor reconnaissance/ sampling missions and
2. missions aimed at diverting or fragmenting an NEO of any size.

The prime objective of precursor reconnaissance missions would be to characterize the diversity of NEOs because objects with different material composition will respond quite differently to perturbation effects. Such relatively low-cost missions using small, lightweight spacecraft and launch vehicles could fly by, rendezvous with, or land on NEOs. The Department of Defense has developed a number of lightweight technologies which could be used for such missions in their present configuration or after some modification, and NASA is actively cooperating in investigating how these technologies can be converted for civilian applications such as NEO probing. The ongoing Clementine mission provides the first fully integrated example.

For development of reliable interception and deflection capabilities, it was proposed by some members of the workshop that experiments will eventually have to be performed on representative objects. From time to time when conditions are favorable, it is possible that appropriate methods to alter orbits of asteroids and comets might be tested. Favorable conditions would include circumstances where no collision with Earth can possibly occur, where the expense can be minimized, and where the observations can be optimized. The expressed viewpoint was that definitive planning for the future will become possible only if such experimental results are available. I believe the importance of this view is underlined by the rapidly accumulating evidence that asteroids and comets are very unsymmetrical and diverse objects that may react in unpredictable ways to perturbation efforts.

The real uncertainty underlying the need for such tests is whether there is any urgency. Impact of large NEOs is a very infrequent phenomenon, but potentially more destructive than any other threat to life as we know it. Whether it is safe to defer action for fifty years while we await a more robust, higher technology space program is anybody's guess. The function of our workshop was to explore issues and lay out options. A few workshop members expressed concerns, however, regarding the possibility of using nuclear explosives in space to deflect or destroy on-coming objects. Some advocated kinetic impact tests on the smallest NEOs. Others felt that the probability of early detection of large, dangerous objects is sufficiently high that any tests, nuclear or otherwise, could be deferred until an actual impact is threatened. There is a clear need for continuing national and international scientific investigation and political leadership to establish a successful and broadly acceptable policy.

Interception Technologies

The kinetic energy of a mountain-sized object traveling typically at twenty miles per second is so enormous that it is difficult to comprehend. The consequences of an impact on the Earth could be so devastating that it is both reasonable and necessary to explore all possible means for avoiding this ultimate catastrophe. The energy of the largest nuclear weapons ever built is very small by comparison, so it is not surprising that nuclear technology provides the only presently realizable hope for imparting enough energy to perturb a NEO from an Earth-impacting trajectory. This is why the workshop had to consider the probable use of nuclear devices if an Earth-threatening object is identified.

Non-nuclear energy payloads or other non-nuclear in-situ options may be useful only for dealing with small NEOs, or for larger objects when many years are available to effect a change. This is true because nuclear devices yield tens of millions times more energy per kilogram of payload, and very large energies are required to perturb massive NEOs significantly, even when the required velocity change to miss the Earth may be only a few inches per second. In summary, workshop participants concluded that any defensive planning for a potential impact should include nuclear technology. Clearly, any such use of nuclear devices would require appropriate national policy as well as international agreements and protocols.

To emphasize a few key points:

Technologies currently exist that could be integrated into systems capable of protecting the Earth from most NEO impacts. Such integrated systems do not currently exist and would require considerable development.

Early warning is the ultimate key to success. Both ground and space-based assets should be considered.
Nuclear explosives would be essential to deflect a large object within the foreseeable future. More benign in-situ methods may become feasible as manned space activities mature and expand.
Efforts should be made to transfer technologies and data capable of helping the NEO interception effort from defense and intelligence sources to the civilian and scientific community.