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Research on Near Earth Objects (Asteroids or Comets)

A number of minor bodies in the Solar system (asteroids and comets) exist whose orbits intersect or pass close to that of the Earth (Near Earth Objects or NEOs). The possibility therefore exists that some of these bodies may impact on Earth sometime in the future, as has happened a number of times in the past as shown by the geological record. The existence of such a risk is attracting increasing attention from the public and the media.

The scientific reality is that the number, orbits, sizes, and physical properties of NEOs are poorly known at this time. Although estimated time scales for impacts with catastrophic consequences range from hundreds of thousands to millions of years, there are good reasons, scientific as well as social, for conducting vigorous research programmes to improve our knowledge of these bodies. Such research has, in fact, already a long history of international cooperation, coordinated by the IAU.

The general policy of the IAU with regard to research on and public information about NEOs is formulated in the Policy Statement issued by the IAU Executive Committee. The practical execution of this policy is organised by the IAU Working Group on NEOs of IAU Division III and the IAU Minor Planet Center.

Search programmes to chart the NEO population have recently increased greatly in efficiency due to the use of modern wide-field detector technology and advanced data processing techniques. Positions of moving objects are being rapidly compared to the data base of existing observations and either assigned to known bodies or labelled as new discoveries. When a few accurate positions of an object have been accumulated over a period of weeks or months, a preliminary orbit can be computed. This orbit is then used to (i): Predict where follow-up observations can be made in the near future to improve the determination of the orbit, size, and physical characteristics of the object, and (ii): compute the trajectory of the body far into the future to check for any close approaches to earth. In such cases, the object in question will of course be followed with particular attention in order to reduce the uncertainty of its predicted future course. The situation may be illustrated by associating the object with a "bundle" of possible orbits which will be rapidly diverging from the nominal orbit solution when based on a short arc of the orbit but narrows down very rapidly after a few years of follow-up observations.

At the current level of search effort, it will happen several times a year that an object is discovered whose initially wide range of possible orbits may encompass the position of the Earth at some time in the future. It is obviously important that the data and computations for such objects be checked and improved as soon as possible in order to reduce the uncertainty of the prediction and confirm or (in the vast majority of cases) rule out the possibility of any danger of a possible actual impact in the future. This is clearly a task for the international community of scientific experts in the field, and this collaboration is organised by the IAU through its Minor Planet Center and Working Group on NEOs.

The keen - and justified - public interest in the subject has occasionally led to premature predictions of imminent disaster from known or unknown comets or (mostly) asteroids. As long as only a small fraction of NEOs has yet been detected and tracked, the danger of an impact by an unknown object remains real. History tells us that catastrophic impacts are exceedingly rare, occurring at intervals of hundreds of thousands or even millions of years. Yet, by definition, the time of impact of a given unknown object remains unpredictable. Astronomers, therefore, must give full and urgent attention to every new discovery, and this they do. On the other hand, every single object that has been fully studied so far has been proved completely harmless in the foreseeable future (many decades to centuries).

There has been much discussion on the need to inform the public promptly when a future close approach has been predicted. A couple of such cases in recent years have resulted in considerable media activity, which has promptly died down when the danger was quickly disproved. It is clear that, as discoveries intensify, it becomes absurd and counterproductive to alert the public to every case under consideration: If a potential impact is a possibility a century from now, surely no harm is done by waiting a few months for confirmation or disproval of the danger. On the other hand, basic scientific principles of free access to scientific data and freedom in publishing scientific results should be respected also in this field of research.

The IAU is currently discussing with its major partners in the field how to deal with this issue. As part of these consultations, the IAU organised, with NASA, ESA, the Spaceguard Foundation, and other partners, an international workshop: International Monitoring Programs for Asteroid and Comet Threat (IMPACT), June 1-4, 1999, in Torino, Italy. Procedures will be refined with time and experience and the description below updated accordingly, but the basic principles are the following:

One result of the Torino IMPACT Workshop was the recognition that the so-called "Torino Scale" is a convenient means to illustrate the status of any particular NEO as regards a potential impact hazard. All known NEOs as of the time of this update have a rating of 0, i.e. "harmless", on the Torino scale.

The most up-to-date summary of all issues of NEO research is found in the Report of the Task Force on potentially hazardous Near Earth Objects. This task force was set up by the UK Minister for Science on 4 January 2000, and the result of its work has been welcomed by the IAU in a Statement of 20 September 2000 and a Press Release of 11 October 2000.

Useful information for readers interested in NEOs is provided by the IAU Minor Planet Center, the NASA NEO sites at Ames and JPL (the main NASA NEO site), the international Spaceguard Foundation which has been created under the auspices of the IAU NEO Working Group to stimulate research in the field, and by the NEODyS Consortium. Links to further sites will be added as appropriate.

A current space mission of special interest in relation to NEOs is the Near Earth Asteroid Rendezvous spacecraft.

Page maintained by iauweb@iau.org, last modified on 2001-04-25.