Dealing With the Impact Hazard
Paris - 22 July 1999 - The Earth is constantly sweeping up particles of various sizes as it travels on its orbit around the Sun. We often see some of them burning in the atmosphere as meteors. From time to time, penetrating objects that are many meters across may cause major explosions in the air.
Recently, public concern has been raised over much less frequent but devastating impacts by km-sized asteroids or comets. It has now been realized that the risk of fatality as a result of such impacts is comparable to that of well-perceived hazards like airplane crashes.
Possibly hazardous objects in the solar system can be discovered by astronomical observations, e.g., when they are recorded as faint streaks of light in long telescopic exposures because of their motions. Astronomers therefore have a special mission relating to the impact hazard, namely, that of discovering and characterizing the dangerous objects, and, hopefully, by verifying the expectation that no major impact is going to occur during the next centuries.
The vast majority of these "dangerous" objects have as yet escaped discovery and, since we do not know their orbits, they may hit at any time. For the time being, only statistical calculations can be made. They show that the risk of the Earth being hit by a km-sized object during the next couple of centuries is one-in-a-thousand. The risks are low but the consequences are large enough to cause concern. In fact, astronomers have now put in place efficient search programs that are already resulting in a fast stream of new discoveries of such objects.
When the newly discovered objects are investigated with regard to future encounters with the Earth, it is sometimes found that, due to the necessarily imprecise, initial orbit determinations, the risk of a collision cannot be entirely ruled out. The likelihood of such a possible disaster may also be estimated. If it is found to be significant when compared to the combined risk posed by all the unknown objects, then the asteroid or comet in question will become subject of careful monitoring. The expectation is that improved knowledge of its orbit will sooner or later show that the impact will not occur. This is at least the outcome of all monitoring programmes so far.
This type of observational work is now occupying a small group of astronomers worldwide. It is an international effort, since the impact hazard is obviously of concern to the entire world. Possible impacting objects are best studied by means of international observing campaigns and there must be an efficient exchange of information among all scientists involved. This is also why the International Astronomical Union (IAU) has engaged itself as co-sponsor of a very well attended workshop in Torino, Italy, on June 1-4. Among the other main co-sponsors were NASA and ESA.
Many related issues were debated during this meeting. For instance, how to secure a fast, efficient search such that nearly all the potentially hazardous asteroids get discovered and safely catalogued before too long; how to collaborate in order to measure their most important physical properties by means of ground-based as well as space-based observations; how to speed up and widen the data channels for an optimal use of the world's combined observational facilities, and, not least, how to inform both the public and political authorities -- if ever needed -- about calculations that point towards sinister events.
Even though US national agencies, i.e., NASA and the US Air Force, are presently carrying a major part of the burden of these observations and calculations and may possibly increase their efforts further, the participants in the Torino meeting decided to make a strong recommendation to all governments to establish ``National Spaceguard Centres'' and to support these financially. In this way, a proper sharing of responsibilities may be realised, so that this important work can be enhanced and reach maximum efficiency. Another urgent action item is the setting up of an expert committee, under the auspices of the IAU, that will check impact predictions and advise about their publication.
Perhaps the most visible and immediately practical result was the adoption of the so-called "Torino impact scale". It was worked out by Prof. Richard Binzel of MIT (Cambridge MA, USA) as a tool for communicating the issues of impact prediction outside the professional circuit. With some superficial similarities to the Richter scale for earthquake intensity, it divides the predictions into classes 0-10. All events that have no likely consequences belong to class 0 and higher numbers correspond to progressively more probable, and/or more serious impacts.
At the present time, no single asteroid is known that has been assigned an impact prediction in a class higher than 0. This is of course fortunate and this is expected to be the normal state of affairs. However, it is also likely that initial uncertainties in the calculation of an orbit of a newly discovered asteroid may temporarily place it in a higher category. This is not a cause for immediate concern, but merely signals the need for more accurate observations, leading to a better determination of the orbit.
With the increased rate of discoveries of asteroids and the efficient schemes of orbital computations now in use, the new Torino Scale will most certainly become of great use and will be frequently cited as reference.
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