Small objects discovery and recovery
Is the next Tunguska Predictable?

by Andrea Milani* - Copyright Tumbling Stone 2001

Let me first state clearly that I fully agree with the goal of the Spaceguard Survey, namely discovering the potential impactors with diameter 1 kilometer or larger. The smaller objects are a minor fraction of the impact risk, since each one of them would only be capable of local damage, if any. Indeed the objects smaller than about 50 meters in diameter may not reach the ground, depending upon composition, and may not even result in damage at the ground level, only fireworks when exploding in the atmosphere. The Tunguska impactor of 1908 (see T.S. number 2: "Turning asteroids into stardust" by Luigi Foschini), roughly 70 meters in diameter, is believed to have been near the smallest size capable of ground damage, at least for an object of ordinary meteoritic composition (the Meteor Crater-Arizona impactor was only 40-50 meters, but of metallic composition. See T.S. number 2: "Learning by images").
However, this does not imply that the information on small Earth-crossing asteroids we are collecting while looking for the larger ones is unimportant.  Obviously it is scientifically valuable to study objects of this size range, on which almost nothing was known only ten years ago. Moreover, the capability of predicting an impact of the Tunguska class has an obvious value in terms of civil defense. If such a prediction were available, very simple measures could be taken to avoid casualties, such as evacuation of the target area; it is even possible to blow up the object (unlike the bigger objects, which can be blown up only in second rate SF movies). 
Thus the question is: do we have this capability of prediction? I would like to illustrate the problem with the example of a recently discovered asteroid. 2001 BA16 was discovered by the LINEAR survey on 19 January 2001; it was followed up by two amateur observatories (Badlands and Sormano) and a professional one (Klet), so that its orbit could be computed and the data were made public by the Minor Planet Center on 24 January at 6:18 AM (I am using Europen time); the data were automatically processed by NEODyS at 10:00 AM. On 24 January at 6:56 PM I received the warning mail from the software robot CLOMON with:

The same evening the Spaceguard Central Node added in its New Announcements web page
http://spaceguard.ias.rm.cnr.it/SSystem/NEOCS?Announcements.html the following message:

Although this message is very low key, everybody in the business of near Earth objects knowns very well that these messages from the Spaceguard Foundation (SGF) have to be taken seriously. The most skilled, and best equipped, observers did try, and on 10 February Carl Hergenroter of the Catalina Survey succeded in the (very difficult) recovery. On 28 February David Tholen from Mauna Kea was able to observe the same object while at apparent magnitude (dict.) between 23 and 24, a feat impossible for essentially anybody else of the asteroid observers, who do not normally have access to the new big telescopes.
Unfortunately these additional observations were not enough to completely remove all the Virtual Impactors (dict.), that is, based upon the observations available we cannot yet exclude the possibility of collision. The current situation is summarized by the risk page posted at the NEODyS site: http://newton.dm.unipi.it/cgi-bin/neodys/neoibo?objects:2001BA16;risk.
The essential information anyway is the following: the most likely collision could take place in the year 2041, and the probability is roughly estimated at 1 in 6000. The other possibilities are both more remote in time and more unlikely. So the small body was rated in the Torino scale (dict.) 0 and this has to be interpreted as a legal requirement of a rule instituted by IAU, following the discussion of the "Impact" meeting held in Torino in 1999. The NEODyS site follows this rule rigorously, thus an announcement is always given with the specification of the Torino scale.
The problem is solved? Are we indeed predicting the next Tunguska class impact, actually even smaller ones? Unfortunately, this story does not really support this conclusion. The problem is indicated by the low value of the impact probability by 2001 BA16; moreover, only another (even smaller) asteroid, 1994 GV, appears on the Risk Page with a probability of impact of the same order (1 in 7000). Another impact solution with comparable (even higher) probability has been computed by Paul Chodas (JPL) for the object 2000 SG344, which is believed to be a human manufact (the third stage of a Saturn V rocket from one of the Apollo launches). The largest object appearing on the risk page is 1998 OX4, which is maybe 200 meters in diameter, but has impact probabilities of the order of 1 in 10 millions.
We can conclude that the list of known Virtual Impactors is severely incomplete. Indeed, if we expect a Tunguska class impact to take place on average once every few hundred years, if our knowledge was complete we should have in the list a Virtual Impactor with a large probability (of the order of 0.1) and for an object of Tunguska size or larger. Most people would be relieved in knowing that we do not really expect anyone of the known asteroids to hit the Earth in the next 50-100 years, and even the minute probabilities we have, refer mostly to objects unable to do substantial damage.
This is a good example of the known fact that the opinion of the majority does not always correspond to the truth. The majority thinks in that way, and they are wrong. We should be relieved only when we know which asteroid is going to hit us, because defensive measures then become possible. If we do not know it, but we expect, on the basis of what we know of the size distribution of asteroids and of the statistics of past impacts, that it is likely to exist, we should be worried. 
Thus the real question is: why? Why are we so far short of the goal of predicting the next Tunguska?
Some answer in the next issue of Tumbling Stone...

Andrea Milani (*) - Director of NEODyS

What can a small cosmic body do to the Earth? Tunguska! by Nanni Riccobono - Copyright Tumbling Stone 2001 Fallen trees in the Tunguska area (1928). From the Tunguska Page of Bologna University On june 30 1908, at 7:17 A. M. the sky over the Tunguska river region was suddenly lit up by a massive fireball coming from the southeast. When it reached about 8 km from the ground, a terrible burst was heard by the tungus, a nomadic population living in that area; soon the forest, the ancient taiga, was on fire and destroyed for 2000 square meters. The energy of the blast has been calculated at the equivalent of 10 megatons of TNT, a thousand time the power of Hiroshima bomb. The jolt was registered by seismometers in Britain and the atmospheric shock wave circled the Earth twice. We still do not know about human casualties in the Tunguska case (Siberia, and especially that region, were quite out of the world at that period), still we know for sure many animals died and so did 60 millions of trees. This is a very brief account of what a small cosmic body is capable to do when it reaches the Earth (you can have detailed information at the Tunguska website of the University of Bologna: http://www-th.bo.infn.it/tunguska). Tunguska’s body had a diameter of 60 meters, a cinch, though it provoked quite a big damage to the taiga. It is calculated that this kind of events take place once every hundred years ...