Near Earth Objects (NEO) science started in 1770, 236 years ago (this number curiously coincides with that of the Symposium), when Messier discovered the comet that is now known under the name of comet Lexell. This comet passed exceptionally close to the Earth, and posed a big problem to those who tried to compute its orbit.
Lexell, a brilliant young mathematician working in Saint Petersburg, succeeded in establishing that the comet was on a 5.5 yr period orbit, that its orbit before 1767 was of longer period and had been modified by a close encounter with Jupiter, and that another encounter with that planet in 1779 would send again the comet in an orbit of much longer period. With his work, the modern era of the dynamics of small solar system bodies was born.
In mid XIXth century LeVerrier understood that the best-fit orbit of comet Lexell computed from the available observations is poorly constrained. He identified a line in the space of orbital elements - what we would nowadays call the Line of Variations (LoV) - in which the point corresponding to the true orbit most probably lies. Current impact monitoring software robots exploit the same concept.
LeVerrier computed the post-1770 time evolution of orbits lying on the LoV, and found them to be extremely sensitive to initial conditions, due to the 1779 jovian encounter. His computations are possibly the first instance of chaotic dynamics in physics and astronomy. However, the profound importance of this work was obscured by the vast fame gained by the other main work that he was doing at the same time: the perturbation computations that led to the discovery of Neptune.
After a long hiatus in the second half of the XIXth century and in the first three quarters of the XXth, the subject of NEOs is attracting again the attention of astronomers. Some of the basic ideas have not changed; however, new observational and computational techniques have allowed us to make significant progress in NEO studies. In fact, besides the Moon, NEOs are our closest neighbors; this allows us to carry out a number of investigations on them that would not be otherwise possible on such small bodies, both from the ground and from space. Due to radar observations during passages close to the Earth, and to spacecraft visits, we now know a number of NEOs with unprecedented detail.
The wealth of data produced by the first generation surveys dedicated to the discovery of NEOs, started in the last decade, have led to detailed studies of their orbital evolution, evidencing the importance of subtle nongravitational effects, the abundant presence of binary objects and of objects with unusual shapes. The next generation surveys, that will begin operations in the next few years, will lead to a hundred-fold increase in the discovery rate, as well as to more accurate astrometry, raising expectations for even more interesting new accomplishments.
As the Symposium has evidenced, we now start to understand how NEOs formed and how they evolve, both dynamically and physically; this opens a window on collisions, a universal astrophysical phenomenon that has left clear markings both on NEOs and on the surfaces of planets, including our one.
This last issue brings us to an aspect that is also emphasized in the title of IAU Symposium 236. In fact, NEO impacts represent a threat over very long time scales. To deal with this, mankind has to put in place a global system.
In this system, NEO astronomy, both from the ground and from space, is the first link of the chain of actions needed to prevent and/or mitigate the effects of a collision. The other links involve civil protection, disaster management, international laws and, above all, political actions. Once in place, this global system will run essentially forever, like the earthquake, volcano and tsunami alert systems nowadays in operation.
Astronomers have started to put in place their link of the chain, by discovering the potentially hazardous objects and putting in operation impact monitoring software robots that allow us to predict the possibilities of collisions with the Earth many decades in advance; space missions are under study that will lead to the development of realistic mitigation strategies. It is now time for the other communities to start putting in place their links of the chain.
Wilhelm von Biela (1782–1856) was descendant of a Czech noble family that found exile in Saxony after Friedrich von Biela died as one of the 27 Protestant noble man executed on the Old Town Square in 1621. Wilhelm finished military school in Dresden and after participation in Napoleonic wars he moved to Prague in 1815 and started to study astronomy under the supervision of Martin Alois David, director of the Prague Clementinum Obseratory.
Together with Joseph Morstadt, Biela recognised similarity of comets seen by Montaigne in 1772 and Pons in 1805, estimated the orbital period to be about 6,75 years and predicted a return in 1826. As captain of the Austrian army, he was stationed in the years 1824–1826 in Fort Josephstadt in East Bohemia, where he established a small private observatory in the bastion No. 37 (identified shortly before the 26th GA according to the coordinates measured by Biela). On February 27th, Biela found this comet and after follow up observations he computed and published an improved orbit showing that the comet is a Near-Earth Object. On the next apparition the comet was found on November 26th, 1832. J. H. Maedler from Berlin mentioned in Astronomische Nachrichten a letter of 22nd October 1837 in which Morstadt suggested this comet as a source of the November meteor shower (today´s Leonids).
The comet split in 1845 into two parts that were observed e.g. by Bradley and Herrick at Yale University and by Maury at the USNO; and by the next return in 1852 by A. Secchi in Rome. Until 1872 the comet was not found, but this year, on 27th November an intense and spectacular meteor rain appeared and confirmed so the Schiaparelli´s publication from 1866 about comets as sources of meteor showers.
Biela discovered also comets of 1823 and 1831, and studied sunspots as impact places “of comets falling into the Sun”.
Cake to celebrate the 1000th anniversary of the supernova of 1006 A.D., brightest star in recorded human history, is consumed in less than 1000 seconds by enthusiastic participants in JD09–Supernovae: One Millennium after SN 1006.