MOTION OF THE MINOR PLANET 4179-TOUTATIS - CAN WE PREDICT ITS COLLISION WITH THE EARTH

Minor planet 4179 Toutatis is an Apollo type object with a very small orbit inclination (i = 0.degrees 47), hence it has a possibility to ap proach closely the Earth tan encounter to within 0.01 a.u. is expected in 2004) and might be a good candidate for a future collision with th e Earth. We collected 640 astrometric observations of Toutatis from th e period 1934-1997 to improve the orbit. We had to include a nongravit ational term into equations of motion expressed by a secular change (a ) over dot of the semimajor axis a of the Toutatis orbit to obtain a f ully satisfactory solution of the orbit determination. A value (a) ove r dot = -0.16 x 10(-10) is two orders smaller than that determined in the case of short-period comets with known nongravitational effects. T o investigate the long-term motion of Toutatis we numerically integrat ed the equations of motion by recurrent power series taking into accou nt perturbations caused by the eight planets from Mercury to Neptun, t reating the Earth and Moon as separate bodies, and also by the four bi ggest asteroids. We randomly varied the orbital elements to examine th e Toutatis' motion for a number of different orbits. We present a new method of the random orbit selection which allows us to find a set of different orbits but representing well all the observations used for t he: orbit correction. Our results confirm a conclusion found by other authors that Toutatis orbit is exceptionally chaotic. Therefore, we ar e not able to predict the motion of Toutatis further than for 300 year s. However, our integrations spanning 1500 years showed that the evolu tion of position of the descending node of Toutatis' orbit might go al so in such a direction that the orbits of Toutatis and of the Earth wo uld intersect in the future. Hence a possibility of the Toutatis-Earth collision is not excluded but it is completely unpredictable. To inve stigate conditions of a hypothetical collision of a minor planet with the Earth we made the following numerical simulation. Based on the Tou tatis' orbit we deduced such orbital elements for a fictitious minor p lanet ''Fatum'' that a shape of the orbit was very similar to that of Toutatis, but we knew in advance that ''Fatum'' would certainly collid e with the Earth in September 2004 and we calculated values of the imp act parameters. We created a set of 638 artificial observations of ''F atum'' in 1988-1997 for the same dates and with the same random observ ational errors like those of Toutatis. Then we corrected the ''Fatum's '' orbit for different observational intervals to examine the exactnes s of the impact prediction in 2004. We found that in 1993 we would be sure that the collision is inevitable, and in 1997 we could determine an impact area on the Earth's surface in range of a square of 100 x 10 0 km. We show that if we knew the impact date so early we could undert ake an action to avoid the collision by trying to change the ''Fatum's '' heliocentric velocity only by one cm/sec.