THE TWELFTH MOSCOW SOLAR SYSTEM SYMPOSIUM 2021 COLLISIONS OF PLANETESIMALS WITH THE EARTH AND THE MOON S. I. Ipatov, M. Ya. Marov Vernadsky Institute of Geochemistry and Analytical Chemistry of Russian Academу of Sciences, Moscow, Russia (siipatov@hotmail.com, marovmail@yandex.ru) Keywords: Planetesimals, migration, velocities of collisions, Earth, Moon, giant planets. Introduction: The problem of migration of celestial bodies in the Solar System is important for understanding of the formation and evolution of the Solar System [1]. Earlier in our studies of migration of bodies to the Earth and the Moon we paid the main attention to the probabilities of collisions of bodies with these celestial objects. The considered bodies migrated from different distances from the Sun. In this presentation we discuss the times elapsed before collisions of bodies-planetesimals with the Earth and the characteristic velocities of their collisions with the Earth and the Moon. Probabilities of collisions of planetesimals with the Earth: The average probability of collisions of planetesimals with the Earth was estimated to be pE=2×10 –6 for planetesimals from the Jupiter and Saturn zones [2]. For bodies that initially had orbits of Jupiter-family comets pE was higher than 4×10 –6 [3-6]. The pE values were, on average, smaller for greater initial values of the semi-major axes ao of orbits at 5≤ao≤40 AU [7-8]. At pE=2×10 –6 and with the assumption that the total mass of planetesimals in the Jupiter and Saturn feeding zone was 100mE (where mE is the Earth's mass) [9-10], we found out that the total mass of planetesimals that have fallen onto the Earth was 2×10 –4 mE. About the same amount of bodies could collide with the Earth from the outer asteroid belt and from behind the Saturn’s orbit. If the ice made up a half of the mass of this material, then the total ice mass delivered to the Earth from behind the snowline was equal to the total mass of the Earth's oceans (~2×10 –4 mE). Similar conclusion was made in 2001 in [11-12], based on the studies of migration of Jupiter-crossing objects presented in [13]. Estimated ice amounts in comets are no higher than 33%. However, some authors believe that the primary planetesimals may have contained more ice than it is now found in comets. Some fraction of water was lost at collisions, especially with the Moon. Therefore, the amount of water accumulated by the terrestrial planets and the Moon may have been lower than the water amount delivered to these celestial objects. Times elapsed before collisions of bodies with the Earth: Bodies initially located at different distances from the Sun reached the Earth at different times. Former Jupiter-crossing bodies could become Earth- crossers within the first million years. The migration time of bodies from the feeding zone of Uranus and Neptune depends on when large embryos of these planets appeared in this zone. The main changes in elements of the orbits of the embryos of the giant planets took place within no more than 10 million years [9-10]. At present orbits and masses of the planets, some bodies from the Uranus and Neptune zone may have fallen onto the Earth in more than 20 million years. At an initial value ao of the semi-major axis of the