In the history of mankind, the sky above us has always fascinated and inspired. Many investigations with different scientific questions have led to great progress towards better understanding of the universe and our Solar System. But many questions are still waiting to be answered – not only in the distant universe, but also in our direct neighborhood. One such question is about the origin of the Moon.
Astronomers have presented several hypothesis how the satellite of the Earth could have been formed. Most likely, the Moon has not been captured and is also not the result of a fission process . Nowadays, most scientists agree on the giant impact hypothesis: Another celestial object named Theia collided with the proto-Earth about 4.5 billion years ago . After the impact, matter in the orbit around our planet could have accumulated to form the Moon. Compared to other planet and satellite pairs, the Moon is peculiarly large. To explain the corresponding angular momentum, Theia must have been as large as Mars . But this hypothesis does not explain all characteristics of the Moon. Whereas the density differs between the Earth and the Moon, the chemical composition, mainly investigated in terms of abundances of some element isotope ratios (e.g. oxygen, titanium or tungsten), is rather similar. This is odd, because most other objects in our Solar System show significant differences that represent their different origin in the Solar System. Therefore, the Moon’s chemical composition should resemble the one of Theia – at least for the assumed impact angle and velocity and mass ratios .
One possible solution: coincidence! The composition of proto-Earth and Theia as collision partners must have been similar. Earlier this was thought to be too unlikely, but new investigations and simulations show that there is a certain probability of about 20% for this incident to happen . Subtle differences in isotope ratios may be the result of a late accretion following the impact [4,5]. But why this accretion led to the isotope ratios astronomers observe nowadays, still remains a riddle.
 A. Mastrobuono-Battisti, H. B. Perets, S. N. Raymond, A primordial origin for the compositional similarity between the Earth and the Moon, Nature 520 (2015), 212–215.
 R. M. Canup, E. Asphaug, Origin of the Moon in a giant impact near the end of the Earth’s formation, Nature 412 (2001), 708–712.
 R. M. Canup, Simulations of a late lunar-forming impact, Icarus 168 (2004), 433–456.
 M. Touboul, I. S. Puchtel, R. J. Walker, Tungsten isotopic evidence for disproportional late accretion to the Earth and Moon, Nature 520 (2015), 530–533.
 T. S. Kruijer, T. Kleine, M. Fischer-Goedde, P. Sprung, Lunar tungsten isotopic evidence for the late veneer, Nature, 520 (2015), 534–537.