Asteroid 1I/'Oumuamua is the first observed interstellar object. Its light-curve amplitude indicates that the object is highly elongated with an axis ratio of at least 5:1. The absence of such elongated asteroids in the Solar system, as well as the apparent lack of observed interstellar objects, are intriguing problems. Here we show that 'Oumuamua may have originated as a slightly-elongated asteroid about 500 × 300 meters in size. Surface erosion, caused by interstellar dust bombardment, subsequently increased the axis ratio. Simply traveling through the interstellar medium for 0.03 to 2 Gyrs would have sufficed to give 1I its present shape. Passing through a 10 pc dust cloud with a grain density of 10 −23 g/cm 3 at 50 km/s would have had a similar effect on 'Oumuamua's form. Smaller objects of around 100 meters in diameter can travel the Galactic disk for merely 30 Myrs before they are disrupted. This could explain the small number of interstellar objects observed to date.

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International audienceVariations of plasma distribution and/or wave spectral features in the ionosphere were suggested by many authors as possible earthquake precursors, and the change of plasma density and temperature above seismic regions were reported in the literature. These quantities are known to influence the lower hybrid resonance (LHR) frequency profiles in the upper ionosphere and the magnetosphere, which, in turn, strongly affects the propagation of quasi-resonance VLF waves with frequencies f close to the maximum of the LHR frequency on the propagation path. This makes the VLF signals a tool of registration of ionospheric perturbations. Using the measurements from the DEMETER satellite for 3 yr we have calculated the maps of LHR frequency over the globe, and the maps of VLF spectral intensity at the frequencies of Alpha navigation transmitters. These maps demonstrate a significant dependence of the spectral intensity in the transmitter conjugate region on the relation between the signal frequency and the LHR frequency above the observation point. Then, using the DEMETER data and the earthquake database from the US geological survey server we have performed statistical analysis of the LHR frequency over seismic regions and found an appreciably different behaviour of the LHR frequency before earthquakes, as compared to its regular behaviour, for several seismic regions. Although this difference is statistically significant, in each particular case the ionospheric perturbations may be related to different processes in the Earth's atmosphere, ionosphere, and the magnetosphere, other than gathering earthquakes. Thus, the unexpected variations in the LHR frequency profile, revealed from the variations of VLF transmitter signals, should only be considered as one indicator in a list of possible earthquake precursors

Context. Breathtaking imagery recorded during the European Space Agency's Rosetta mission confirmed the bilobate nature of comet 67P/Churyumov-Gerasimenko's nucleus. Its peculiar appearance is not unique among comets. The majority of cometary cores imaged at high resolution exhibit a similar build. Various theories have been brought forward as to how cometary nuclei attain such peculiar shapes.Aims. We illustrate that anisotropic mass loss and local collapse of subsurface structures caused by non-uniform exposure of the nucleus to solar irradiation can transform initially spherical comet cores into bilobed ones. Methods. A mathematical framework to describe the changes in morphology resulting from non-uniform insolation during a nucleus' spin-orbit evolution is derived. The resulting partial differential equations that govern the change in the shape of a nucleus subject to mass loss and consequent collapse of depleted subsurface structures are solved analytically for simple insolation configurations and numerically for more realistic scenarios. Results. The here proposed mechanism is capable of explaining why a large fraction of periodic comets appear to have peanut-shaped cores and why light-curve amplitudes of comet nuclei are on average larger than those of typical main belt asteroids of the same size.