![]() ![]() After being captured, they typically orbit the Earth for no more than a few years before breaking free to reclaim an independent orbit about the Sun. None of these stays long, because gravitational tugs from our much larger permanent moon and the Sun make their orbits unstable. One study has suggested that at any one time, the Earth is likely to be accompanied by at least one temporary mini-moon greater than one metre in size that makes at least one loop around the Earth before escaping. So-called “mini-moons” like this one come and go, and 2020 CD₃ is probably already on its final loop before breaking free. The white band is the orbit of Earth’s main, permanent, moon. Perspective view of the orbit of 2020 CD3 about the Earth. Occasionally, they come near or collide with the Earth, but in this case a collision would not have been a catastrophe for us because 2020 CD₃ is so small that it would have broken up in the atmosphere before reaching the ground. The object 2020 CD₃ is essentially just a tiny member of a class of asteroids whose orbits cross the Earth’s orbit. ![]() ![]() Subsequent observations enabled its orbit to be calculated, and at 22:53 Universal Time (UT) on February 25, the Minor Planet Center announced the discovery, designating it as 2020 CD₃ and confirming that it is temporarily bound to the Earth. The body was first spotted by US astronomers Theodore Pruyne and Kacper Wierzchos using a 1.52-metre (60 inch) telescope at Mount Lemmon Observatory near Tuscon, Arizona on February 15. It is extremely faint – it is estimated to be only between one and six metres across – and won’t be with us for much longer. But while excitement about the discovery is growing, it is important to keep in mind that this moon isn’t as impressive as our main satellite. The Minor Planet Centre has just announced that the Earth has been orbited by a second moon for the past three years or so. The Open University provides funding as a founding partner of The Conversation UK. He is Educator on the Open University's free learning Badged Open Course (BOC) on Moons and its equivalent FutureLearn Moons MOOC, and chair of the Open University's level 2 course on Planetary Science and the Search for Life. He is author of Planet Mercury - from Pale Pink Dot to Dynamic World (Springer, 2015), Moons: A Very Short Introduction (Oxford University Press, 2015) and Planets: A Very Short Introduction (Oxford University Press, 2010). He has received funding from the UK Space Agency and the Science & Technology Facilities Council for work related to Mercury BepiColombo, and is currently funded by the European Commission under its Horizon 2020 programme for work on planetary geological mapping (776276 Planmap). He is co-leader of the European Space Agency's Mercury Surface and Composition Working Group, and a Co-Investigator on MIXS (Mercury Imaging X-ray Spectrometer) that is now on its way to Mercury on board the European Space Agency's Mercury orbiter BepiColombo. He is writing this in his own time, because currently he is on strike with fellow members of the University and College Union in a dispute mainly about pension funding and casualization. Professor of Planetary Geosciences, The Open Universityĭavid Rothery is Professor of Planetary Geosciences at the Open University.
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