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Beyond Earthly Skies

A Low-Density Mid-Size KBO

17 Apr 2015, 22:00 UTC
A Low-Density Mid-Size KBO
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2002 UX25 is a mid-size Kuiper Belt Object (KBO) with an estimated diameter of about 650 km. It orbits the Sun with an orbital period of roughly 280 years. 2002 UX25 has a tiny moon in orbit around it. Measuring the moon’s orbit allows the mass of 2002 UX25 to be estimated. With the size and mass known, 2002 UX25 is found to have a remarkably low density of only 0.82 ± 0.11 g/cm³. This makes 2002 UX25 the largest known object in the Kuiper Belt with a measured density below that of pure water-ice (~1.0 g/cm³). Its surface gravity is only 1/125th the gravity on Earth.Small KBOs with diameters less than ~350 km have densities below that of pure water-ice due to their high porosities. However, an object as large as 2002 UX25 is unlikely to have a porosity of more than ~20 percent because ice is more compressed due to the higher pressure in such a large object. Basically, smaller objects tend to have lower densities, while larger objects tend to have higher densities. The low density of 2002 UX25 and its inability to support a high porosity suggest that a very small fraction of its mass is in ...

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