Mueller, who works with the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, has been interested in learning about small bodies in the solar system since his Ph.D. thesis, which he completed in 1997 - where he tried to apply infrared measurements of well-known targets to objects that were less well known, but still scientifically interesting. He's been working to characterize Ryugu (in collaboration with the Japan Aerospace and Exploration Agency, or JAXA) since 2008.
"Mission targets like Itokawa (Hayabusa mission in 2005) or Ryugu (Hayabusa-2 mission) always attracted my attention for many reasons," Mueller added, providing a list: "(1) The possibility to compare my model predictions with 'ground-truth' at some point; (2) the relation to space projects (I worked in the European Space Agency for several years); (3) the connection between near-Earth objects and Earth (NEAs as a risk for Earth, but also as the origin of life, water and heavy element supply); (4) to find out more about the building blocks of the planets."
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Specifically for Ryugu, Mueller says the latest research will help engineers adjust their instrument settings, do risk assessments and develop plans for what the spacecraft will do when it gets there. Some of the things they have covered include Ryugu's estimated size, brightness (known as albedo), rotation period and spin axis, thermal properties and where grains of different sizes are distributed.
But there are challenges with observing a small object from so far away. The new paper notes that because Ryugu is nearly spherical, it made it hard to get a light curve. So the astronomers combined radiometric and lightcurve inversion techniques to come up with an estimation of Ryugu's physical and thermal properties.