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Thermography in space: Solving the mystery of how asteroids form

Hayabusa2 has the same basic design as the first Hayabusa, but with an array of new capabilities. Among them is a type of camera designed by NEC called a thermal infrared (TIR) imager, which senses the asteroid’s temperature. This imager is so accurate that it can measure temperatures of between minus 40˚C and plus 150˚C to within 0.3˚C — even in outer space where it cannot be serviced by human hands.
Thermographic cameras are often used on the news — when reporting on heat stress during summer, for example — to display temperature distribution intuitively by showing warmer areas in red and cooler areas in blue. The Hayabusa2 mission team employed the same technique. They used the TIR imager to image temperature fluctuations throughout Ryugu over the course of the day, and created a color-coded map of temperature variations on the asteroid. Nothing like it had ever been done before.

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Ryugu’s temperature profile as imaged by Hayabusa2’s thermal infrared (TIR) imager at an altitude of 20km on June 30, 2018, immediately after its arrival at the asteroid.
©JAXA, Ashikaga University, Rikkyo University, Chiba Institute of Technology, University of Aizu, Hokkaido University of Education, Hokkaido Kitami Hokuto High School, National Institute of Advanced Industrial Science and Technology, National Institute for Environmental Studies, University of Tokyo, German Aerospace Center, Max Planck Society, University of Stirling

This image revealed that the boulders making up Ryugu are extremely porous, like a sponge — a finding published in the 17 March 2020 issue of the British journal Nature, one of the world’s leading scientific publications.

Porous boulders gets warm and cool rapidly, whereas dense boulders are slow to warm up but also cool only gradually. Because of that property, a boulder’s temperature shows how porous it is. Using the TIR imager to monitor how the asteroid’s temperature fluctuated over the course of the day revealed that Ryugu is covered in boulders far more porous than the meteorites that strike Earth.

Asteroids like Ryugu are repositories of the history of the solar system, but scientists previously had almost no opportunity to observe them up-close. No one really knew how they were formed. Hayabusa2’s observations have led to a new hypothesis about their origin. Ryugu’s parent body, it is now believed, consisted of highly porous boulders formed from cosmic fluffy dust; this then disintegrated upon collision with another body, and its fragments coalesced to form what is now Ryugu. Thus an asteroid’s origins can be deduced from its temperature profile as revealed by the TIR imager.

Researched and written by Ayano Akiyama
Published: September 18, 2020

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