Space dust weighing with radar

Over 1,000 kilograms of so-called interplanetary dust are thought to fall on Earth every day. This dust is essentially an untold number of small faint meteors, discarded remnants of asteroids and comets that pass close to Earth. Two ways to study faint meteors are radar and optical observations, each with advantages and limitations. Astronomers have combined specific observations with both methods and can now use radar to make the kind of observations that previously only optical telescopes could do.

Our solar system is a hectic place: in addition to the large bodies we all know, there are an innumerable number of rocky asteroids and icy comets. These remain mostly positioned in their orbits away from Earth, but many also roam the solar system. As they do so, they lose some material due to shock, deformation or heating. For this reason, the Earth is surrounded by small particles that we call interplanetary dust. By investigating the size and composition of interplanetary dust, astronomers can indirectly investigate the activity and composition of parent bodies.

“In space, interplanetary dust is practically invisible. However, around 1,000 kilograms fall to Earth every day in the form of tiny meteors that appear as bright streaks in the night sky, “said astronomer Ryou Ohsawa of the Tokyo University Institute of Astronomy.” We can observe them with ground-based radars and optical instruments. Radar is useful in that it can cover large areas and collect large readings, but optical telescopes can provide more detailed information useful for our studies. So we decided to close this gap to increase our observation capacity. “

Earth’s radar is very good at detecting the movement of meteors, but it doesn’t reveal much information about the mass or composition of the meteors. Optical telescopes and sensors can deduce those details based on the light emitted by the falling meteors due to interaction with the atmosphere. However, telescopes have a limited field of view and until recently did not have the sensitivity to see faint meteors. Ohsawa and his team wanted to give radar observatories the powers of optical ones. After a few years they finally succeeded.

“We thought that if we could observe enough meteors at the same time with both radar and optical structures, the details of the meteors in the optical data could match patterns that were previously invisible even in the radar data,” Ohsawa said. “I am pleased to announce that this is indeed the case. We have recorded hundreds of events over several years and have now gained the ability to read meteor mass information from subtle signals in radar data. “

In 2009, 2010 and 2018, the team used the radar facility for medium and high atmosphere (MU), operated by Kyoto University and located in Shigaraki, Shiga Prefecture, and the Kiso Observatory, managed by the University. of Tokyo, on the Nagano Prefecture side of Mount Ontake. They are 173 kilometers apart, which is important: the closer the structures are, the more accurately the data can be correlated. MU points straight up, but Kiso can be angled, so it was aimed 100km above MU’s site. The team saw 228 meteors with both structures and this was enough to derive a statistically reliable relationship to link the radar and optical observations.

“The data analysis was laborious,” Ohsawa said. “A sensitive instrument called the Tomo-e Gozen wide-field camera mounted on the Kiso telescope captured over a million images per night. This is too much for us to analyze manually, so we have developed software to automatically recognize faint meteors. From what we have learned here we hope to extend this project and start using radar to study the composition of meteors. This could help astronomers explore comets and aspects of solar system evolution like never before. “