Meanwhile : An asteroid’s fleeting shadow

Audio report: written by reporters, read by AI

Moon Hong-kyu

The author is a principal researcher at the Korea Astronomy and Space Science Institute. 

A bright full moon once turned a deep, amber red. In the early hours of Sept. 8 last year, a total lunar eclipse unfolded as Earth’s shadow swallowed the moon whole. The eclipse lasted about an hour and a half over Korea’s night sky. Earth’s shadow was wide enough to cover the moon twice.

Seven months later, on April 19, the celestial stage shifted from Earth to an asteroid. A dark, coal-like object named Hermione, whose long axis is comparable to the distance between Gwangju and Busan, passed in front of a faint star known only by a numerical designation. Because the asteroid resembles a misshapen potato, the duration of its shadow varied by location. In the southern part of the Korean Peninsula, the star vanished and reappeared for as long as one minute.

The phenomenon, known as a stellar occultation, allows scientists to capture a fleeting moment using high-speed digital cameras. Without launching a probe, they can determine the size and outline of a tiny asteroid. It is an unexpected scientific gain. Hermione also has a small moon roughly the size of the distance between Seoul Station and Jamsil Station. Researchers are attempting to trace its orbit as well.

That is far from simple. The moon’s shadow sweeps across a path as long as the distance from Seoul to New York at a speed that outpaces the cooling of a cup of coffee. To capture such a moment, observers rely on GPS signals to measure time down to a few milliseconds.

In the early hours of that day, teams from the Korea Astronomy and Space Science Institute, a local government observatory and a youth training center positioned themselves along the southern path of the asteroid’s shadow. Across Japan, Taiwan and China, about 40 observation sites formed a dense east-west line spanning Hainan Province and the Japanese archipelago to capture the shadow of the asteroid’s moon.

Weather conditions, however, did not cooperate. Rain left observers in Korea empty-handed, and western Honshu and Kyushu faced similar conditions. In contrast, Taiwan and eastern Japan enjoyed clear, almost midsummer skies.

The multinational effort was also a rehearsal for Japan’s asteroid mission “Density+.” The disappointment was palpable. Researchers now await data from regions where the skies held. Results may not arrive until early summer.

This article was originally written in Korean and translated by a bilingual reporter with the help of generative AI tools. It was then edited by a native English-speaking editor. All AI-assisted translations are reviewed and refined by our newsroom.