Based on the data, it was determined that the elongated asteroid measures 12.6 x 6.8 km.
Keystone / Valentin Flauraud
The Schafmatt Observatory has discovered an asteroid moon. Experts are calling it a sensational discovery.
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“It suddenly became clear to me that I had discovered something truly special,” says amateur astronomer Josef Käser. “I was simply delighted.” He hadn’t expected to discover a moon near an asteroid on that summer evening.
But that’s how it was, at 1:00 am on July 13. Josef Käser’s goal at the Schafmatt Observatory was to determine the size and shape of an asteroid – known in technical terms as (108968) 2001 PE40. This is determined by the exact time the asteroid can occult a star. As expected, the asteroid succeeded in doing so for 1.1 seconds.
But then the completely unexpected happened: About 1.5 seconds after the star’s reappearance, the starlight disappeared a second time, for a fraction of a second.
Fellow astronomers in Eberfing, Germany, recorded the same observation and confirmed that it is only one possible conclusion: the occulting asteroid is orbited by a previously unknown moon.
The brief occultation of the star by the moon – the term used when the star is briefly no longer visible –was not only visible to the naked eye; Josef Käser recorded the occultation in detail and later analysed it.
Based on the data, it was determined that the elongated asteroid measures 12.6 x 6.8 km. Its newly discovered moon, in contrast, measures only 2.9 x 1.83 km and orbits the asteroid at a distance of 23.9 kilometers.
Career highlight
Josef Käser has been an amateur astronomer for 50 years and has recorded stellar occultations on numerous occasions. But the discovery of a new, unknown moon is the highlight of his astronomical career. “If you imagine that such an asteroid has been hurtling through space for 400 million years, and that it might only obscure the light of another star for a little more than a second when viewed from Schafmatt, then I find it fascinating how much precision is required to discover an asteroid moon.”
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The website of Orion, the Swiss journal for astronomy enthusiasts, calls it a “sensational discovery.” Marc Eichenberger, President of the Swiss Astronomical Society (SAG), doesn’t want to go quite that far. In football terms, he says this discovery is comparable to winning the Swiss championship, but not to winning the Champions League.
“This discovery is a very nice achievement for Switzerland, but it’s not as if such a discovery only happens once in 100 years,” he said. A few asteroid moons have been spotted around the world in recent years. Each of these discoveries is a piece of the puzzle in small planet research.
Translated from German by DeepL/jdp
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