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A large, potentially dangerous asteroid for Earth is spinning faster and faster for an unknown reason

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The astronomical observations did not fit the elaborated models if a constant acceleration in the rotation of the asteroid was not assumed.

A team of scientists led by the Arecibo Observatory, in Puerto Rico, and the University of Central Florida, in the USA, found an unexpected acceleration in the rotation of the asteroid 3200 Phaethon, discovered in 1983. Although its orbit is precisely known and it is not a threat for the near future, its large size, about 5.4 km in diameter, and its proximity to Earth, classify it as potentially dangerous. Phaethon makes a complete turn around itself every 3.6 hours, and its rotation period decreases by about 4 milliseconds per year, astronomers reported.

The Japan Exploration Agency Aerospace (JAXA) will send Phaethon on 2024 the DESTINY+ space mission, so that in addition to testing technologies for deep space exploration, at 2028 make observations of the asteroid and interplanetary dust and carry out a study on that celestial body, responsible for the meteor shower known as the ‘Geminids’. In particular, the determination of the physical characteristics of this asteroid generates a lot of interest for astronomers, since the data obtained will be essential for the success of a future mission.

For his analyses, Sean Marshall, a planetary scientist from Arecibo, has resorted to optical light curves that show variations in brightness at as the asteroid rotates, as well as observations of the moment it appears to pass in front of a star, causing the star to briefly disappear, to determine Phaethon’s size, shape, and rotation.

Thanks to the optical light curves captured between 1989 and 2021 and stellar occultations from 2019 to 2021 , Marshall derived a model showing that Phaethon has a somewhat rounded top, with a ridge around its equator. However, he had difficulty matching the light curve observations from the end of 2021 with the developed model. “Shape model predictions did not match the data”, he commented.

“The times when the model looked brightest were clearly out of sync with those when it actually was. I realized that this could be explained by the fact that Phaethon’s rotation period changed slightly at some point. time before the 2021 observations, perhaps due to comet-like activity, when it was near perihelion , in December of 2020,” explained the astronomer.

The scientist discovered that the model could fit the data of 2021 if a constant rotational acceleration was assumed.Furthermore, that accelerated model slightly improved the fit in terms of data from previous years. The calculated acceleration is equivalent to a decrease in the period of rotation of Phaethon by about 4 milliseconds per year. That change, insignificant , could only be detected after 32 years of observation and thousands of Phaethon rotations.

“This is good news for the DESTINY+ team, as constant change means that Phaethon’s orientation, by the time of a spacecraft flyby, can be accurately predicted, so it will be known which regions will be illuminated by the Sun”, underlined Marshall.

The researchers announced this discovery about the acceleration of the aster oide, the eleventh with a measured change in its rotation period and the largest of them, at the 54th annual meeting of the Division of Planetary Sciences of the American Astronomical Society.

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