A team of scientists led by Arecibo Observatory and the University of Central Florida has measured a change in the rotation period of the potentially hazardous near-Earth asteroid 3200 Phaethon, a future spacecraft target.

Phaethon is just the 11th asteroid with a measured change in its rotation period, and it is the largest of them.

The discovery is an example of progress in global efforts to characterize potentially hazardous asteroids and shows planetary defense programs at work.

The researchers announced the discovery at the 54th annual meeting of the American Astronomical Society’s Division for Planetary Sciences.

Phaethon has an average diameter of about 5.4 kilometers, making it one of the largest asteroids that comes close enough to Earth to be classified as potentially hazardous. However, Phaethon’s orbit is known very accurately, and it poses no threat to Earth for the foreseeable future.

Phaethon rotates once every 3.6 hours, and that rotation period is decreasing by about 4 milliseconds per year. The next-largest asteroid with a measured change in its rotation period is near-Earth asteroid 1685 Toro, with a diameter of about 3.5 kilometers.

Due to its interesting characteristics, the Japanese Aerospace Exploration Agency (JAXA) selected Phaethon as the target of the upcoming DESTINY+ mission, which is scheduled to launch in 2024 and fly by Phaethon in 2028. The DESTINY+ mission’s objectives are to make observations of Phaethon and interplanetary dust and to demonstrate technologies for deep space exploration.

Phaethon has been frequently observed with optical light curves, showing variations in its brightness as it rotates, and it was observed by radar from NASA’s Goldstone Deep Space Communications Complex. Phaethon also has been observed via stellar occultations, in which the asteroid appears to pass in front of a star, as seen from certain locations on Earth, causing the star to briefly disappear.

Arecibo planetary scientist Sean Marshall has been leading efforts to use these observational data to determine the size, shape and rotation state of Phaethon, in support of the DESTINY+ mission.

With radar data, optical light curves from 1989 through 2021, and occultations from 2019 through 2021, Marshall derived a shape model that shows Phaethon to be top-shaped – somewhat rounded with a ridge around its equator, similar to the shapes of recent spacecraft targets 101955 Bennu and 162173 Ryugu.

While trying to finalize the shape model, Marshall unexpectedly had difficulty fitting the most recent light curve observations, from late 2021.

“The predictions from the shape model did not match the data,” Marshall says. “The times when the model was brightest were clearly out of sync with the times when Phaethon was actually observed to be brightest. I realized this could be explained by Phaethon’s rotation period changing slightly at some time before the 2021 observations, perhaps from comet-like activity when it was near perihelion in December 2020.”

After investigating more carefully, Marshall says he found that the full set of data, from 1989 through 2021, could be fit by a model with constant rotational acceleration. This accelerating model provided a much better fit to the data from 2021 and also slightly improved the model’s fits to data from earlier years.

The measured acceleration is 3.7×10-8 rad/day2, which is equivalent to Phaethon’s rotation period decreasing by about 4 milliseconds per year. This change, though small, is enough to be noticeable in an extensive set of observational data spanning 32 years and thousands of rotations of Phaethon.

“This is good news for the DESTINY+ team, since a steady change means that Phaethon’s orientation at the time of the spacecraft’s flyby can be predicted accurately, so they will know which regions will be illuminated by the Sun,” Marshall says.

Phaethon was discovered by Simon Green and John Davies in 1983 in data from the Infrared Astronomical Satellite (IRAS) – the first asteroid to be discovered in spacecraft data. Shortly after its discovery, Fred Whipple recognized it as the parent body of the Geminid meteor shower that is visible from Earth in mid-December.

Originally designated as 1983 TB, the asteroid was later named Phaethon, the son of the sun god Helios in Greek mythology. This name is appropriate because its orbit takes it within 21 million kilometers (13 million miles) of the Sun – less than half the perihelion distance of the planet Mercury. Observations have shown that Phaethon has an extremely blue spectrum, a rarity among asteroids.

Phaethon’s closest approach since its discovery occurred in December 2017, when it passed 10 million kilometers (6.4 million miles, or 27 lunar distances) from Earth. During five nights around that close approach, Phaethon was observed with Arecibo Observatory’s NASA-funded planetary radar system, yielding a detailed set of radar images. This was Arecibo’s first major planetary radar campaign after Hurricane Maria caused heavy damage across Puerto Rico, less than three months earlier. The Arecibo Observatory team was able to restore the telescope to operational status in time to observe Phaethon. Arecibo had the most powerful and most sensitive planetary radar system that has ever been built.

There had been an earlier hint of Phaethon’s changing rotation period in an optical light curve from 1989, which was first reported in Hanuš et al. 2016 (Astronomy & Astrophysics 592). This Phaethon model was also out of sync with that light curve observation, but there were not yet enough other data to explain this discrepancy, Marshall says.

The list of authors for this discovery includes observers from all over the world who contributed Phaethon observations from 2018 onward that were incorporated into the shape model. This list includes: Sean Marshall and Maxime Devogèle (Arecibo Observatory & University of Central Florida); Patrick Taylor (National Radio Astronomy Observatory); Christopher Magri (University of Maine at Farmington); Jin Beniyama (University of Tokyo); Tomohiko Sekiguchi (Hokkaido University of Education); Daisuke Kuroda (Kyoto University); Seitaro Urakawa (Japan Safeguard Association); Fumi Yoshida (University of Occupational and Environmental Health  Japan); Tomoko Arai (Chiba Institute of Technology); Brian Warner (Center for Solar System Studies); Petr Pravec, Hana Kučáková, Kamil Hornoch, and Peter Kušnirák (Academy of Sciences of the Czech Republic); Josef Hanuš (Charles University); Marin Ferrais (Laboratoire D’Astrophysique de Marseille); Emmanuel Jehin (Université de Liège); Eduard Kuznetsov, Dmitrij Glamazda, Galina Kaiser, Andrej Shagabutdinov, and Yuliya Vibe (Ural Federal University); Aleksander Serebryanskiy, Maxim Krugov, and Inna Reva (Fesenkov Astrophysical Institute); Myung-Jin Kim and Dong-Heun Kim (Korea Astronomy and Space Science Institute).

Arecibo Observatory is a facility of the U.S. National Science Foundation. The facility is managed by the University of Central Florida (UCF), under a cooperative agreement with Universidad Ana G. Méndez and Yang Enterprises, Inc. Arecibo’s planetary radar program is fully supported by NASA’s Near-Earth Object Observations Program in NASA’s Planetary Defense Coordination Office through a grant awarded to UCF.