A team of scientists led by the Arecibo Observatory and the University of Central Florida have measured a change in the rotation period of the potentially dangerous near-Earth asteroid 3200 Phaethon, a future spacecraft target.
Phaethon is only 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 dangerous asteroids and shows planetary defense programs at work.
The researchers announced the discovery at the 54th Annual Meeting of the Division of Planetary Sciences of the American Astronomical Society.
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 dangerous. However, Phaethon’s orbit is known very precisely and poses no threat to Earth for the foreseeable future.
Phaethon rotates once every 3.6 hours, and this rotation period decreases by about 4 milliseconds per year. The second 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 Japan Aerospace Exploration Agency (JAXA) has selected Phaethon as the target of the next DESTINY+ mission, scheduled to launch in 2024 and flyby Phaethon in 2028. The DESTINY+ The objectives of the mission 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 has been observed by radar at NASA’s Goldstone Deep Space Communications Complex. Phaethon has also been observed via stellar occultations, in which the asteroid appears to pass in front of a star when viewed from certain locations on Earth, causing the star to briefly disappear.
Arecibo planetary scientist Sean Marshall led efforts to use this observational data to determine Phaethon’s size, shape and rotational state, in support of the DESTINY+ mission.
With radar data, optical light curves from 1989 to 2021, and occultations from 2019 to 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 lightcurve observations, from late 2021.
“The shape model predictions did not match the data,” says Marshall. “The times when the model was at its brightest were clearly out of sync with the times when Phaethon was actually observed to be at its brightest. I realized this could be explained by Phaethon’s rotation period changing slightly at some point before the 2021 observations, possibly due to comet-like activity while it was close to perihelion in December 2020.”
After studying more carefully, Marshall says he discovered that the entire data set, from 1989 to 2021, could be fitted by a constant rotational acceleration model. This acceleration model provided a much better fit to 2021 data and also slightly improved the model fits to data from previous 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 a large observational dataset spanning 32 years and thousands of Phaethon rotations.
“This is good news for the DESTINY+ team, as constant change means Phaethon’s orientation at the time of the spacecraft’s flyby can be accurately predicted, so they will know which regions will be illuminated by the Sun. “Marshall said.
Phaethon was discovered by Simon Green and John Davies in 1983 in Infrared Astronomical Satellite (IRAS) data – 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 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 distance to the perihelion 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. For five nights around this close approach, Phaethon was observed with the Arecibo Observatory’s NASA-funded planetary radar system, producing a detailed set of radar images. This was Arecibo’s first major planetary radar campaign after Hurricane Maria caused heavy damage in Puerto Rico less than three months earlier. The Arecibo Observatory team was able to restore the telescope to working order in time to observe Phaethon. Arecibo had the most powerful and sensitive planetary radar system ever built.
There had been an earlier hint of Phaethon’s changing rotational period in a 1989 optical lightcurve, which was first reported in Hanuš et al. 2016 (Astronomy and Astrophysics 592). This Phaethon model was also out of sync with this lightcurve observation, but there was not yet enough other data to explain this discrepancy, Marshall says.
The list of authors of this discovery includes observers from around the world who contributed observations of Phaethon from 2018 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 Safeguarding Association); Fumi Yoshida (University of Occupational and Environmental Health of Japan); Tomoko Arai (Chiba Institute of Technology); Brian Warner (Center for Solar System Studies); Petr Pravec, Hana Kučáková, Kamil Hornoch and Peter Kušnirák (Czech Republic Academy of Sciences); Josef Hanuš (Charles University); Marin Ferrais (Marseille Astrophysics Laboratory); Emmanuel Jehin (University of Liège); Eduard Kuznetsov, Dmitrij Glamazda, Galina Kaiser, Andrej Shagabutdinov and Yuliya Vibe (Ural Federal University); Aleksander Serebryanskiy, Maxim Krugov and Inna Reva (Fesenkov Institute of Astrophysics); Myung-Jin Kim and Dong-Heun Kim (Korean Institute of Astronomy and Space Science).
The Arecibo Observatory is a facility of the US National Science Foundation. The facility is operated by the University of Central Florida (UCF), under a cooperative agreement with Ana G. Méndez University and Yang Enterprises, Inc. Arecibo’s planetary radar program is fully supported supported by NASA’s Near-Earth Object Observing Program in NASA’s Planetary Defense Coordination. Office through a grant awarded to UCF.
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