3200 Phaethon, one of the largest potentially hazardous asteroids, rotates once every 3.6 hours, and that rotation period is decreasing by about 4 milliseconds per year. This near-Earth asteroid is just the 11th asteroid with a measured change in its rotation period, and it is the largest of them. The next-largest asteroid with a measured change in its rotation period is the near-Earth asteroid 1685 Toro.
An artist’s illustration of what the near-Earth asteroid Phaethon might look like up close. Image credit: Heather Roper.
Phaethon was discovered on October 11, 1983 by astronomers Simon Green and John Davies in data from NASA’s Infrared Astronomical Satellite (IRAS).
Also known as 1983 TB, it is the first asteroid to be discovered in spacecraft images and is the target of JAXA’s upcoming DESTINY+ mission.
Phaethon is categorized as a so-called Apollo asteroid, as its orbital semi-major axis is greater than that of the Earth’s at 190 million km (118 million miles, or 1.27 AU).
With a volume-equivalent diameter of 5.3 km (3.3 miles), it is approximately spheroidal with an equatorial ridge, resembling the shapes of 101955 Bennu and 162173 Ryugu.
Phaethon is the third largest near-Earth asteroid classified as ‘potentially hazardous’ after asteroids 1999 JM8 and 4183 Cuno.
It is thought to be the source of the Geminid meteor shower because its orbit is very similar to the orbit of the Geminid meteors.
Phaethon is blue in color and sometimes acts like a comet. Its elongated, 524-day orbit takes it well within the orbit of Mercury, during which time the Sun heats the asteroid’s surface up to 750 degrees Celsius (1,390 degrees Fahrenheit).
These colorized radar images show the near-Earth asteroid Phaethon. At time of closest approach on December 16, 2017, Phaethon was about 1.1 million miles (1.8 million km) away. The encounter is the closest the asteroid will come to Earth until 2093. Image credit: Arecibo Observatory / NASA / NSF.
“The earliest available lightcurve of Phaethon is from October 9, 1989,” said Dr. Sean Marshall, a planetary scientist at Arecibo Observatory, and his colleagues.
“In 2016, scientists noticed that their shape model’s rotation phase differed from that 1989 observation by about 20 minutes.”
“They noted that this may be from Phaethon’s rotation period changing due to activity, but they could not say whether this was real or just a timing error in that one lightcurve, since the next lightcurves are from 1994.”
“A model with a constant rotation period provided a good fit to all of the data from 1994 through 2020. However, this model could not adequately fit the lightcurves that were acquired from four different observatories in 2021.”
“There was a difference of about 15 minutes between the predicted and observed rotation phases — clearly greater than what the observations’ uncertainties could permit.”
“We initially considered that Phaethon’s rotation period may have changed before the 2021 observations, perhaps due to activity when it was near perihelion in December 2020,” they added.
“However, we found that a constant rotational acceleration provides a good fit to all of the data from 1989 through 2021, clearly accounting for the discrepancies seen in 1989 and 2021 while also slightly improving the model’s agreement with the data during other apparitions.”
The authors find that Phaethon’s rotation period in December 2017 was 3.603944 hours.
The asteroid’s spin rate is increasing at a rate of 2.1*10-6 deg/day2, which corresponds to its 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,” Dr. Marshall said.
The astronomers reported their findings on October 7, 2022 at the 54th Annual Meeting of the American Astronomical Society’s Division for Planetary Sciences (DPS54).
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S. Marshall et al. 2022. The changing rotation period of 3200 Phaethon. DPS54, abstract # 514.07
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