Delay-Doppler images of Phaethon, acquired with Arecibo Observatory’s planetary radar system in December 2017 (Taylor et al. 2019, Planetary and Space Science 167). These images have a range resolution of 75 meters per pixel.
Credit: University of Central Florida
Near-Earth asteroid 3200 Phaethon is potentially hazardous, with an average diameter of about 5.4 kilometers. It is one of the largest asteroids that come close to Earth to be classified as potentially dangerous.
A team of scientists led by Arecibo Observatory and the University of Central Florida has measured a change in the rotation period of the asteroid 3200 Phaethon, a future spacecraft target. The finding demonstrates how planetary defense programs are progressing in categorizing potentially dangerous asteroids.
Phaethon’s orbit is known very accurately, and it poses no threat to Earth for the foreseeable future. It rotates once every 3.6 hours, decreasing that rotation period by about four milliseconds per year.
Phaethon has regularly been spotted using optical light curves, which indicate variations in its brightness as it rotates. It has also been observed using NASA’s Goldstone Deep Space Communications Complex radar. Phaethon has also been spotted by stellar occultations, in which, as seen from specific points on Earth, an asteroid appears to pass in front of a star, briefly obliterating it.
To support the DESTINY+ mission, which is scheduled to launch in 2024 and fly by Phaethon in 2028, Arecibo planetary scientist Sean Marshall is leading efforts to use these observational data to determine the size, shape and rotation state of Phaethon. Marshall developed a shape model using information from radar, optical light curves from 1989 to 2021, and occultations from 2019 to 2021. The model depicts Phaethon as top-shaped or somewhat rounded with a ridge around its equator.
While finalizing the shape model, the team unexpectedly had difficulty fitting the most recent light curve observations from late 2021.
Marshall said, “The predictions from the shape model did not match the data. The times when the model was brightest were clearly out of sync with those when Phaethon was 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, we found that the full data set, 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 slightly improved the model’s fits to data from earlier years.”
The measured acceleration is 3.710-8 rad/day2, which translates to a four-millisecond annual decrease in Phaethon’s rotation period. Despite its tiny size, this alteration has been observed in an extensive observational data collection that spans 32 years and thousands of Phaethon revolutions.
Marshall says, “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.”
There had been an earlier hint of Phaethon’s changing rotation period in an optical light curve from 1989, which was first reported by Hanuš et al. 2016.
Marshall says, “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.”
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