The near-Earth asteroid 3200 Phaethon is potentially dangerous, with an average diameter of about 5.4 kilometers. It is one of the largest asteroids to come close to Earth to be classified as potentially hazardous.
A team of scientists led by the Arecibo Observatory and the University of Central Florida has measured a change in the rotation period of the asteroid 3200 Phaethon, a future target of the spacecraft. The finding demonstrates how planetary defense programs are making progress in categorizing potentially dangerous asteroids.
Phaethon’s orbit is known very precisely and poses no threat to Earth in the foreseeable future. It rotates once every 3.6 hours, decreasing this rotation period by about four milliseconds per year.
Phaethon has been regularly seen 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 detected 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 this observational data to determine the size, shape and Phaethon’s state of rotation. Marshall developed a shape model using radar information, optical light curves from 1989 to 2021, and occultations from 2019 to 2021. The model depicts Phaethon as superior or slightly rounded with a ridge around its equator.
While finalizing the shape model, the team had unexpected difficulty fitting the most recent light curve observations from late 2021.
Marshall said: “The shape model predictions 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 that this could explain why Phaethon’s rotation period changed slightly sometime before the 2021 observations, perhaps due to comet-like activity when it was near perihelion in December 2020.”
“After investigating more carefully, we found that the entire data set, from 1989 to 2021, could be fitted by a model with constant rotational acceleration. This accelerated model provided a much better fit to the data from 2021 and slightly improved model fits to data from earlier years.”
The measured acceleration is 3.710-8 rad/day2, which translates into an annual decrease of four milliseconds in Phaethon’s rotation period. Despite its small size, this alteration has been observed in an extensive collection of observational data spanning 32 years and thousands of revolutions of Phaethon.
Marshall says: “This is good news for the DESTINY+ team, as a constant shift means that Phaethon’s orientation at the time of the spacecraft flyby can be accurately predicted, so it will know which regions will be illuminated by the Sun”.
There had been an earlier indication of Phaethon’s rotation period change in an optical light curve since 1989, which was first reported by Hanuš et al. 2016
Marshall says, “This model of Phaethon was also out of sync with this light curve observation, but there was still not enough other data to explain this discrepancy.”
The discovery was announced at the 54th annual meeting of the Division of Planetary Sciences of the American Astronomical Society.