At 5.4 kilometers in diameter, Phaethon is one of the largest asteroids that may pose a threat to earth.
Earth is a vulnerable place. Our blue marble is so far the only planet known to have life on its surface. Throughout billions of years since its formation, our planet has been impacted countless times by rocks and comets. Some of these impacts were catastrophic, ending nearly all life on Earth. The Dinosaurs witnessed such an impact some 66 million years ago. This collision led to their extinction. Dinosaurs, however, did not evolve as humans did. The goal of our species is to become an interstellar civilization. To do this, we must ensure that life on Earth is not threatened by comets or asteroid impacts. Eventually, we will travel and colonize other planets, but baby steps are ahead.
This is why studying comets, asteroids, and interstellar objects is of paramount importance. Recently we witnessed the massive success of the DART mission, NASA’s first planetary defense operation. Although the asteroid that was impacted by the spacecraft did not pose a threat to our world, others might.
Studying asteroids
Researchers at the University of Central Florida and Arecibo Observatory have detected a change in the rotation period of 3200 Phaethon. This space rock is a potentially hazardous near-Earth asteroid. Phaethon is just the 11th asteroid to show an observed change in its rotation period, and it is the largest of them all. The discovery represents progress in identifying potentially hazardous asteroids and shows planetary defense programs in action.
At 5.4 kilometers in diameter, Phaethon is one of the largest asteroids that may pose a threat to earth. Phaethon’s orbit, however, is very accurate, and it does not pose a threat to Earth for the foreseeable future. There is one rotation per 3.6 hours for Phaethon, and this rotation period decreases by about four milliseconds per year. 1685 Toro, with a diameter of about 3.5 kilometers, is the next largest asteroid with a measured rotation period change. A stellar object with interesting characteristics, Phaethon was selected by the Japanese Aerospace Exploration Agency (JAXA) as the mission’s target for the upcoming DESTINY+ mission scheduled for launch in 2024.
What DESTINY+ will do
Apart from making observations of Phaethon and interplanetary dust, DESTINY+ will demonstrate deep space exploration technologies. In addition to being observed with optical light curves, Phaethon has also been observed with radar from NASA’s Goldstone Deep Space Communications Complex. As seen from certain points on Earth, Phaethon has also been observed through stellar occultations, during which the asteroid passes in front of a star and causes it to disappear for a brief period. As part of the DESTINY+ mission, Arecibo planetary scientist Sean Marshall is using these observational data to determine Phaethon’s size, shape, and rotation status. In his shape model of Phaethon, Marshall used radar data, optical light curves from 1989 to 2021, and occultations from 2019 to 2021.
A similar shapre
In a similar fashion to recent spacecraft targets 101955 Bennu and 162173 Ryugu, Phaethon is somewhat rounded with a ridge around its equator. A late 2021 light curve observation, which Marshall was trying to finalize, was unexpectedly difficult to fit. According to Marshall, the predictions from the shape model did not match the data. “The times when the model was brightest were clearly out of sync with the times when Phaethon was actually observed to be brightest. Marshall 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.
Furthermore, Marshall found that a model with constant rotational acceleration could fit the entire data set from 1989 through 2021. Data from 2021 was better fit by the accelerating model, and earlier years’ data was also slightly improved. The measured acceleration is 3.7×10-8 rad/day2, which translates into Phaethon’s rotation period decreasing by approximately four milliseconds per year. Although the difference is small, it is significant enough to be noticed in 32 years of observational data spanning thousands of rotations.
