It was in 1983 when Simon Green and John Davies spotted Phaethon, the 5.4 kilometers-wide asteroid in data from the Infrared Astronomical Satellite. The asteroid came close to Earth in 2017, passing from a distance of just 10 million kilometers away from the planet.
The potentially hazardous asteroid has been behaving strangely and scientists have observed a change in the rotation period of this near-Earth object. While this is the 11th asteroid with a measured change in its rotation period, it is the largest ever discovered. The orbit of the asteroid is known with pinpoint accuracy, but astronomers are not sure why its rotation is changing.
The new findings were announced at the 54th annual meeting of the American Astronomical Society’s Division for Planetary Sciences. While it poses no threat to Earth, the asteroid has emerged as a target for the Japanese Aerospace Exploration Agency’s (JAXA) DESTINY+ mission that will launch in 2024.
Phaethon rotates once every 3.6 hours, but new observations reveal that the rotation period is decreasing by about 4 milliseconds per year. While that time change is barely noticeable, it can be seen in an extensive set of observational data spanning 32 years and thousands of rotations of Phaethon.
The NEO has been regularly observed with optical light curves, showing variations in its brightness as it rotates, and it was observed by radar from Nasa’s Goldstone Deep Space Communications Complex.
Based on radar data optical light curves from 1989 through 2021, and occultations from 2019 through 2021, astronomers at the Arecibo observatory derived a shape model of the object. The model 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.
“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. 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,” Marshall said in a release.
The team found that the data was fitting accurately on a model with constant rotational acceleration. The new data can be used by the Jaxa team to analyse the asteroid’s orientation.
“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,” Marshall added.
