Oct. 13 () –
Astronomers have measured a change in the rotational period of the near-Earth asteroid 3200 Phaethon, potentially dangerousfuture objective of the Japanese space mission DESTINY+.
Phaethon is only the 11th asteroid with a measured change in its rotation period, and it is the largest of them.
A team of scientists led by the Arecibo Observatory and the University of Central Florida announced the discovery at the 54th annual meeting of the Division for Planetary Sciences of the American Astronomical Society.
Phaethon has an average diameter of about 5.4 kilometers, making it one of the largest asteroids to come close enough to Earth to be classified as potentially dangerous. However, Phaethon’s orbit is known very precisely and does not pose a threat to Earth for the foreseeable future.
Phaethon rotates once every 3.6 hours, and that rotation period is decreasing by about 4 milliseconds per year.. The next largest asteroid with a measured change in its rotation period is near-Earth asteroid 1685 Torus, with a diameter of about 3.5 kilometers.
Due to its interesting features, the Japan Aerospace Exploration Agency (JAXA) has selected Phaethon as the target for the next DESTINY+ mission, which is scheduled to launch in 2024 and will fly over Phaethon in 2028. The DESTINY+ mission objectives are to conduct observations of Phaethon and interplanetary dust and to demonstrate technologies for deep space exploration.
Phaethon has been frequently observed with optical light curves, which show variations in its brightness as it rotates, and was observed by NASA’s Goldstone Deep Space Communications Complex radar. Phaethon has also been observed through stellar occultations, in which the asteroid appears to pass in front of a star, seen from certain locations on Earth, causing the star to briefly disappear.
Arecibo planetary scientist Sean Marshall has been leading efforts to use these observational data to determine Phaethon’s size, shape, and rotational state. in support of the DESTINY+ mission.
Using radar data, optical light curves from 1989 to 2021, and occultations from 2019 to 2021, Marshall derived a shape model showing that Phaethon has a somewhat rounded upper shape with a ridge around its equator, similar to the shapes of the recent spacecraft targets 101955 Bennu and 162173 Ryugu.
While trying to finalize the shape model, Marshall unexpectedly had difficulty to fit the most recent light curve observations, from late 2021.
“The shape model predictions didn’t match the data,” Marshall says. it’s a statement. “The times when the model was brightest were clearly out of sync with the times when Phaethon was observed to be brightest. I realized this could be explained by Phaethon’s rotation period changing slightly at some point before the 2021 Observations, perhaps due to comet-like activity when near perihelion in December 2020.”
After further investigation, Marshall says he found that the entire data set, from 1989 to 2021, could be fitted to a model with constant rotational acceleration. This accelerated model provided a much better fit to the 2021 data and also slightly improved the model fits to data from previous years.
The measured acceleration is 3.7 x 10-8 rad/day2, which is equivalent to Phaethon’s rotational period decreasing by about 4 milliseconds per year. This change, although small, is enough to be noticed in an extensive observational dataset spanning 32 years and thousands of rotations of Phaethon.
“This is good news for the DESTINY+ team, as constant change means Phaethon’s orientation at the time of the spacecraft’s flyby can be accurately predicted, so they will know which regions will be illuminated by the Sun.” Marshall says.
Phaethon was discovered by Simon Green and John Davies in 1983 in Infrared Astronomical Satellite (IRAS) data, the first asteroid discovered in spacecraft data. Shortly after its discovery, Fred Whipple recognized it as the main body of the Geminid meteor shower that is visible from Earth in mid-December.
Originally designated 1983 TB, the asteroid was later named Phaethon, the son of the sun god Helios in Greek mythology. This name is appropriate because its orbit takes it within 21 million kilometers (13 million miles) of the Sun, less than half the perihelion distance of the planet Mercury. Observations have shown that Phaethon has an extremely blue spectrum, a rarity among asteroids.
Phaethon’s closest approach since its discovery occurred in December 2017, when it passed within 10 million kilometers (6.4 million miles or 27 lunar distances) of Earth. For five nights around that close approach, Phaethon was observed with the Arecibo Observatory’s NASA-funded Planetary Radar System, which produced a detailed set of radar images. This was Arecibo’s first major planetary radar campaign.
