Researchers from the U.S. National Science Foundation’s Arecibo Observatory have been able to measure a change in the rotation period of the near-Earth asteroid, 3200 Phaethon. The “rock comet” is thought to be hazardous, and is the largest of 11 asteroids whose rotation period has been found to have changed. This is an important discovery as global efforts step up to mark out potentially hazardous asteroids and develop a planetary defense program. The announcement was made at the 54th Annual Meeting of The Division for Planetary Sciences (DPS) of the American Astronomical Society (AAS).

What is 3200 Phaethon?

Asteroids have played an important part of Earth’s history, from the extinction of dinosaurs, to shaping the geology of our planet, enriching it with minerals such as gold and rare earths. Astroid Mining has even been proposed on asteroids such as 16 Psyche.

3200 Phaethon fascinates scientists for very different reasons. It is an active Apollo asteroid whose orbit takes it closer to any other major asteroid. It was discovered on October 11, 1983, by Simon F. Green and John K. Davies, as they searched through data from the Infrared Astronomical Satellite (IRAS). It was provisionally named 1983 TB, and given its current name, 3200 Phaethon, in 1985. 

It has a diameter of 6.25 km and it is the Geminids meteor shower’s parent body. 32000 Phaethon’s observation arc is over 35 years, and the asteroid has a well-determined orbit. 

A close approach is highly constrained within the next 400 years, meaning it is not a threat within the foreseeable future. Arecibo’s radar detected that the asteroid approached to within 0.120895 AU (18,085,600 km; 11,237,900 mi) of Earth on December 10, 2007. In July 2009, 3200 Phaethon came to perihelion, showing itself to be brighter than previously thought. The STEREO-A spacecraft calculated that its unexpected brightening made it twice as bright as expected. In 2017, it was within 10 million km of Earth, with an accuracy of around 700m. 

The asteroid is classified as a “potentially hazardous asteroid” because of three things:

  • Its absolute magnitude H is greater or equal to 22, making it large enough to pose a threat

  • The Earth minimum orbit intersection distance (-MOID), which describes the minimum distance between 3200 Phaethon’s orbit and that of Earth, is 0.01945 astronomical units (AU) (2,910,000 km; 1,808,000 mi), meaning it can come close enough to be a threat. 

Source: Wikipedia

3200 Phaethon has a rotation period of about 3.6 hours, decreasing by around 4 milliseconds a year. 1685 Toro, the second largest asteroid with a measured change in rotation period, has a diameter of around 3.5 km. 

The DESTINY+ Mission  

The Japanese Aerospace Exploration Agency (JAXA) chose 3200 Phaethon as one of the subject of its DESTINY+ (Demonstration and Experiment of Space Technology for INterplanetary voYage with Phaethon fLyby and dUst Science) mission. 

Source: JAXA

The spacecraft will be launched in 2024, from the Uchinoura Space Center by an Epsilon S launch vehicle into low Earth orbit, and will then spend 1.5 years raising its orbit with ion engines. It will make a lunar flyby of around 300,000 km (190,000 mi) in order to accelerate into an interplanetary orbit. It is expected to fly by 3200 Phaethon in 2028 and other minor near-Earth bodies around the asteroid, measure interplanetary and interstellar dust, and demonstrate technologies for future deep space exploration.

Source: JAXA

NASA’s Goldstone Deep Space Communications Complex has observed the asteroid using radar, collecting information on optical light curves and changes in rotation. There have also been observations of the asteroid by way of stellar occultations. 

Learning More About 3200 Phaethon

Senal Marshall, an observatory scientist with Arecibo, has led efforts to use this data to calculate 3200 Phaethon’s size, shape and rotation changes. Marshall and the Arecibo team gathered together radar data from 1989 to 2021, and stellar occultations between 2019 and 2021. Marshall’s figured out that 3200 Phaethon was top-shaped, much like 101955 Bennu and 162173 Ryugu. A top-shaped asteroid is rounded and has a ridge around its equator. Marshall got stuck when developing his shape model, because the light curve observations from the latter part of 2021 were not predictable by his shape model. Somehow, the most recent data, which had the brightest observations of 3200 Phaethon, could not be fitted into the model. Marshall realized that this could be a result of the asteroid’s rotation period changing somewhat before last year’s observations. He postulated that this change came about in December 2020 when there was comet-like activity as 3200 Phaethon neared perihelion. So, he retooled his model so that the rotation period was constantly accelerating throughout the 1989 to 2021 period. With an accelerating model, he was able to fit the 2021 data and even improve the overall fit of the data.

Marshall measured an acceleration of 3.7×10-8 rad/day2, which is the asteroid’s rotation period as it declines by 4 milliseconds a year. Over a period of decades, encompassing thousands of rotations, this mild decline results in large divergences. By capturing this acceleration, scientists now have a fuller understanding of the asteroid’s orientation.