Researchers at the US National Science Foundation’s Arecibo Observatory were able to measure a change in rotation period on the near-Earth asteroid 3200 Phaethon. Comet Rock is considered dangerous and is the largest of 11 asteroids whose rotation period has changed. It’s an important discovery as global efforts intensify to mark potentially dangerous asteroids and develop a planetary defense program. The announcement was made at the 54th annual meeting of The division for Planetary sciences (DPS) of American Astronomical community (AAS).
Asteroids have played an important part in Earth’s history, from the extinction of the dinosaurs to shaping our planet’s geology, enriching it with minerals such as gold and rare earth elements. Asteroid mining has even been proposed for asteroids like 16 Psyche.
3200 Phaethon has fascinated scientists for many different reasons. It is an active Apollo asteroid whose orbit brings it closer to any other large asteroid. It was discovered on October 11, 1983 by Simon F. Green and John C. Davies while sifting through data from the Infrared Astronomy Satellite (IRAS). It was temporarily named the 1983 TB and received its current name, the 3200 Phaethon, in 1985.
It has a diameter of 6.25 km and it is the parent body of the Geminid meteor shower. The observational arc of Phaethon 32000 is over 35 years, and the asteroid has a well-defined orbit.
Close approach is severely limited within the next 400 years, meaning it is not a threat in the foreseeable future. The Arecibo radar detected the asteroid approaching 0.120895 AU (18,085,600 km; 11,237,900 mi) from Earth on December 10, 2007. In July 2009, 3200 Phaethon came to perihelion, indicating that it was more brighter than previously thought. The STEREO-A spacecraft calculated that its unexpected illumination made it twice as bright as expected. In 2017, it was within 10 million kilometers of Earth, with an accuracy of about 700 meters.
The asteroid is classified as a “potentially hazardous asteroid” because of three things:
Its absolute magnitude H is greater than or equal to 22, making it large enough to pose a threat
The Minimum Earth Orbit Crossing Distance (-MOID), which describes the minimum distance between Phaethon’s 3200 orbit and that of Earth, is 0.01945 astronomical units (AU) (2,910,000 km; 1,808,000 mi), which means, that he can get close enough to be a threat.
The 3200 Phaethon has a rotation period of about 3.6 hours, decreasing by about 4 milliseconds per year. 1685 Toro, the second largest asteroid with a measured change in rotation period, has a diameter of about 3.5 km.
The Japan Aerospace Exploration Agency (JAXA) selected 3200 Phaethon as one of the objects of its FATE+ (Space technology demonstration and experiment for interplanetary travel with Phaethon fLyby and Dust Science) mission.
The spacecraft will be launched in 2024 from the Uchinoura Space Center on an Epsilon S launch vehicle into low Earth orbit and then spend 1.5 years raising its orbit with ion thrusters. It will make a lunar flight of about 300,000 km (190,000 miles) to accelerate into interplanetary orbit. It is expected to fly by 3200 Phaethon in 2028 and other small near-Earth bodies around the asteroid, measure interplanetary and interstellar dust, and demonstrate technologies for future deep space exploration.
NASA’s Goldstone Deep Space Communications Complex has been observing the asteroid using radar, gathering information on optical light curves and changes in rotation. There are also observations of the asteroid through stellar occultations.
Senal Marshall, a scientist at the Arecibo Observatory, led the effort to use this data to calculate the size, shape and changes in rotation of 3200 Phaethon. Marshall and Arecibo’s team collected radar data from 1989 to 2021 and stellar eclipses between 2019 and 2021. Marshall found that 3200 Phaethon has a peak shape similar to 101955 Bennu and 162173 Ryugu. A tip-shaped asteroid is rounded and has a ridge around the equator. Marshall got stuck in developing his shape model because the light curve observations from the latter part of 2021 were not predicted by his shape model. Somehow the most recent data, which contained the brightest observations of 3200 Phaethon, could not be fitted into the model. Marshall realized that this could be the result of the asteroid’s rotation period changing shortly before last year’s sightings. He postulated that this change occurred in December 2020, when there was comet-like activity as 3200 Phaethon approached perihelion. So he reworked his model so that the rotation period accelerated steadily over the period from 1989 to 2021. 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 decelerates by 4 milliseconds per year. Over a period of decades spanning thousands of rotations, this slight dip leads to large deviations. By capturing this acceleration, scientists now have a more complete understanding of the asteroid’s orientation.