The Geminids are an extraordinary meteor shower in that their progenitor is an asteroid, making it the only meteor shower with such an origin. 3200 Phaethon, the asteroid that is responsible for the Geminids, measures only 5.10 km in diameter, which further adds to the uniqueness of this phenomenon; considering the sheer quantity of meteors that we witness during this shower, would expect the asteroid to be much larger. Discovered in 1983 using the Infrared Astronomical Satellite, Phaethon was named after the Greek myth of Phaethon, son of the sun god Helios due to its close proximity to our Sun. In summary, the Geminids are a rare and remarkable meteor shower in that their source is an asteroid, rather than a comet like most other showers.
Asteroid 3200 Phaethon has been known to act like a comet for a while now, showing a brightening and forming a tail when it’s near the Sun and being the source of the Geminid meteor shower. Scientists had thought this was due to dust escaping from the asteroid when it gets scorched by the Sun. However, new research using two NASA solar observatories has revealed that Phaethon’s tail is actually made of sodium gas, not dust, which is an unprecedented discovery. This gas is believed to be due to a process known as sublimation which is caused by the Sun’s heat.
Analysis shows that Phaethon’s comet-like activity cannot be explained by any kind of dust, according to California Institute of Technology PhD student Qicheng Zhang, the lead author of a paper published in the Planetary Science Journal. This paper presents the results of a detailed analysis of data from the observatory of the Japanese Aerospace Exploration Agency’s Hayabusa-2 spacecraft. The researchers found that the dust properties of Phaethon, a near-Earth object, were different from those of comets, suggesting that its activity is not driven by dust particles. They also discovered that Phaethon’s surface had a large amount of dark material, possibly caused by hydrothermal activity.
The Solar and Heliospheric Observatory (SOHO) released a two-hour sequence of images on May 15, 2022, showing 3.4-mile-wide asteroid Phaethon passing close to the Sun, at a distance of 13 million miles. The images demonstrate the relative movement of the asteroid to the background stars. SOHO routinely observes the Sun, but also monitors many other objects passing near the Sun, such as comets and asteroids. In addition, random white specks seen in the images are energetic particles, i.e. cosmic rays, which constantly bombard the SOHO camera.
In 2009, NASA’s Solar Terrestrial Relations Observatory (STEREO) detected a short tail extending from Phaethon when the asteroid was at its closest point to the Sun, called perihelion, during its 524-day orbit. The tail had not been seen before because it only forms when Phaethon is too close to the Sun to observe, except with solar observatories. STEREO also observed the tail on later solar approaches in 2012 and 2016, and its appearance supported the idea that dust was escaping the asteroid’s surface when heated by the Sun.
However, in 2018, another solar mission imaged part of the debris trail from the Geminid meteor shower and found something unexpected. The Parker Solar Probe, another NASA mission, observed that the trail contained far more material than Phaethon could shed during its close approaches to the Sun.This finding challenges the previous understanding of Phaethon’s dust production and suggests that there may be other sources of material contributing to the Geminid meteor shower.
After observing the comet-like behavior of Phaethon, Zhang’s team questioned if something else other than dust was responsible. Zhang hypothesized that sodium emission might be responsible for its brightening near the Sun, which was supported by a prior study that suggested the Sun’s heat could vaporize sodium within the asteroid and cause comet-like activity. To test their theory, they conducted an experiment to analyze the spectral emission of Phaethon as it flew past the Sun and observed a strong sodium emission signature in the resulting data. This indicated that sodium was indeed a factor in Phaethon’s behavior and helped explain why it appears to act like a comet in some ways.
Zhang’s team has been trying to understand the composition of Phaethon’s tail for a long time, and he was hoping to get definitive answers during the asteroid’s latest perihelion in 2022. To achieve this, they used the Solar and Heliospheric Observatory (SOHO) spacecraft, which has special color filters that can detect sodium and dust. Additionally, they searched through archival images from STEREO and SOHO, finding evidence of a tail during 18 of Phaethon’s close solar approaches between 1997 and 2022. Through their careful research, Zhang and his team have been able to gain valuable insight into the nature of Phaethon’s tail, and their work is an important piece of the puzzle in understanding this mysterious comet-like asteroid.
Observations from SOHO revealed that the asteroid’s tail was bright in the sodium filter, but not in the dust filter. Additionally, the shape of the tail and its intensification as it passed closer to the Sun suggested that it was made of sodium, and not dust. These observations indicate that Phaethon’s tail is composed of sodium, not dust. This discovery is important because it provides insight into the composition of Phaethon’s tail and gives scientists a better understanding of the physics behind its formation and evolution.
The recent discovery of the tail of the comet Phaethon being composed of sodium, as opposed to dust, is groundbreaking. According to team member Karl Battams of the Naval Research Laboratory, this result was unexpected and was made possible by two heliophysics spacecrafts, SOHO and STEREO, that were not designed to observe such phenomena. This evidence indicates that our understanding of comets is far from complete, and that there are still mysteries surrounding them that are yet to be unlocked.
The Large Angle and Spectrometric Coronagraph (LASCO) on the Solar and Heliospheric Observatory (SOHO) imaged asteroid Phaethon as it passed near the Sun in May 2022. It used two different filters to capture the asteroid, an orange filter that is sodium-sensitive and a blue filter that is dust-sensitive. The orange filter showed the asteroid with a surrounding cloud and small tail, which suggests that sodium atoms from the surface of the asteroid were glowing in response to sunlight. In contrast, the blue filter showed no sign of Phaethon, indicating that it was not producing any detectable dust. This demonstrates the ability of LASCO to detect different physical components of an asteroid such as gas and dust, providing valuable insight into its composition.
Zhang and his colleagues are currently raising an interesting query – whether some of the comets discovered by SOHO and citizen scientists studying SOHO images, as part of the Sungrazer Project, are actually not comets at all. According to Zhang, these mysterious objects could actually be a type of heated rocky asteroid, something similar to the asteroid known as Phaethon which is thought to be heated up by the Sun. This suggests that these ‘comets’ are not the icy bodies usually associated with comets and rather a type of asteroid. While this is still only a theory at this point, Zhang and his colleagues will continue to study these objects in more detail in order to determine what they really are.
The mystery of how the Geminid meteor shower is supplied with material each December has remained unanswered, until now. Zhang and his team suggest that a disruption event occurred thousands of years ago, which caused the asteroid Phaethon to eject around a billion tons of material that makes up the Geminid debris stream. While this event remains unknown, the general consensus is that it was caused by a piece of the asteroid being broken apart due to the stresses from Phaethon’s rotation. This provides a possible explanation for the Geminid meteor shower, despite the fact that Phaethon does not shed much dust.
