New finding about 'weird' asteroid 3200 Phaethon made it weirder

"Our analysis shows that Phaethon’s comet-like activity cannot be explained by any kind of dust," California Institute of Technology Ph.D. student Qicheng Zhang, lead author of the paper, said in a statement.

Why wasn't the tail noticed before?

Phaethon's tail forms only when it is too close to the Sun - this is something that regular telescopes can't capture. In 2009, NASA's Solar Terrestrial Relations Observatory (STEREO) spotted a short tail extending from Phaethon as it reached its closest point to the Sun(perihelion). Later, in 2012 and 2016, STEREO saw the tail develop on later solar approaches. The appearance substantiated the theory that dust was escaping the asteroid's surface when heated by the Sun.

However, in 2018, a solar mission that imaged part of the Geminid debris trail found something peculiar. NASA's Parker Solar Probe showed that the trail contained more material than what could be shed during Phaethon's close approaches to the Sun.

"Comets often glow brilliantly by sodium emission when very near the Sun, so we suspected sodium could likewise serve a key role in Phaethon’s brightening," Zhang said.

Zhang was intrigued. He used the Solar and Heliospheric Observatory (SOHO) spacecraft, which contains color filters that can detect sodium and dust, and observed the tail during Phaethon’s latest perihelion in 2022.

The tail was made of sodium, not dust

According to SOHO's observations, the asteroid's tail appeared bright only in the filter that detected sodium, not that detected dust. Also, the tail changed its shape and intensity when Phaethon passed by the Sun, matching precisely what was expected if it were made of sodium.

"Not only do we have a really cool result that kind of upends 14 years of thinking about a well-scrutinized object," said team member Karl Battams of the Naval Research Laboratory, "but we also did this using data from two heliophysics spacecraft – SOHO and STEREO – that were not at all intended to study phenomena like this."

According to the release, the mystery of Phaethon might be solved. Still, an important one remains: If Phaethon doesn’t shed much dust, how does the asteroid supply the material for the Geminid meteor shower we see each December?

An upcoming Japan Aerospace Exploration Agency (JAXA) mission called DESTINY+ (Demonstration and Experiment of Space Technology for Interplanetary voyage Phaethon fLyby and dUst Science) could provide more answers. Later this decade, the DESTINY+ spacecraft is expected to fly past Phaethon, "imaging its rocky surface and studying any dust that might exist around this enigmatic asteroid".

The paper is published in the Planetary Science Journal.

Study Abstract:

Sunskirting asteroid (3200) Phaethon has been repeatedly observed in Solar Terrestrial Relations Observatory (STEREO) Heliospheric Imager 1 (HI1) imagery to anomalously brighten and produce an antisunward tail for a few days near each perihelion passage, phenomena previously attributed to the ejection of micron-sized dust grains. Color imaging by the Solar and Heliospheric Observatory (SOHO) Large Angle Spectrometric Coronagraph (LASCO) during the 2022 May apparition indicates that the observed brightening and tail development instead capture the release of sodium atoms, which resonantly fluoresce at the 589.0/589.6 nm D lines. While HI1's design bandpass nominally excludes the D lines, filter degradation has substantially increased its D line sensitivity, as quantified by the brightness of Mercury's sodium tail in HI1 imagery. Furthermore, the expected fluorescence efficiency and acceleration of sodium atoms under solar radiation readily reproduce both the photometric and morphological behaviors observed by LASCO and HI1 during the 2022 apparition and the 17 earlier apparitions since 1997. This finding connects Phaethon to the broader population of sunskirting and sungrazing comets observed by SOHO, which often also exhibit bright sodium emission with minimal visible dust, but distinguishes it from other sunskirting asteroids without detectable sodium production under comparable solar heating. These differences may reflect variations in the degree of sodium depletion of near-surface material and thus the extent and/or timing of any past or present resurfacing activity.