As the rocky asteroid Phaeton approaches the Sun, it throws out a short but bright tail, like a real ice comet. New observations have shown that this tail arises from the asteroid's intense heating, which causes sodium to evaporate.
Asteroids are composed mostly of hard rock, rocky and metallic, while comets are composed of ice. Every time a comet passes close to the Sun, it begins to evaporate. Volatile matter and released dust particles are ejected from its surface, stretching out in the form of a bright tail. This is unusual for asteroids, but there are some curious exceptions. One of them is the five-kilometer-long near-Earth asteroid (3200) Phaeton.
Phaethon moves around the Sun on a greatly elongated ellipse, now going further than Mars, now converging closer to the orbit of Mercury. Its trajectory coincides with a bright stream of Geminids meteors, flaring in the Earth's sky in December. Therefore, it is believed that Phaeton serves as the parent body, fragments of which formed this stream. And in 2009, the STEREO space telescope noticed that the asteroid ejects a short tail as it approaches the Sun.
Because of the proximity of the star to observe it with conventional telescopes can not be, but created specifically to study the Sun STEREO confirmed the appearance of Phaeton's tail during the following rapproximities. It was assumed that the asteroid is flying dust particles, but the tail looked too bright for this.
Carl Schmidt and his colleagues noticed that the asteroid heats up to 800 ° C as it approaches the Sun. This temperature is enough for its surface to vaporize sodium, creating an exceptionally bright tail. Previously, this mechanism was shown only on computer models and in laboratory experiments. Now, the scientists have reviewed the archival observations of Phaeton passages through the perihelion, made by the "solar" telescopes STEREO and SOHO, as well as held new ones during its rapprochement with the star in 2022.
The SOHO apparatus is equipped with filters to distinguish both dust particles and sodium. Its images clearly show that Phaeton's tail is made up of sodium. "Many other "comets" approaching the Sun may also turn out to be not the usual ice comets, but rocky asteroids like Phaeton," said Qicheng Zhang, one of the authors of the new work. Note that asteroids that exhibit some "cometary" characteristics, the scientists allocate to a separate group of comets of the main belt, or active asteroids.
However, Phaeton still holds its main mystery - the connection with the Geminid stream. If it does not emit even significant amounts of dust, then why did the huge amount of matter that formed this cluster come off it? Perhaps the asteroid experienced a catastrophic event in the past - for example, spinning too hard and splitting apart - losing billions of tons of mass. Another possibility could be due to evaporation of the same sodium, the violent evaporation of which can tear apart a celestial body. However, this remains to be determined definitively.
