Asteroids too can have tails like comets but ....

Comets and asteroids are two distinct classes of objects, and it used to be fairly easy to tell them apart. When comets approach the Sun, they brighten, and the ice bound up in the rock sublimates, loosening dust as it does so. The gas of the sublimated ice and the dust form twin tails that stream away from the Sun at slightly different angles.

Asteroids have little or no ice, and therefore were thought not to have tails. However, in recent years, astronomers have found a growing number of asteroids with comet-like characteristics, including a tail. These are known as active asteroids, and their discovery blurs the line between how objects in our Solar System behave.

The strange asteroid  responsible for Earth's annual shower of Geminid meteors just got even more peculiar. Every orbit, as it approaches the Sun and heats up, 3200 Phaethon grows a comet-like tail of material, which streams away in a pale fog.

Phaethon's tail was thought to consist of tiny grains of dust, but new observations reveal that's not the case. Rather, the asteroid belches out sodium gas – just like the planet Mercury, but smaller. It's a finding that significantly changes our understanding of this strange object, as well as other tailed rocks that skim close to the Sun, suggesting that what we think are comets may, in at least some cases, truly be sodium-spewing asteroids.

In fact, Phaethon has always been regarded as a bit of an oddball ever since its discovery in the 1980s, and scientists realized its 524-day orbit matched up with the appearance of the Geminids in our night sky. Comets are usually the objects responsible for Earth's meteor showers, too: when Earth passes through the debris left behind by a comet, that debris burns up as it falls into our atmosphere, creating a glorious light show.

One of the problems with the dust hypothesis explaining Phaethon's tail is that the dust production was insufficient to explain the amount of material required to produce the Geminids. So researchers went looking to see if there were other explanations.

Comets often glow brilliantly by sodium emission when very near the Sun, so they suspected sodium could likewise serve a key role in Phaethon's brightening.

Two years ago, modeling and experiment work suggested that Phaethon's tail might also be sodium that sublimates when the asteroid approaches the Sun. 

The research team collected data from two solar observatories, NASA's Solar Terrestrial Relations Observatory (STEREO) and the joint ESA NASA Solar and Heliospheric Observatory (SOHO). At first, STEREO wasn't able to detect the wavelengths in which sodium fluoresces. Luckily, as the spacecraft aged, its Heliospheric Imager 1 filter has degraded to increase sensitivity to sodium.

Their observations matched with that of a sodium tail rather than a dust tail! 

This suggests that the tail is indeed predominantly sodium gas, which means we may have been looking at some other Solar System objects incorrectly, too.

Researchers now think that a lot of those other sunskirting 'comets' may also not be 'comets' in the usual, icy body sense, but may instead be rocky asteroids like Phaethon heated up by the Sun!

https://iopscience.iop.org/article/10.3847/PSJ/acc866