This is known as an “anti-tail,” and appears to point in the opposite direction to the usual comet tail—seemingly breaking the laws of physics.
Comet ZTF, a green-colored comet currently approaching the central solar system, due to fly past Earth on its journey toward the sun, also has a clearer, traditional tail facing away from the sun, which has been visible during its long approach towards us. The anti-tail, however, is a brief anomaly, caused by a strange optical illusion.
“A comet is like a dirty snowball,” Keith Horne, a professor of astronomy at the University of St. Andrews in Scotland, told Newsweek. “Comets and planets both orbit the sun, but unlike the circular orbits of planets, comets follow highly elliptical orbits.”
Comets spend most of their time on the outskirts of the solar system, but once per orbit they zoom in close to the sun and then head out again.
“As they approach the sun, the solar radiation heats the comet’s surface, evaporating dust and ice,” Horne said.
As a comet is heated by the radiation from the sun (or any other star), the outer layer warms and starts to melt.
“This leaves it with lots of loose particles, both charged and neutral, that come away from it while it is moving,” Ian Whittaker, a senior physics lecturer in physics at Nottingham Trent University in the U.K., told Newsweek. “The neutral particles come off in a cone behind the comet as it moves—a bit like being behind a big truck on the motorway while it’s raining, all the excess water hits whatever is directly behind it.”
“The charged particles will do the same unless there are any electric or magnetic fields nearby. The sun itself has a magnetic dipole (like the Earth) and has a magnetic field flowing out into the solar system. So the charged particles coming off of the comet are picked up by this magnetic field and sent directly away from the sun (anti-sunward).”
This forms the traditional backward-facing tail streaming out from behind a comet.
“The third comet tail is made up of the neutral particles but is not actually traveling towards the sun, it is just the relative position of the comet, Earth, and sun,” Whittaker said.
He continued: “If we go back to the analogy of the truck traveling on a lot of surface water, the spray comes out and mostly goes behind, but in a slight cone shape. If we were now in front and looked backward, we would not see the main spray hitting the car behind but a slight amount either side of the truck where the edge of the cone is. The apparent third comet tail is the same thing, we are seeing neutral particles flowing away from the comet in a cone and from where we are on the Earth we just see both edges of this cone so it looks like two tails when it is really one.”
According to Christopher Conselice, a professor of astrophysics at the University of Nottingham in the U.K., this is an optical illusion caused by the strange alignment of the comet.
“This ‘anti-tail’ consists of large dust particles,” he told Newsweek. “These dust particles are not strongly affected by the pressure coming from the sun due to light which creates the normal tails comets usually show.”
“Because of this, the particles stay within the orbital plane of the comet and become ejected into a large disk surrounding the comet, which can appear to be a tail when seen from Earth. This ‘anti-tail’ can usually only be seen for a short period of time and many famous comets never show one, so it is somewhat unusual. These ‘anti-tails’ are therefore not real tails in a sense, but simply how we view the comet given our orientation to its orbit.”
Anti-tails are a relatively uncommon sight, as most comets do not develop sufficiently for an anti-tail to be visible.
Despite being an optical illusion, the strange illogical tail is an impressive sight nonetheless. Comet ZTF recently became visible to the naked eye, and will pass by the Earth at a distance of 0.28 AU, or around 26 million miles, on February 1.
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