Why Do Comet Tails Point Away From the Sun?

comet's tail
C/2020 F3 (NEOWISE) Comet

Did you observe C / 2020 F3 (NEOWISE) comet? Or have you observed other comets before? If you notice, the comet tails always points away from the sun. But why? Before that, we must know the basis of photons first.


Photons are representations of electromagnetic radiation. One of the electromagnetic radiation is visible light. Photons do not have mass but have energy. Consider the following equation.

\begin{aligned} E=mc^2 \end{aligned}

E = Energy
m = mass
c = Speed ​​of light

Do you know the equation? It seems almost everyone must have seen it right? Based on these equations, how can photons have energy but don’t have mass? The answer is because photons have momentum. Basically, momentum is the ability of an object to push another object. The complete formula of the above famous formula is as follows.

\begin{aligned} E^2=m^2c^4+p^2c^2 \end{aligned}

E = Energy
m = mass
c = Speed ​​of light
p = Momentum

Photons only have momentum and have no mass. Then the equation that applies to photons is as follows.

\begin{aligned} E^2&=p^2c^2 \\ E&= pc \end{aligned}

E = Energy
p = Momentum
c = Speed ​​of light

Therefore, because the photon has momentum, the photon can ‘push’ something in front of it just like in this tool below.


A radiometer is a device to measure the ‘strength’ of electromagnetic radiation. One of them is the following tool.

Copyright: Applied Science

Light photons are reflected by the white blades and absorbed by the black blades. It can be seen that on the inside, the black blades are pushed back and what makes it rotate clockwise. This happens because the black blades get more momentum from the light photons because it absorbs it.

Now let’s talk about comet tails.

Comet Tails

Comets are chunks of frozen gas, rocks, and dust. Generally, comets have very eccentric orbits, meaning that their orbits are very long, unlike the orbits of planets that tend to be circular.

halley comet orbit
Halley comet orbit

When a comet is far from the sun ice will form and accumulate on the comet. And when the comet approaches the sun, the ice on the comet will melt and evaporate. The evaporated ice which we later see as a coma and tail of a comet.

The majority of the comet tails consist of dust and ions, meaning that the tail constituent particles are very small. If you think about it roughly, the light can make the blades on the radiometer rotate, then it must be able to push these tiny particles, right?

The sun emits very, very many photons each time. This much photon is certainly enough to push the comet tails away from the sun.

Also read
Asteroid Belt 101: Definition, Origin, Shapes, and Misunderstanding


Photons which are the smallest units of light do not have mass, but they have momentum. All objects that have momentum can give force, or in other words, it can ‘push’ other objects. This also applies to sunlight and comet tails. The comet tails consist of dust and small ions. This means the reason why the comet tails are pointing away from the sun is that the sun’s rays have enough momentum to push the comet tails away from the sun.

Resources and References:

  1. Mass–Energy Equivalence – Wikipedia
  2. How a Crookes Radiometer Works – Applied Science
  3. Comets – NASA Solar System Exploration
  4. How Comets Are Born – NASA Visualization Explorer
  5. Tails of Wonder! – NASA Space Place
  6. The Outer Planets – LASP University of Colorado Boulder

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