From this exhibition by an astronaut on the moon dropping a hammer and a feather and showing they move at the same speed, why do the particles of the comets tail move away from the comet, slow down and fall away as it disintegrates? Shouldn't they move at the same speed of the comet? For example if the feather was attached to the hammer and it was falling and broke off wouldn't it fall at the same speed, not start to trail behind and eventually fall away?


3 Answers 3


The tail of a comet is not actually "slowing down and falling away" from the comet, like you might expect to see when smoke streams out from behind a moving object on earth. The tail of a comet is actually being pushed away from the sun by the solar winds and radiation. That's why the tail of a comment always points away from the sun, and doesn't stream out behind it.

If there were no other forces at work, the coma would actually travel at about the same speed at the comet. As the material surrounding the core of the comet vaporized, it would form a coma ("atmosphere") made up of sublimated gas mixed with dust, moving along at the same speed (more or less(1)) as the comet… like you observed in the video.

enter image description here (source)

(1) In reality, without the solar winds, the coma of the comet would continue to move outward, as its expulsion velocity would easily overcome the comet's weak gravitational pull.


The tail particles from a comet are ejected, so they have a distinct velocity than that of the comet. Besides that, they orbit the Sun in an orbit similar to that of the comet when they were ejected, they don't slow down.

About the hammer and the feather, it is not the same to just break off than to be ejected. If the hammer blows the feather up while falling, the feather will delay a lot more than the hammer to fall.

  • $\begingroup$ If the particles are ejected wouldn't they be ejected with a spherical distribution not just towards tail direction? $\endgroup$
    – chaonomy
    Dec 14, 2013 at 2:44
  • 1
    $\begingroup$ No, as they are ejected as a result of solar radiation (solar wind) $\endgroup$
    – Envite
    Dec 14, 2013 at 11:50
  • $\begingroup$ The ejection speed of the comet material is negligible compared to the solar winds carrying it into space. So while it does impart a bit of velocity, it does not explain the "slow down and fall away as it disintegrates" effect the author is asking about. $\endgroup$ Dec 16, 2013 at 14:51
  • $\begingroup$ It is negligible compared to the solar wind itself, as you point out, but not compared to the comet's movement. $\endgroup$
    – Envite
    Dec 16, 2013 at 20:00

There are a few more elements which you might consider. Like, when dropping a hammer along with a feather in earth, the feather descends slower than the hammer. Reason: although the force of gravity is same, their mass, and the resistance from external forces faced by them is an entirely different story. The air between the object and the ground must also be taken into consideration. Whereas, moon lacks atmosphere and hence they fall at relatively same speed (with only low gravity acting upon them).

As in http://en.wikipedia.org/wiki/Comet_tail

As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. The streams of dust and gas thus released form a huge, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the Sun.

Similar to the air acting on the detached feather (as said above), the solar radiation pressure and solar winds act on the tail debris of the comet. For instance, you drop an article from a moving vehicle, it falls to the ground rather than flying in the air at the same speed. Deceleration and opposite forces like solar winds cause it to slow down.

UPDATE: The reason why mass is taken into consideration is due to the fact that mass is directly proportional to gravity. So, increasing the mass means increased gravitational pull. That is why heavier objects like hammer fall faster than lighter objects like feather. Explaining this would be a matter of physics and not astronomy which is irrelevant to this website and should be discussed in the physics stack exchange website.

  • $\begingroup$ Mass is not important. Air resistance is. A paper sheet and the same sheet made a paper ball have the same mass and fall with different speeds on air. $\endgroup$
    – Envite
    Dec 14, 2013 at 11:51
  • $\begingroup$ I was actually referring to the difference in mass between the hammer and the feather relative to the mass of atmosphere or air which causes the resistance. A sheet and a paper ball differ in mass slightly. But, what if the sheet is made of aluminium or steel. given the same area, they would fall at relatively greater speed.. $\endgroup$
    – Mahe
    Dec 14, 2013 at 13:31
  • $\begingroup$ A sheet and a ball made from the same sheet does not differ in mass at all. $\endgroup$
    – Envite
    Dec 14, 2013 at 19:06
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    $\begingroup$ It is FALSE that heavier objects fall faster than lighter ones. They have increased gravitational pull but also increased inertia, and the outcome is that they fell (without resistence) at EXACTLY the SAME speed. Flagging for deletion. $\endgroup$
    – Envite
    Dec 15, 2013 at 4:44
  • $\begingroup$ @Envite Flags are there to notify moderators of serious issues with user contributions, not their accuracy or helpfulness (or lack thereof) to other users, that's what the voting is there for. ;) $\endgroup$
    – TildalWave
    Dec 16, 2013 at 5:08

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