Astronomy Stack Exchange is a question and answer site for astronomers and astrophysicists. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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?

share|improve this question
up vote 11 down vote accepted

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.

(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.

share|improve this answer

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.

share|improve this answer
If the particles are ejected wouldn't they be ejected with a spherical distribution not just towards tail direction? – chaonomy Dec 14 '13 at 2:44
No, as they are ejected as a result of solar radiation (solar wind) – Envite Dec 14 '13 at 11:50
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. – Robert Cartaino Dec 16 '13 at 14:51
It is negligible compared to the solar wind itself, as you point out, but not compared to the comet's movement. – Envite Dec 16 '13 at 20:00

So, increasing the mass means increased gravitational pull. That is why heavier objects like hammer fall faster than lighter objects like feather. This sounds true up to conluding that the hammer falls faster for this reason. The mass of the hammer needs the extra amount of gravitational pull to its mass to the earth, or moon. In a similar way, the amount of pull on the feather is proportional to its mass, and so on for object, at least on a relationship like earth and mars to a feather or hammer with such a vast difference in their relative masses. This phenomenon was made famous by Galileo when he dropped a few things off of the leaning of pisa or some ther building like it and observed that the two objects he dropped though of different masses hit the ground at the same time. At least part of the reason a feather drops nore slowly than a hammer of the same mass is the effects of aerodynamics where the feather has a much more resistant outer shell, or shape, than the hammer of similar mass. The effects of wind currents are much different on feathers than hammers, and these effects are studied by use of terms like coefficients of friction. On a comet's coma, the density of materials, or amount of nuclei per given volume exceeds the tail's, and so as a hammer is denser than the feather, the tail of the moves at a slightly different speed than the comet itself.

share|improve this answer
No. Heavier objects like hammer do not fall faster than lighter objects like feather due to their masses. It is ENTIRELY dependant upon the aerodynamics. – Envite Dec 15 '13 at 21:10
Actually, Galileo didn't drop things from the tower of Pisa (even Wikipedia mentions this) but let them roll down on a slope which made it much easier for him to compare arrival times. And @Envite is right about speed being independent of mass in this case. – István Zachar Dec 16 '13 at 16:30

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

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.

share|improve this answer
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. – Envite Dec 14 '13 at 11:51
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.. – Mahe Dec 14 '13 at 13:31
A sheet and a ball made from the same sheet does not differ in mass at all. – Envite Dec 14 '13 at 19:06
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. – Envite Dec 15 '13 at 4:44
@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. ;) – TildalWave Dec 16 '13 at 5:08

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.