<p><strong>Note:</strong> This question is related to <a href="https://astronomy.stackexchange.com/questions/47474/31410">Average DeltaV applied to a comet during outgassing?</a></p>
<p>Rocket engines have a parameter called &quot;specific impulse,&quot; which is the measurement (in seconds) of how efficient the propellant consumption is. As comets propel themselves via geysers near perihelion, I am wondering how efficient this process is in terms of specific impulse. Has this been directly measured?</p>

CC BY-SA 4.0
2021-11-12T17:24:45.043Z
What do you mean by "how efficient" it is, or by "comets propel themselves"? Neither of these is clear. Comets outgas. It causes velocity changes. But efficiency - how would you define it?
Rory Alsop
CC BY-SA 4.0
2021-11-12T17:44:48.240Z
@RoryAlsop See https://en.wikipedia.org/wiki/Specific_impulse, how much mass is ejected per unit velocity change.
WarpPrime
CC BY-SA 4.0
2021-11-12T18:28:06.830Z
I don't have any misunderstanding as to what specific impulse is. But what does your question mean? What are you trying to ask?
Rory Alsop
CC BY-SA 4.0
2021-11-14T10:50:46.037Z
Since $v_e = g_0  I_{sp}$ this is equivalent to asking what the *effective velocity* of cometary outgassing is and avoid the confusion with rockets. You could just as for typical or average velocity.
uhoh
CC BY-SA 4.0
2021-11-14T13:17:27.043Z
It's been modeled https://link.springer.com/content/pdf/10.1007/s10569-020-09973-5.pdf not sure if it's ever been measured.
Daddy Kropotkin
<comets><gas>
Specific impulse of cometary outgassing?
WarpPrime
# Specific impulse of cometary outgassing?

(Asked by: username_0 on 2021-11-12)

**Note:** This question is related to [Average DeltaV applied to a comet during outgassing?](https://astronomy.stackexchange.com/questions/47474/31410)

Rocket engines have a parameter called "specific impulse," which is the measurement (in seconds) of how efficient the propellant consumption is. As comets propel themselves via geysers near perihelion, I am wondering how efficient this process is in terms of specific impulse. Has this been directly measured?

**Tags:** comets, gas

> **Comments:**
> **username_1**: What do you mean by "how efficient" it is, or by "comets propel themselves"? Neither of these is clear. Comets outgas. It causes velocity changes. But efficiency - how would you define it?  
> **username_0**: @RoryAlsop See https://en.wikipedia.org/wiki/Specific_impulse, how much mass is ejected per unit velocity change.  
> **username_1**: I don't have any misunderstanding as to what specific impulse is. But what does your question mean? What are you trying to ask?  
> **username_2**: Since $v_e = g_0  I_{sp}$ this is equivalent to asking what the *effective velocity* of cometary outgassing is and avoid the confusion with rockets. You could just as for typical or average velocity.  
> **username_3**: It's been modeled https://link.springer.com/content/pdf/10.1007/s10569-020-09973-5.pdf not sure if it's ever been measured.  

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