<p>The basic answer to your question though is that Halley's comet is unlikely to be very old (in the sense that it has not spent a large amount of time orbiting the Sun as closely as it does now). Most comets are perturbed into their current orbits from way beyond the orbit of Pluto and then live comparatively brief lives as they gradually disintegrate.</p>
<p>According to <a href="https://solarsystem.nasa.gov/asteroids-comets-and-meteors/comets/1p-halley/in-depth/" rel="noreferrer">this NASA source</a> the comet should lose about 1-3 metres depth of surface every time it swings around the Sun. Let's say it started off with a 10 km radius; this would mean it could complete 3,000-10,000 orbits before vanishing. Since the orbital period is 76 years, this implies a lifetime of 228,000 to 760,000 years. Thus if Halley's comet has been in its current orbit for less time than this then there is no mystery.</p>
<p>An interesting calculation is presented by <a href="https://ui.adsabs.harvard.edu/abs/1985MNRAS.213..103H/abstract" rel="noreferrer">Hughes (1985)</a> who estimates the current mass of the comet, its estimated mass-loss per orbit around the Sun, and then compares that with the estimated total mass in the meteor stream you mention.
The conclusion is that the comet has made about 2300 previous passages around the Sun and so is only about 175,000 years old (in its current orbit) and will probably survive for another 2300 orbits.</p>

CC BY-SA 4.0
2023-01-12T22:42:06.680Z
One way to measure some of this directly would be a robotic space probe launched to reach the comet. This would likely be very pricey and time consuming though. @ProfRob
Deko Revinio
CC BY-SA 4.0
2023-01-12T22:47:26.420Z
@Xenon errrrr, a space craft has been sent to Halley's comet. https://en.m.wikipedia.org/wiki/Giotto_(spacecraft)
ProfRob
CC BY-SA 4.0
2023-01-12T23:29:06.860Z
Re *so is only about 175,000 years old* -- as a comet, that is. Presumably it's about 4.5 billion years old, but only 175,000 years (or maybe 760,000 years; still a short time compared to 4.5 billion years) after its orbit was perturbed so as to make it into a short-term comet.
David Hammen
CC BY-SA 4.0
2023-01-13T12:54:57.700Z
@Xenon Also, we have historical records of Halley's comet going back a long way, which allow us to estimate the changes in its orbit due to perturbations and mass loss. JPL Horizons actually has 30 trajectory files for it, see [here](https://ssd.jpl.nasa.gov/api/horizons.api?format=text&MAKE_EPHEM=NO&COMMAND=%271P%3B%27) & [here](https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=1P).
PM 2Ring
CC BY-SA 4.0
2023-01-13T16:32:38.287Z
@ProfRob maybe another mission would be good. Estimate exactly how much has been lost since Giotto's visit.
Deko Revinio
CC BY-SA 4.0
2023-01-13T21:38:52.497Z
@Xenon:  It wouldn't surprise me if such a mission happens in 2060 or so.  If nothing else, NASA would probably like another shot at it;  a flyby mission for the 1986 appearance was cancelled due to budget cuts, and then a satellite designed to study the comet from LEO was destroyed in the *Challenger* disaster.
Michael Seifert
CC BY-SA 4.0
2023-01-13T21:42:55.753Z
@MichaelSeifert: we shall see. We shall see indeed. https://www.seeker.com/lets-plan-for-a-rendezvous-with-halleys-comet-1767783657.html
Deko Revinio
CC BY-SA 4.0
2023-01-12T20:07:26.853Z
2023-01-13T11:56:28.497Z
2023-01-13T11:56:28.497Z
ProfRob
<p>Comets can be fairly large initially, multiple kilometers across. However, just as parrots become <a href="https://www.youtube.com/watch?v=SvTBkjL6Dxo" rel="noreferrer">ex-parrots</a>, comets eventually become ex-comets. They shed volatiles, sometimes shed pieces, and yet other times even disintegrate. Some of the things we think of as asteroids might well be comets that have lost their volatiles, leaving <a href="https://www.scientificamerican.com/article/if-comets-melt-why-do-the/" rel="noreferrer">just the non-volatile (e.g. rocky and dusty) material behind</a>.</p>

CC BY-SA 4.0
2023-01-13T15:29:09.900Z
And sometimes they [crash into giant planets](https://en.wikipedia.org/wiki/Comet_Shoemaker%E2%80%93Levy_9)
Barmar
CC BY-SA 4.0
2023-01-12T17:53:19.347Z
2023-01-12T17:53:19.347Z
David Hammen
<p>According to the first answer of <a href="https://astronomy.stackexchange.com/questions/1216/did-a-piece-of-halleys-comet-strike-the-earth-1-500-years-ago?rq=1">This question</a> parts of Halley's comet hit the Earth every spring as part of the Eta Aquarid meteor shower. If this is true, then how is the comet still in one piece? If parts of this thing are breaking off every year as it travels through space, that means the comet must be getting smaller. If I am not mistaken, a comet can only be so big.
This question addresses just how, in all the time Halley's comet has come back before, and broken parts off of itself each time, can it still be in one piece.</p>

CC BY-SA 4.0
2023-01-12T17:39:18.640Z
Another way to phrase your question is how many grains of sand (that is, future meteor material) are in a comet. That will provide an idea of how long a comet will last before it dwindles to nothing.
JohnHoltz
CC BY-SA 4.0
2023-01-13T04:01:26.103Z
Does this answer your question? [What's the estimated remaining life of Halley's Comet?](https://astronomy.stackexchange.com/questions/1592/whats-the-estimated-remaining-life-of-halleys-comet)
Nilay Ghosh
CC BY-SA 4.0
2023-01-13T15:04:01.047Z
Too many U's in your name of the shower... too small a change at my rep
CGCampbell
CC BY-SA 4.0
2023-01-14T21:48:36.327Z
Note that even though the Eta Aquarid meteor shower occurs every year, the comet isn't producing material for it every year. The debris was created a long time ago and the Earth passes through it every year instead. This doesn't fully answer your question (which others have done better), but figured this might be a misapprehension.
ajd138
CC BY-SA 4.0
2023-01-14T22:06:35.313Z
One issue with your question I don't see addressed is this misunderstanding "If parts of this thing are breaking off every year as it travels through space".  The meteor shower isn't caused by yearly ejections from Halley's Comet, it is caused by the Earth passing through the trail left by Halley's Comet on its previous passes.
Jim
CC BY-SA 4.0
2023-01-17T16:14:13.180Z
@Jim it may be a misunderstanding, however, the trail we pass through is still technically made of material from the comet. I am currently conducting some research on the subject because it is quite fascinating to me.
Deko Revinio
<comets><meteor-shower><halleys-comet>
How is Halley's comet still in one piece?
Deko Revinio
# How is Halley's comet still in one piece?

(Asked by: username_0 on 2023-01-12)

According to the first answer of [This question](https://astronomy.stackexchange.com/questions/1216/did-a-piece-of-halleys-comet-strike-the-earth-1-500-years-ago?rq=1) parts of Halley's comet hit the Earth every spring as part of the Eta Aquarid meteor shower. If this is true, then how is the comet still in one piece? If parts of this thing are breaking off every year as it travels through space, that means the comet must be getting smaller. If I am not mistaken, a comet can only be so big.
This question addresses just how, in all the time Halley's comet has come back before, and broken parts off of itself each time, can it still be in one piece.

**Tags:** comets, meteor-shower, halleys-comet
> **Comments:**
> **username_1**: Another way to phrase your question is how many grains of sand (that is, future meteor material) are in a comet. That will provide an idea of how long a comet will last before it dwindles to nothing.  
> **username_2**: Does this answer your question? [What's the estimated remaining life of Halley's Comet?](https://astronomy.stackexchange.com/questions/1592/whats-the-estimated-remaining-life-of-halleys-comet)  
> **username_3**: Too many U's in your name of the shower... too small a change at my rep  
> **username_4**: Note that even though the Eta Aquarid meteor shower occurs every year, the comet isn't producing material for it every year. The debris was created a long time ago and the Earth passes through it every year instead. This doesn't fully answer your question (which others have done better), but figured this might be a misapprehension.  
> **username_5**: One issue with your question I don't see addressed is this misunderstanding "If parts of this thing are breaking off every year as it travels through space".  The meteor shower isn't caused by yearly ejections from Halley's Comet, it is caused by the Earth passing through the trail left by Halley's Comet on its previous passes.  
> **username_0**: @Jim it may be a misunderstanding, however, the trail we pass through is still technically made of material from the comet. I am currently conducting some research on the subject because it is quite fascinating to me.  

---

## 2 answers

### Answer by username_6 on 2023-01-12. Score: 31

The basic answer to your question though is that Halley's comet is unlikely to be very old (in the sense that it has not spent a large amount of time orbiting the Sun as closely as it does now). Most comets are perturbed into their current orbits from way beyond the orbit of Pluto and then live comparatively brief lives as they gradually disintegrate.

According to [this NASA source](https://solarsystem.nasa.gov/asteroids-comets-and-meteors/comets/1p-halley/in-depth/) the comet should lose about 1-3 metres depth of surface every time it swings around the Sun. Let's say it started off with a 10 km radius; this would mean it could complete 3,000-10,000 orbits before vanishing. Since the orbital period is 76 years, this implies a lifetime of 228,000 to 760,000 years. Thus if Halley's comet has been in its current orbit for less time than this then there is no mystery.

An interesting calculation is presented by [Hughes (1985)](https://ui.adsabs.harvard.edu/abs/1985MNRAS.213..103H/abstract) who estimates the current mass of the comet, its estimated mass-loss per orbit around the Sun, and then compares that with the estimated total mass in the meteor stream you mention.
The conclusion is that the comet has made about 2300 previous passages around the Sun and so is only about 175,000 years old (in its current orbit) and will probably survive for another 2300 orbits.

> **Comments:**
> **username_0**: One way to measure some of this directly would be a robotic space probe launched to reach the comet. This would likely be very pricey and time consuming though. @ProfRob  
> **username_6**: @Xenon errrrr, a space craft has been sent to Halley's comet. https://en.m.wikipedia.org/wiki/Giotto_(spacecraft)  
> **username_7**: Re *so is only about 175,000 years old* -- as a comet, that is. Presumably it's about 4.5 billion years old, but only 175,000 years (or maybe 760,000 years; still a short time compared to 4.5 billion years) after its orbit was perturbed so as to make it into a short-term comet.  
> **username_8**: @Xenon Also, we have historical records of Halley's comet going back a long way, which allow us to estimate the changes in its orbit due to perturbations and mass loss. JPL Horizons actually has 30 trajectory files for it, see [here](https://ssd.jpl.nasa.gov/api/horizons.api?format=text&MAKE_EPHEM=NO&COMMAND=%271P%3B%27) & [here](https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=1P).  
> **username_0**: @ProfRob maybe another mission would be good. Estimate exactly how much has been lost since Giotto's visit.  
> **username_9**: @Xenon:  It wouldn't surprise me if such a mission happens in 2060 or so.  If nothing else, NASA would probably like another shot at it;  a flyby mission for the 1986 appearance was cancelled due to budget cuts, and then a satellite designed to study the comet from LEO was destroyed in the *Challenger* disaster.  
> **username_0**: @MichaelSeifert: we shall see. We shall see indeed. https://www.seeker.com/lets-plan-for-a-rendezvous-with-halleys-comet-1767783657.html  

---

### Answer by username_7 on 2023-01-12. Score: 8

Comets can be fairly large initially, multiple kilometers across. However, just as parrots become [ex-parrots](https://www.youtube.com/watch?v=SvTBkjL6Dxo), comets eventually become ex-comets. They shed volatiles, sometimes shed pieces, and yet other times even disintegrate. Some of the things we think of as asteroids might well be comets that have lost their volatiles, leaving [just the non-volatile (e.g. rocky and dusty) material behind](https://www.scientificamerican.com/article/if-comets-melt-why-do-the/).

> **Comments:**
> **username_10**: And sometimes they [crash into giant planets](https://en.wikipedia.org/wiki/Comet_Shoemaker%E2%80%93Levy_9)  

---

