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According to the first answer of This question 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.

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    $\begingroup$ 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. $\endgroup$
    – JohnHoltz
    Jan 12, 2023 at 17:39
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    $\begingroup$ Does this answer your question? What's the estimated remaining life of Halley's Comet? $\endgroup$ Jan 13, 2023 at 4:01
  • $\begingroup$ Too many U's in your name of the shower... too small a change at my rep $\endgroup$
    – CGCampbell
    Jan 13, 2023 at 15:04
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    $\begingroup$ 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. $\endgroup$
    – ajd138
    Jan 14, 2023 at 21:48
  • $\begingroup$ 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. $\endgroup$
    – Jim
    Jan 14, 2023 at 22:06

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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 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) 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.

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  • $\begingroup$ 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 $\endgroup$ Jan 12, 2023 at 22:42
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    $\begingroup$ @Xenon errrrr, a space craft has been sent to Halley's comet. en.m.wikipedia.org/wiki/Giotto_(spacecraft) $\endgroup$
    – ProfRob
    Jan 12, 2023 at 22:47
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    $\begingroup$ 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. $\endgroup$ Jan 12, 2023 at 23:29
  • $\begingroup$ @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 & here. $\endgroup$
    – PM 2Ring
    Jan 13, 2023 at 12:54
  • $\begingroup$ @ProfRob maybe another mission would be good. Estimate exactly how much has been lost since Giotto's visit. $\endgroup$ Jan 13, 2023 at 16:32
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Comets can be fairly large initially, multiple kilometers across. However, just as parrots become ex-parrots, 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.

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Comets are extremely volatile because mostly they consist of ice and dust, once they reach the perihelion i.e closest point to the Sun in an orbit around the Sun they sublimate forming the Coma and the tails this is also the reason meteor showers out of comets happen, also whenever a comet reaches some celestial object the Tidal forces of the object melt some pieces of the comet, and Halley's comet's orbit is in a short-term orbit (because it's orbit was perturbed due to the Solar system being chaotic) so it comes close to the Sun frequently. So in reality it is actually reducing in mass every time it comes at perihelion


But however it's rate of loss of mass is slow compared to it's entire mass. So recent research suggest that Halley's comet might evaporate in a few thousand years.

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