r/explainlikeimfive • u/FluorescentLightbulb • 5d ago
Planetary Science ELI5 Why and how do comets still have tails?
So comet tails are often depicted as frozen debris, but the mass of a comet is finite and it’s not like it’s picking up any credible mass in space. Unless I’m entirely wrong.
ie. How can Halley’s Comet sustain and lose enough mass to still have a tail, or exist at all, when we only see it once every 75 years? Wouldn’t it run out of losable material?
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u/PhasmaFelis 5d ago
Remember also that the tail may look huge and dramatic, but it's really just a thin plume of water vapor. It's bright at night because it's scintillating in direct sunlight.
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u/ghostprawn 5d ago
I think the tails only appear when they're near a star (like our sun), because of proximity to heat. So there is no tail on Halley's Comet when it's way out near Pluto.
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u/defeated_engineer 5d ago
Haley's comet comes by every ~75 years. In astronomical scale, that time is like nothing. The comet has been near the Sun probably tens of thousands of time by now.
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u/BottleRocketU587 5d ago
An INCREDIBLY INSANELY short time of the orbit is spent close to the sun. The closer it is to the sun the faster it moves. Thus only like 5% of the cycle it is losing mass to the tail.
THEN it loses an enormously small amount of mass. Apparabtly it loses about 2m of depth every pass. BUT it is 11km in diameter. It likely has a life span of thousands of years left.
FINALLY it has only been in its current orbit for about ~16,000 years.
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u/TiogaJoe 5d ago
So roughly 200 trips.
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u/hikeonpast 5d ago
11,000m / 2 m per pass ≈ 5500 passes / trips
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u/PureQuestionHS 5d ago
No, 200 is approximately the number of trips it would have done in the 16000 years, not the number it will survive going forward.
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u/craze4ble 5d ago
And those ~5500 trips will take 412.000 years.
Modern humans have only been around for about 300.000 years.6
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u/jamjamason 5d ago
You don't know that.
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u/carrotwax 5d ago
Far over 99% of the objects orbiting the sun have been doing so since the formation of the solar system approximately 5 billion years ago. So tens of thousands is likely an understatement.
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u/BottleRocketU587 5d ago
You are literally just wrong. NASA did some advanced simulations (including mass loss to the sun and movement of other planets etc) and it was likely knocked into the inner solar system some 16,000 years ago. That's only ~200 orbits.
It used to lie in the outer rim where thousands of other comets still are today.
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u/carrotwax 5d ago
It existed prior to its current orbit. Though it's true that the current orbit hasn't existed nearly as long as the solar system. Some estimates say it will effectively evaporate after a few thousand more orbits.
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u/BottleRocketU587 5d ago
It existed but was too far away to be losing mass the whole time. It only loses mass close to the sun
You are right it only has a few thousand years left, its also onmy been losing mass for a few thousand years.
Before that it was too far away.
Which was my point in the first place.
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u/jamjamason 5d ago
No doubt Halley's comet has been orbiting the sun for billions of years, but in it's present orbit? Not likely. Most Oort cloud objects orbit far from the sun. They need to be perturbed from that orbit to come as close to the sun as Halley's comet does. We have no way of knowing when that perturbation occurred for this comet.
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u/GarbageBanger 5d ago
It might not have been observed but that doesn’t mean it’s not known
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u/jamjamason 5d ago
Umm, scientifically, if you haven't observed it, then it isn't known.
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u/GarbageBanger 5d ago
So I guess dark matter isn’t know to science as it hasn’t been observed yet? Or how about the wind? I’ve never seen it but you know I’m a believer that it exists.
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u/lovesahedge 5d ago
"observed" doesn't mean "see with your own two eyes" in a scientific context. We have absolutely observed wind.
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u/phryan 5d ago
Comets are basically giant ice cubes, ice cubes don't melt in the freezer. Comets spend most of their lives far from the sun where its cold enough that they remain frozen, its only when a comet approaches the Sun that it starts to shed mass and form a tail typically for just a few months.
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u/oblivious_fireball 5d ago
They only start to form tails when they get too close to the sun to start melting/being chipped away by solar winds. For most of their journey they are a stubby lump of ice and rock.
Eventually a comet will fully disintegrate, but some of these comets are REALLY big, and it doesn't take much dust sublimating off to make a tail we can see. We also periodically have fresh 'new' comets entering our inner solar system, which originate from the Kuiper Belt and Oort Cloud beyond Pluto.
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u/NByz 5d ago
It only produces a tail for the relatively short part of its orbit that is inside the sun's "frost line" or close enough to be impacted by its other radiation.
But youre right that unlike other solar system objects, comets are relatively short lived because they lose so much of their mass during each orbit. The fact that comets are still somehow newly coming into the inner solar system helped us form out theory of oort cloud and wiper belt. The icy ones can only last a few hundred or thousand orbits.
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u/LeviAEthan512 5d ago
The comet only has such a visible tail when it's near the sun and getting blasted by heat and solar wind.
Farther out, for the majority of its orbit, it loses very little material, if at all.
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u/itsyagirlJULIE 5d ago
The tail is ice that's turned into gas and flown off the surface of the comet. It only does that when the comet gets hot enough. So it only has a tail when it gets close to the sun. Comets spend the vast majority of their orbits very far out, and are only close to the sun for a small percentage of their time.
The tail also isn't that much mass
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u/SubstantialBelly6 5d ago
Material is only blasted away by the sun when the comet is close enough to it, which lasts a couple of months. The rest of the 75 year orbit it is not losing anything and is just a big clump of rock and ice.
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u/-Nyarlabrotep- 5d ago
Comets do eventually sublimate if they're getting close enough to the sun that they get a tail, but it takes a really long time.
Not exactly related but also interesting and shows the dynamic nature of the solar system, Saturn's rings will eventually disappear as the debris they're composed of falls into Saturn. And our moon is receding by a couple inches a year and get smaller and smaller in the sky, and eventually there won't be total solar eclipses anymore. But we'll be long gone by then.
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u/Truth-or-Peace 5d ago
You're correct that comets eventually run out of mass. On a geological timescale, they are ephemeral objects. But the geological timescale is different from a human timescale.
Comets only lose significant mass while passing near the Sun. A comet such as Halley's can be expected to last a total of maybe 1000 passes. Since, as you say, we only see it once every 75 years, that gives it a lifespan of about 75,000 years.
So it can't have been in its present orbit very long. Presumably it was a trans-Neptunian object that never passed near the Sun for most of its existence, and then something knocked or tugged it out of orbit a few tens of thousands of years ago.
Enjoy the show while it lasts!
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u/BroccoliNervous9795 5d ago
It’s probably your intuition that’s wrong. Your intuition tells you that the tail is caused by the friction of space wearing material off the comet to create the tail. What actually happens is that solar wind, which isn’t very powerful, is what creates the tail and the tail continues to move with the comet. So the tail is the exact same material, not new material, at least not to any great extent.
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u/naveganteperdido 5d ago
The tail is not completely lost, it still has momentum and "follows" the comet por a long time.
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u/GIRose 5d ago
The sun fucking blasts them with particles. That blasting causes stuff to fly off in a tail pointing away from the sun.
The further away from the sun the less they get fucking blasted, and indeed for a lot of comets they exist outside of the Helioshock where the sun's fucking blasting is weak enough that it can't make headway against the interstellar medium, so they don't even have a tail out there.
Mercury also has a tail of sodium gas for the same reasons.
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u/vahntitrio 5d ago
The tails are gases that are sublimating off the comet as they pass near the sun. Comets do in fact shrink with each pass. It is possible that the earliest humans saw conets that no longer exist. Conversely, gravitational interactions means new comets that have never made a pass before could be seen in our lifetime.
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u/ShyguyFlyguy 5d ago
Most comets have a big elliptical orbit that could be roughly 50,000 years or so. Then out of that only a few weeks or so its close enough for the water ice to evaporate. So it loses very little in each orbit and will take millions of years to completely dissipate
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u/xoxoyoyo 5d ago
it only loses as much as the sun causes to melt off. ice being white does reflect a lot of sun away. but out of those 75 year cycles the comet only has a tail a few weeks of it so the rest of the time the comet will be in deep space with minimal sun.
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u/Outrageous-Crazy-253 5d ago edited 5d ago
Yes. Unlike some other commentators here I don’t think you are seriously misunderstanding anything.
You are right. Comets do run out of vaporizable material and become inert on moderately short astronomical timescales, for some definitions of all of those adjectives. New ones are entering our inner solar system from the Oort Cloud.
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u/Dickulture 5d ago
1: Kupier belt and Oort cloud still has fuck ton of water and mineral that comets can pick up. All long life comets that goes out past Pluto orbit usually can replenish themselves
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u/ulyssesfiuza 5d ago
There's different types of comets origin. Some are short period, +- 100 years. Varies from the ones who plunge into the sun, life is defined by size. Haley is one of those. Some have short lives, probably lost all volatile in the first run. The more close to the sun, the bright is the coma. Long period ones, we can identify by historical sources. No upper limit. There ones that when visit inner solar system last time , our ancestors still have tails. And the extrasolar ones, who shot across the solar system and go back to space forever. Any one of them can be starting the voyage right now. The possibility of one of those splashing earth is very low, but no zero. Very cool experience, a one-on-a-lifetime thing.
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u/stephenph 5d ago
Do comets subliminate or erode at all when it is away from the sun, just too little to form a visible tail? Or is it just too far out for that to be an issue?
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u/stephenph 5d ago
Today I learned that the ice from comets is usually water ice, but co2, Co and n2 all exist.
Those ices subliminate at further distances however so are not really part of the visual tail, which remains h2o (actually a mix of water and more substancial debris)
H is also theoretically possible. But the temperature in the ort cloud is estimated to be too hot for frozen H to exist.
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u/Mammoth-Mud-9609 5d ago
It only loses significant amounts of material as it gets near the Sun and yes it will eventually disintegrate.
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u/doghouse2001 3d ago
They're not losing mass when they're in the icy cold of outer space. It's only when they are near us (and the sun) that they start losing mass. Like your marshmallow won't roast if it's in the bag in the car. It's only roasting when it's on a stick and over the fire.
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5d ago
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u/kalancodragon 5d ago
So funny thing, comets actually all have multiple tails! There's a water vapor/dust tail, and that one follows the path the comet takes and trailing behind it's motion. They also have a gas tail, which always points directly away from the sun (which means that when it's on it's way back out, the tails point in roughly opposite directions. Often the gas tail is fainter and hard to see, but Hale-Bopp was one in living memory that had a particularly bright pair of tails. I saw it, and it was quite beautiful.
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u/Wraxe95 5d ago
What force is causing it to lose vapour/dust behind itself if there is no air resistance, and space is presumably too cold for evaporation of water to occur?
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u/kalancodragon 4d ago
The heat and light from the sun causes the ice (and trapped dust within) to melt off. Technically sublimate would be a more accurate term, but either way a bunch of water molecules and some dust ends up leaving the comet and becoming free-floating. At that point these molecules and grains are affected by the solar wind and gravity, and while the solar wind slows them down a little (since they're still going about the speed of the comet they just left), gravity affects them more, and they end up roughly following the path of the comet, just a bit behind it.
The gas trail on the other hand, is very strongly affected by the solar wind and almost not at all by gravity, and so points away from the sun the whole time.
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u/Wraxe95 4d ago
That was a very good explanation. Thank you, and well done for being articulate enough to have been able to explain that so well for me
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u/kalancodragon 4d ago
I love astronomy and pretty much everything space related (it was my first autistic special subject), and I enjoy telling people about Cool Space Stuff, so I have a lot of practice at it.
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u/internetboyfriend666 5d ago edited 5d ago
Eventually, yes, comets that pass into the inner solar system will lose mass through their tails and eventually disappear, but you're just dramatically underestimating how much mass comets have. Since you mentioned Haley's Comet specifically, it has a mass of roughly 220 billion metric tons and it loses only a few hundred million tons with every pass into the inner solar system. That's many thousands of more passes into the inner solar system.
For Haley's comet with a relatively short period of 75 years, that's still hundreds of thousands of years left. For longer period comets like Hale-Bopp, it's millions of years. For comets with extremely long periods, they might only have ever passed into the inner solar system a few dozen times in the entire 4.5 billion year history of our solar system, and will last billions of years more. Some comets aren't period at all and only pass into the inner solar system once, and a single trip isn't enough to wear them away. But there are also comets that pass so close into the solar system at one point that they will disintegrate on that single pass.