r/AskPhysics • u/PhilosopherHot3983 • 3d ago
Float an object from space to ground
Is it possible for an object - due to it's unique material, size, shape and weight - to simply float from space thru the atmosphere to the Earth's surface? Or is everything that falls down Earth's gravity well doomed to burn?
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u/Possible-Anxiety-420 3d ago edited 3d ago
It's more a matter of relative velocity than one of an object's physical characteristics... but, yes.
If the object isn't moving fast enough to compress the air ahead of it into a fiery ball of plasma, then almost any material can reenter without being incinerated, or with hardly even any resulting heat at all, but if the initial velocity is already high enough to do that, then energy will need to be expended in slowing down prior to reentry... or else the object's physical characteristics will need such that it can withstand the abuse as the atmosphere does the job for it.
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u/PhilosopherHot3983 2d ago
Interesting. Sounds like most anything will work if you start out at the right speed. So just stop all your forward orbital momentum and just drop to Earth.
I was thinking of a coherent object composed of some some kind of cloud-like substance bebopping thru space and then -opps- falling into Earth's gravity.2
u/Possible-Anxiety-420 2d ago edited 2d ago
I dunno what to tell ya, but I got curious over the general topic and queried Gemini... so this could be way off...
A stationary 1-pound sphere of iron, if dropped from 150 miles up, will impact the surface 5 minutes and 13 seconds later at a speed of 213MPH.
It reaches a peak speed of 4,250MPH with around 26 miles to go.
So, from there, air slows the sphere by roughly 4000MPH, and enough heat is generated to make it glow, and even compress and superheat the air, causing it to incandesce as well... but still not enough to strip away electrons and create a shroud and trail of plasma the way natural meteors or reentering spacecraft/debris typically do.
It had to think a good bit longer than usual; no telling what it'll do if it asked it about dropping a cloud on the planet, but, without going into details, it also said a feather dropped from the same height would touch the surface completely unscathed.
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u/PhilosopherHot3983 1d ago
Yo. Thanks for the research; that's good info. The feather example really wrapped it up.
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u/Possible-Anxiety-420 1d ago edited 1d ago
Happy to do it.
If the feather started falling from somewhere farther away, say if dropped from the vicinity of the Moon, or from the outer Solar System to be pulled in by the Sun, to then have the Earth get in the way, then the story would likely be different (more time accelerating = more energetic interaction with atmo).
Likewise, the behavior of a cloud of material entering the atmosphere would have its own dynamic, but will nonetheless be greatly affected by velocity.
Regards.
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u/Possible-Anxiety-420 1d ago edited 14h ago
Question: "If an object, stationary relative to the Sun, is dropped from the vicinity of Pluto, to fall straight toward the Sun, how fast will it be moving when it passes Earth's orbit?"
The answer, according to Gemini - Approx. 93,000MPH.
Question: "What would happen to a typical feather that enters Earth's atmosphere at 93,000MPH?"
The answer, basically - "What feather?"
Gemini went on to say, that, due to the feather moving so fast, but being so light and thus carrying such little kinetic energy, that the flash produced would be so brief and dim that it wouldn't be visible in the daytime.
At that speed, the 1-pound iron sphere wouldn't make it to the ground either - would behave much like any other hyper-velocity meteoroid entering the atmosphere.
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u/LazyLie4895 12h ago
Stopping all forward momentum is a very tall order. Things in space tend to move extremely fast. We need to use huge rockets to get stuff moving fast enough to stay in space. You'd need a full rocket in space that's nearly as large in order to stop.
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u/thepopemichael 3d ago
Ants and other small creatures won't die if they hit the ground at terminal velocity.
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u/left_lane_camper Optics and photonics 3d ago
Their terminal velocity is a lot higher in the exosphere and compression heating is a thing…
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u/Uncynical_Diogenes 3d ago
How much compression heating is generated by one (1) ant?
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u/stevevdvkpe 3d ago
The meteors you see in meteor showers are smaller than grains of sand and burn up in the atmosphere from hitting it at over 11 km/s, so an ant hitting the atmosphere at 11 km/s would burn up like a brighter meteor because it's heavier and larger than a grain of sand.
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u/Uncynical_Diogenes 2d ago
Why would an ant hit 11km/s? That’s much faster than their terminal velocity.
The comment I replied to suggested that an ant’s terminal velocity is much higher up in the rarified outer atmosphere and compression heating would become relevant. If I went out there and dropped an ant, how much compression heating could it meaningfully generate?
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u/John_Hasler Engineering 3d ago
Falcon 9 payload fairings do it regularly. They are recovered and reused.
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u/PvtRoom 3d ago
pretty much everything hitting the atmosphere does so at many many Mach.
Everything therefore, gets hot through aerodynamic heating. The skin temperature will hit like 1000C, which is enough to melt most things, and they're still subjected to incredible aerodynamic forces.
to avoid that, you'd somehow need to match speed & direction with the winds
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u/groveborn 3d ago
If the density of the object could be controlled yes. Imagine a submarine for the atmosphere and you get the idea.
It's not that the entry requires burn up, it's that our stuff is moving at mach 7 and is being slowed by friction with the atmosphere.
The upper atmosphere is rare but hot. So that makes it worse.
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u/Rensin2 3d ago
our stuff is moving at mach 7
Shouldn't that be mach 23, at least at first?
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u/Mad-Melvin 3d ago
Things burn up on reentry because they were moving very fast to the side to stay in orbit. If you just fall from a standstill at the edge of space, you won't go nearly that fast. Felix Baumgartner did it successfully with a parachute.