Comment by pX0r
8 years ago
"Sorry data, air may be the eventual oil."
Trivia question: How many round-trips from Neptune would it take to cause a 1% dip in Earth's air content?
Bonus question: Since the Earth is not making any more Xenon, are we losing some of this resource to the deep space every time we nudge a satellite?
A lot; also keep in mind that air is not a finite resource, it's continually generated from e.g. electrolysis (h2o -> o) and other processes (carbon + oxide = c02, plenty of carbon on earth, plenty of oxygen). Plus as another commenter mentioned, we're already losing some air all the time anyway.
Tangentially related question - what happens to the gas used as a reaction mass in thrusters in orbit - when it's used to speed up I guess it falls down cause velocities mostly cancel out, but when it's used to slow down the ship, and engines are fired retrograde - the reaction mass has orbital velocity, right?
Does it stay in some orbit forever, like a solid object would? Can it cause gas "Kessler syndrome", with gas rings around Earth's most common reaction mass orbits?
If we choose our orbits and burn times so that this gas piles up in particular place on particular orbit, can we then reuse that as "air" for these engines from the article?
The exhaust velocity in a xenon ion thruster is 20-50 km/s. Most of the time it’s on an earth escape trajectory (~11 km/s in low orbit).
The smaller an object is the less time it takes for drag from the super tenuous atmosphere up in orbit to slow it down so it falls back to Earth. Gas molecules are very low mass and I expect that the exhaust for any given thruster will be gone quickly, even in the higher levels of LEO.
Xenon's very heavy, most of it would eventually come back down to Earth - probably sooner rather than later. Most of what we lose to deep space is hydrogen and helium. And almost none of that is from space missions, anyway, it's just Brownian motion.
Isn't it a matter of speed rather than mass? If the xenon is ejected faster than the escape velocity, it seems like it would get off Earth's gravity.
In fact I think it would have to be roughly twice the escape velocity since the spacecraft is already going near it in one direction. According to Wikipedia[1] the exhaust velocity of an ion thruster is between 20 to 50 km/s when the Earth escape velocity is 11km/s [2]
[1]: https://en.wikipedia.org/wiki/Ion_thruster [2]: https://en.wikipedia.org/wiki/Escape_velocity
so I would assume most of it is lost in space
Velocity alone doesn't answer the question. Direction matters.
My assumption (knowing nothing but basic Physics), is that the xenon is ejected in a direction slightly toward the earth, and mostly directly in the opposite direction of the current travel, because that's what would be necessary to counteract drag and keep a satellite on the same path.
This means that if the satellite is going almost 11km/s one direction, the xenon will have that much less speed compared to the earth. And the trajectory will be slightly toward the earth.
I would assume that makes it substantially more likely that the xenon falls back to earth.
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Wow, you're right, I vastly underestimated the exhaust velocity.
You'd still have to account for its interaction with the atmosphere, but my point is moot.
It's not Xenon you have to worry about, it's Helium. Once we run out we'll be too heavy and fall into the sun.