5 ms·

Is it prohibitively expensive to just boost them into the sun or elsewhere, while in low orbit?

JumpCrisscross 3y ago
> prohibitively expensive to just boost them into the sun or elsewhere, while in low orbit? It’s easier to exit the Solar System than it is to boost mass into the Sun from any Earth orbit [1][2]. The atmosphere providing braking for “free” is a huge propellant saver. [1] https://en.m.wikipedia.org/wiki/File:Solar_system_delta_v_map.svg https://en.m.wikipedia.org/wiki/File:Solar_system_delta_v_ma... [2] https://space.stackexchange.com/questions/3612/calculating-solar-system-escape-and-and-sun-dive-delta-v-from-lower-earth-orbit https://space.stackexchange.com/questions/3612/calculating-s...
- o11c 3y ago
  That's commonly cited, but actually a wrong implication of delta-v. It's expensive to get close to the Sun at low speeds (e.g. if you could land on it, or if you want a close orbit). It's trivial to slingshot around the moon and impact the Sun at high speed.
  - JumpCrisscross 3y ago
    > actually a wrong implication of delta-v Could you share a sample pair of trajectories that demonstrates this claim?
  - mnw21cam 3y ago
    The Earth's orbital velocity around the Sun is ~30km/s. You need to cancel almost all of that out in order to shift to a trajectory that intersects the Sun, and that trajectory still has you impacting the Sun at quite some speed. The Moon's orbital velocity around the Earth is ~1km/s. A slingshot manoeuvre around the Moon isn't going to help nearly enough here. One of the most efficient ways to get to the Sun is to leave Earth in the same direction it is going (and yes you can use a Moon slingshot to help here if you want) which will result in your rocket going further away from the Sun than the Earth. Once you're out in the region of the outer planets, you'll be travelling a lot slower and won't need to cancel out nearly as much velocity. Neptune's orbital velocity is ~5.5km/s, and if your rocket has an apohelion around there and a perihelion around Earth then you'll be travelling slower than that. However, it still takes a huge amount of delta-v to get out to the region of Neptune and then cancel out your velocity relative to the Sun. Compare that to the amount of delta-v required to de-orbit an object from low Earth orbit. The object's velocity around the Earth is going to be about 7.9km/s, but it only requires a delta-v of around 100m/s to put it into a nice predictable atmospheric entry. That's comparatively nothing.
  - JKCalhoun 3y ago
    Not so trivial though to slingshot around the Moon? (Though ISS is in orbit, I believe there is some ways still to climb from ISS to get to the Moon.)
  - perihelions 3y ago
    Not enough delta-v from a lunar gravity assist—not even close. It's extremely difficult to impact the sun (at any speed), or get into a trajectory which does that.
    - andrewflnr 3y ago
      How does delta-v prevent you from aiming in a direction that intersects with the sun? That's all that's required for trash disposal. You aerobrake in the solar atmosphere, if your want to think of it that way
      - lxgr 3y ago
        To reach the sun, you need to shed a lot of kinetic energy - and given that you’re in a vacuum on most of the way there, the only way to do that is through a lot of deceleration, which is really the same thing as retrograde acceleration, and maybe some gravitational slingshots, but these aren’t free either (you need delta-v to execute them) and therefore have limited gains. (Technically it’s cheaper by a factor of three to go to escape velocity out of the solar system and then plunge back into the sun, but at that point, why go through the trouble and come back if your concern is trash disposal?)
        lupusreal 3y ago
        > To reach the sun, you need to shed a lot of kinetic energy - and given that you’re in a vacuum on most of the way there, the only way to do that is through a lot of deceleration, which is really the same thing as retrograde acceleration If you can fenagle yourself into a highly elliptical solar orbit, then a relatively small retrograde burn at apoapsis could get you into the sun.
        JumpCrisscross 3y ago
        > If you can fenagle yourself into a highly elliptical solar orbit Getting to that orbit is more expensive than leaving the system.
      - 3y ago
  - JumpCrisscross 3y ago
    > trivial to slingshot around the moon Slingshots steal orbital momentum. When one slingshots around Jupiter, one is stealing Jupiter’s orbital momentum about the Sun. (It only works in one direction.) Slingshotting about the moon to gain velocity relative to the Sun doesn’t work. That said, I’d love to see an orbital solution for cheap decay into a solar-impact trajectory. (Orbital mechanics are complex enough that nobody should feel comfortable entirely precluding subliminal sets of solutions.)
    - lxgr 3y ago
      > Slingshotting about the moon to gain velocity relative to the Sun doesn’t work. (It only works in one direction.) Most slingshot maneuvers do gain or lose momentum relative to the Sun! And they do work in both directions (gain and lose) – we've used them for quite a few probes, e.g. MESSENGER. > Slingshotting about the moon to gain velocity relative to the Sun doesn’t work. It does, as used by e.g. STEREO [2]. All that said, they're still not "free" in terms of delta-v by any definition. They provide an efficiency gain, but sending stuff to the sun is still prohibitively expensive for anything other than lightweight scientific probes. [1] https://en.wikipedia.org/wiki/MESSENGER https://en.wikipedia.org/wiki/MESSENGER [2] https://en.wikipedia.org/wiki/STEREO https://en.wikipedia.org/wiki/STEREO
    - eru 3y ago
      > Slingshotting about the moon to gain velocity relative to the Sun doesn’t work. Why not? And you don't actually want to gain velocity, you'd want to change direction. I agree that shooting stuff into the sun is not worth it, if you just want to get rid of it.
      - JumpCrisscross 3y ago
        > Why not? And you don't actually want to gain velocity, you'd want to change direction If you fall towards an object and then fall away from it, there is no net force. You can’t bleed or gain delta-V simply by falling into an object and then falling away from it. The reason planetary slingshots work is you’re “dragged” along with their orbital velocity about the Sun. You can be clever about using that to reduce the delta-V to the Sun. But it’s still more than system escape velocity.
deleted 3y ago
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lmm 3y ago
Yes. LEO is "halfway to anywhere" but only halfway, boosting up to a high earth orbit requires significant amounts of fuel, getting all the way out of earth's gravity well takes more, and to get to the sun would take even more than boosting all the way out of the solar system. If they put them in an orbit "next to" the ISS then atmospheric drag would eventually carry them down until they burned up (even the ISS itself needs to be regularly boosted or it would eventually fall down) but that way it's unpredictable and uncontrolled (and there's always the risk of them hitting a satellite or something), a planned re-entry burnup is generally thought to be safer.
bagels 3y ago
Yes. Dropping in to the atmosphere is safer and infinitely cheaper than any other choice but not sending things to orbit in the first place.

> prohibitively expensive to just boost them into the sun or elsewhere, while in low orbit? It’s easier to exit the Solar System than it is to boost mass into the Sun from any Earth orbit [1][2]. The atmosphere providing braking for “free” is a huge propellant saver. [1] https://en.m.wikipedia.org/wiki/File:Solar_system_delta_v_map.svg https://en.m.wikipedia.org/wiki/File:Solar_system_delta_v_ma... [2] https://space.stackexchange.com/questions/3612/calculating-solar-system-escape-and-and-sun-dive-delta-v-from-lower-earth-orbit https://space.stackexchange.com/questions/3612/calculating-s...

o11c 3y ago
That's commonly cited, but actually a wrong implication of delta-v. It's expensive to get close to the Sun at low speeds (e.g. if you could land on it, or if you want a close orbit). It's trivial to slingshot around the moon and impact the Sun at high speed.
- JumpCrisscross 3y ago
  > actually a wrong implication of delta-v Could you share a sample pair of trajectories that demonstrates this claim?
- mnw21cam 3y ago
  The Earth's orbital velocity around the Sun is ~30km/s. You need to cancel almost all of that out in order to shift to a trajectory that intersects the Sun, and that trajectory still has you impacting the Sun at quite some speed. The Moon's orbital velocity around the Earth is ~1km/s. A slingshot manoeuvre around the Moon isn't going to help nearly enough here. One of the most efficient ways to get to the Sun is to leave Earth in the same direction it is going (and yes you can use a Moon slingshot to help here if you want) which will result in your rocket going further away from the Sun than the Earth. Once you're out in the region of the outer planets, you'll be travelling a lot slower and won't need to cancel out nearly as much velocity. Neptune's orbital velocity is ~5.5km/s, and if your rocket has an apohelion around there and a perihelion around Earth then you'll be travelling slower than that. However, it still takes a huge amount of delta-v to get out to the region of Neptune and then cancel out your velocity relative to the Sun. Compare that to the amount of delta-v required to de-orbit an object from low Earth orbit. The object's velocity around the Earth is going to be about 7.9km/s, but it only requires a delta-v of around 100m/s to put it into a nice predictable atmospheric entry. That's comparatively nothing.
- JKCalhoun 3y ago
  Not so trivial though to slingshot around the Moon? (Though ISS is in orbit, I believe there is some ways still to climb from ISS to get to the Moon.)
- perihelions 3y ago
  Not enough delta-v from a lunar gravity assist—not even close. It's extremely difficult to impact the sun (at any speed), or get into a trajectory which does that.
  - andrewflnr 3y ago
    How does delta-v prevent you from aiming in a direction that intersects with the sun? That's all that's required for trash disposal. You aerobrake in the solar atmosphere, if your want to think of it that way
    - lxgr 3y ago
      To reach the sun, you need to shed a lot of kinetic energy - and given that you’re in a vacuum on most of the way there, the only way to do that is through a lot of deceleration, which is really the same thing as retrograde acceleration, and maybe some gravitational slingshots, but these aren’t free either (you need delta-v to execute them) and therefore have limited gains. (Technically it’s cheaper by a factor of three to go to escape velocity out of the solar system and then plunge back into the sun, but at that point, why go through the trouble and come back if your concern is trash disposal?)
      - lupusreal 3y ago
        > To reach the sun, you need to shed a lot of kinetic energy - and given that you’re in a vacuum on most of the way there, the only way to do that is through a lot of deceleration, which is really the same thing as retrograde acceleration If you can fenagle yourself into a highly elliptical solar orbit, then a relatively small retrograde burn at apoapsis could get you into the sun.
        JumpCrisscross 3y ago
        > If you can fenagle yourself into a highly elliptical solar orbit Getting to that orbit is more expensive than leaving the system.
    - 3y ago
- JumpCrisscross 3y ago
  > trivial to slingshot around the moon Slingshots steal orbital momentum. When one slingshots around Jupiter, one is stealing Jupiter’s orbital momentum about the Sun. (It only works in one direction.) Slingshotting about the moon to gain velocity relative to the Sun doesn’t work. That said, I’d love to see an orbital solution for cheap decay into a solar-impact trajectory. (Orbital mechanics are complex enough that nobody should feel comfortable entirely precluding subliminal sets of solutions.)
  - lxgr 3y ago
    > Slingshotting about the moon to gain velocity relative to the Sun doesn’t work. (It only works in one direction.) Most slingshot maneuvers do gain or lose momentum relative to the Sun! And they do work in both directions (gain and lose) – we've used them for quite a few probes, e.g. MESSENGER. > Slingshotting about the moon to gain velocity relative to the Sun doesn’t work. It does, as used by e.g. STEREO [2]. All that said, they're still not "free" in terms of delta-v by any definition. They provide an efficiency gain, but sending stuff to the sun is still prohibitively expensive for anything other than lightweight scientific probes. [1] https://en.wikipedia.org/wiki/MESSENGER https://en.wikipedia.org/wiki/MESSENGER [2] https://en.wikipedia.org/wiki/STEREO https://en.wikipedia.org/wiki/STEREO
  - eru 3y ago
    > Slingshotting about the moon to gain velocity relative to the Sun doesn’t work. Why not? And you don't actually want to gain velocity, you'd want to change direction. I agree that shooting stuff into the sun is not worth it, if you just want to get rid of it.
    - JumpCrisscross 3y ago
      > Why not? And you don't actually want to gain velocity, you'd want to change direction If you fall towards an object and then fall away from it, there is no net force. You can’t bleed or gain delta-V simply by falling into an object and then falling away from it. The reason planetary slingshots work is you’re “dragged” along with their orbital velocity about the Sun. You can be clever about using that to reduce the delta-V to the Sun. But it’s still more than system escape velocity.

deleted 3y ago

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lmm 3y ago

Yes. LEO is "halfway to anywhere" but only halfway, boosting up to a high earth orbit requires significant amounts of fuel, getting all the way out of earth's gravity well takes more, and to get to the sun would take even more than boosting all the way out of the solar system. If they put them in an orbit "next to" the ISS then atmospheric drag would eventually carry them down until they burned up (even the ISS itself needs to be regularly boosted or it would eventually fall down) but that way it's unpredictable and uncontrolled (and there's always the risk of them hitting a satellite or something), a planned re-entry burnup is generally thought to be safer.

bagels 3y ago

Yes. Dropping in to the atmosphere is safer and infinitely cheaper than any other choice but not sending things to orbit in the first place.