5 ms·

Assuming you don't go so close that tidal effects tear the probe apart, the g-forces should only be those imposed by the probe's thrusters firing during the ass

by caf 2y ago

Assuming you don't go so close that tidal effects tear the probe apart, the g-forces should only be those imposed by the probe's thrusters firing during the assist. When the thrusters aren't firing it's just in free-fall.

kadoban 2y ago
Hm, that can't be how that works, can it? How do you get any extra velocity then if there's never any acceleration beyond what your thrusters provide?
- messe 2y ago
  It's called the Oberth Effect. In short, in Orbital mechanics, burning your thrusters deeper within a gravity well, results in a greater increase in kinetic energy than burning them further out. This is because momentum ~ v, while kinetic energy ~ v^2. If you're travelling faster—as you would be as you approach the black hole and fall deeper and deeper in your orbit—then you can expend to same amount of momentum to receive a disproportionally larger increase in kinetic energy. Because your potential energy falls off with distance to the black hole at the same rate regardless of the speed you're travelling at, your total energy upon escaping the black hole is much larger than it would be had you burned your thrusters outside its gravity well.
  - deleted 2y ago
    [deleted]
  - HeatrayEnjoyer 2y ago
    But you are still burning the same amount of chemical energy...
    - ibeforee 2y ago
      Wikipedia says: In terms of the energies involved, the Oberth effect is more effective at higher speeds because at high speed the propellant has significant kinetic energy in addition to its chemical potential energy.[2]: 204 At higher speed the vehicle is able to employ the greater change (reduction) in kinetic energy of the propellant (as it is exhausted backward and hence at reduced speed and hence reduced kinetic energy) to generate a greater increase in kinetic energy of the vehicle.
    - perihelions 2y ago
      It's super un-intuitive, but you're indirectly harnessing the gravitational potential energy of lowering your propellant into a gravity well and leaving it there. The overall orbital energy gain of the spacecraft can exceed the chemical energy of the fuel.
      - lxgr 2y ago
        Wow, that's an amazing intuitive explanation of "where the extra energy comes from" in an Oberth effect burn that I haven't heard before. Thank you!
  - dotancohen 2y ago
    The phenomenon described is not due to the Oberth effect. What would be happening would be that the human in the spacecraft, and the spacecraft, are accelerating at exactly the same rate because they are accelerating due to gravity. Thus, the human feels no pressure accelerating him (no outside force acting upon him) from e.g. his seat. And his internal organs feel no pressure accelerating them from each other. They (the craft and the human and all his internal organs) are in free fall together and feel no forces acting upon them despite the whole system (craft-human-organs) being accelerated to tremendous velocities.
    - perihelions 2y ago
      Your parent comment is the textbook definition of the Oberth Effect phenomenon. I think you've misread something.
      - dotancohen 2y ago
        I mean to say that the Oberth effect is one phenomenon, and that the lack of a feeling of acceleration is another effect. The Oberth effect itself is not responsible for the lack of feeling of acceleration during the assist.
- exe34 2y ago
  If you sit on a merry-go-round, and spin it very fast, you feel the "centrifugal force" trying to keep you in an inertial frame. That's because you're having to hold on to the ride. If you're in a spacecraft in orbit around Earth, you don't feel the force keeping you in a circular motion, because both you and the craft are experiencing the same force. The worse thing about going past a blackhole would be tidal forces, which would exert differential stretching to your craft and you. I don't know the numbers for a blackhole that small. Also you'd need to aim very precisely - if you miss, it's a long way round to get back there and if you aim too well, it might take a chunck out of your craft and your left foot.
  - ibeforee 2y ago
    I was wondering that actually if the hole goes through your leg does it leave a hole or does your whole body get sucked in. I imagine it is the whole body unless you are travelling really fast at the time. Like near speed of light. Because the gravity outside the event horizon will still be crazy strong going out for several km (earth is a good comparison in the gravity is still fairly strong about 6000km from the centre)
    - pavel_lishin 2y ago
      Depends on the size of the hole. If it's small enough, all you might get is one long bruise due to tidal/gravity effects, without losing a single atom of your body
      - dotancohen 2y ago
        This is incorrect. Any matter within about three Schwarzschild radii will never escape the black hole - i.e. will be carried along with it - even if the black hole passes you at the speed of light. And if the black hole passes you at a velocity lower than the speed of light, then the radius grows. That matter will begin orbiting the black hole and will leave your body just as fast as the black hole left.
        2y ago

by caf 2y ago

kadoban 2y ago

Hm, that can't be how that works, can it? How do you get any extra velocity then if there's never any acceleration beyond what your thrusters provide?

messe 2y ago
It's called the Oberth Effect. In short, in Orbital mechanics, burning your thrusters deeper within a gravity well, results in a greater increase in kinetic energy than burning them further out. This is because momentum ~ v, while kinetic energy ~ v^2. If you're travelling faster—as you would be as you approach the black hole and fall deeper and deeper in your orbit—then you can expend to same amount of momentum to receive a disproportionally larger increase in kinetic energy. Because your potential energy falls off with distance to the black hole at the same rate regardless of the speed you're travelling at, your total energy upon escaping the black hole is much larger than it would be had you burned your thrusters outside its gravity well.
- deleted 2y ago
  [deleted]
- HeatrayEnjoyer 2y ago
  But you are still burning the same amount of chemical energy...
  - ibeforee 2y ago
    Wikipedia says: In terms of the energies involved, the Oberth effect is more effective at higher speeds because at high speed the propellant has significant kinetic energy in addition to its chemical potential energy.[2]: 204 At higher speed the vehicle is able to employ the greater change (reduction) in kinetic energy of the propellant (as it is exhausted backward and hence at reduced speed and hence reduced kinetic energy) to generate a greater increase in kinetic energy of the vehicle.
  - perihelions 2y ago
    It's super un-intuitive, but you're indirectly harnessing the gravitational potential energy of lowering your propellant into a gravity well and leaving it there. The overall orbital energy gain of the spacecraft can exceed the chemical energy of the fuel.
    - lxgr 2y ago
      Wow, that's an amazing intuitive explanation of "where the extra energy comes from" in an Oberth effect burn that I haven't heard before. Thank you!
- dotancohen 2y ago
  The phenomenon described is not due to the Oberth effect. What would be happening would be that the human in the spacecraft, and the spacecraft, are accelerating at exactly the same rate because they are accelerating due to gravity. Thus, the human feels no pressure accelerating him (no outside force acting upon him) from e.g. his seat. And his internal organs feel no pressure accelerating them from each other. They (the craft and the human and all his internal organs) are in free fall together and feel no forces acting upon them despite the whole system (craft-human-organs) being accelerated to tremendous velocities.
  - perihelions 2y ago
    Your parent comment is the textbook definition of the Oberth Effect phenomenon. I think you've misread something.
    - dotancohen 2y ago
      I mean to say that the Oberth effect is one phenomenon, and that the lack of a feeling of acceleration is another effect. The Oberth effect itself is not responsible for the lack of feeling of acceleration during the assist.
exe34 2y ago
If you sit on a merry-go-round, and spin it very fast, you feel the "centrifugal force" trying to keep you in an inertial frame. That's because you're having to hold on to the ride. If you're in a spacecraft in orbit around Earth, you don't feel the force keeping you in a circular motion, because both you and the craft are experiencing the same force. The worse thing about going past a blackhole would be tidal forces, which would exert differential stretching to your craft and you. I don't know the numbers for a blackhole that small. Also you'd need to aim very precisely - if you miss, it's a long way round to get back there and if you aim too well, it might take a chunck out of your craft and your left foot.
- ibeforee 2y ago
  I was wondering that actually if the hole goes through your leg does it leave a hole or does your whole body get sucked in. I imagine it is the whole body unless you are travelling really fast at the time. Like near speed of light. Because the gravity outside the event horizon will still be crazy strong going out for several km (earth is a good comparison in the gravity is still fairly strong about 6000km from the centre)
  - pavel_lishin 2y ago
    Depends on the size of the hole. If it's small enough, all you might get is one long bruise due to tidal/gravity effects, without losing a single atom of your body
    - dotancohen 2y ago
      This is incorrect. Any matter within about three Schwarzschild radii will never escape the black hole - i.e. will be carried along with it - even if the black hole passes you at the speed of light. And if the black hole passes you at a velocity lower than the speed of light, then the radius grows. That matter will begin orbiting the black hole and will leave your body just as fast as the black hole left.
      - 2y ago