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I understand that rotating momentum around a fixed axis can help with the center of balance. But is it enough?

In my case, a space tug has to burn its engine for 1 min, accelerate a 100-ton piece of asteroid, and then disconnect from it. There will be no chance for trajectory corrections during orbit transfer from NEO asteroid to the Earth's atmospheric entry. Would this precise trajectory change be even possible when applied to the irregular shape of an asteroid rock?

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    $\begingroup$ the key words you should look into for this are "spin-stabilization." I don't think the version of your question (can I spin-stabilize a 100-ton rock) is answerable without additional detail, though. $\endgroup$
    – Erin Anne
    Feb 1, 2023 at 2:28
  • $\begingroup$ @ErinAnne - thank you for your answer. We cannot get definite answers when asteroid mining is a concern. But these questions will have to be studied and answered. Let's say, we are not pushing rock from an asteroid quarry. Let's say this is Earth and you have tools and machinery to chisel this rock. What kind of precision would you need? $\endgroup$ Feb 1, 2023 at 2:44
  • $\begingroup$ Or closer to Earth example. You have spacecraft in orbit with two engines. One engine malfunctioned. Now, when you do an orbit transfer, you have only one engine firing which creates off-balance. Can you adjust the nozzle of this engine to compensate and have precise orbit transfer? $\endgroup$ Feb 1, 2023 at 2:58
  • $\begingroup$ I saw this in the HNQ list and guessed that it was an uhoh question. $\endgroup$ Feb 1, 2023 at 16:12
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    $\begingroup$ @TheMatrixEquation-balance if you are going to push something that is spinning... you need to have it in a fixture/structure of some sort or else bring object to total zero spin and then push it along, let it go ballistic and THEN spin it up. But why bother spinning it if does not buy you much practical benefit? $\endgroup$
    – BradV
    Feb 1, 2023 at 18:35

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Two observations:

  1. Any reasonable way of nudging a NEO towards Earth requires doing a burn months or even years in advance. Compared to the burn time (here 1min), that means we have a lot of time to do adjustments before and after the main burn.

  2. A 100-ton rock does not have a very large moment of inertia. Rotating it around is no big deal.

So what's required of the space tug is just to have an attitude control system with small RCS thrusters, which it is going to need anyway if it's going to do precision docking with a rock. Additionally, two axis engine gimbaling would be nice.

The procedure would then be approximately like this:

  1. Dock with the rock.
  2. Do tiny RCS burns to give the asteroid a little angular velocity. With a couple of axis this should be enough to figure out exactly where the centre of mass is located.
  3. Orient the asteroid in the same way.
  4. Gimbal the engine to align with the centre of mass. This can also be done with the claws/feet/anchors attached.
  5. Do the main burn.
  6. Remain attached for a couple of hours, confirming the new velocity.
  7. Do tiny corrections with the RCS thrusters
  8. Detach.
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  • $\begingroup$ "SE - stop firing the good guys" - thank you very much for your answer. This is what I wanted to confirm. Although, it is still a mystery to me how they've been successfully performing these course correction maneuvers since 60-th, especially gravitational assist maneuvers. From an ignorant point of view, it would require micron level precision. $\endgroup$ Feb 1, 2023 at 13:34
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    $\begingroup$ "A 100-ton rock does not have a very large moment of inertia. Rotating it around is no big deal.": for comparison, the ISS is about 420 metric tons. It was accidentally flipped around 540 degrees by an unintended thruster firing on the Nauka module. $\endgroup$ Feb 1, 2023 at 14:28
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    $\begingroup$ @TheMatrixEquation-balance Why "micron level precision"? Space is really big, getting something wrong by a few kilometres rarely matters. $\endgroup$
    – TooTea
    Feb 1, 2023 at 14:30
  • $\begingroup$ @TooTea - "Precision" tolerances. This is what I am trying to understand. For example, if you push a 100-ton piece of NEO asteroid Bennu - and you want to change trajectory in such a way, that this 100-ton piece - hit the Earth's atmosphere for “Aerocapture” orbital transfer maneuver. What would be my room for error (precision) in terms of fraction of degree of an angle (for applied acceleration force)? $\endgroup$ Feb 1, 2023 at 16:05
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    $\begingroup$ Iteration is key. You make a trajectory change maneuver, monitor the result, and if necessary, make another one. Repeat as needed until arrival at destination. The earlier you make the corrections and the more accurate your adjustment and monitoring, the more efficient you'll be, but with a little extra propellant you can get by with sloppier maneuvers. $\endgroup$ Feb 2, 2023 at 1:24

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