Background: I have an icy moon on a collision course with my fantasy world. Mages on this world have the ability to slow the speed of the moon gradually. I want them to use this power to lock the moon into orbit around the planet.

Note that 'moon' in this case refers to a moon-sized object, which is actually orbiting the sun like a comet.

Based on my understanding of how to capture an asteroid (from here and here [for the visuals of the orbit]), the process the mages would follow goes something like this:

  1. The mages slow the oncoming moon sufficiently for it to achieve a closed orbit around the planet.
  2. The mages must wait for the moon to pass the planet once. This might result in aerobreaking.
  3. Once the moon has passed the planet, the mages slow it more, gradually making its orbit more circular.

If this process is correct, there will be other questions about the second stage, so ignore for now any effects aerobraking or a close pass might have on the planet. The question is as follows:

Is this process correct? Do the mages have to wait for the moon to pass them to apply the third step, or can they achieve a closed orbit and immediately begin making it circular, thus avoiding the second step completely? That's my main concern. However, if there are other aspects (or all) of the process which are wrong, please let me know.


  • I am not at this time interested in numbers or formulas. Those will come in following questions. This question deals only with the process to capture the moon via slowing it.
  • Ignore the reasoning for such a plan. Don't worry about why the mages want to capture the moon, instead of just slowing it so that it bypasses the planet completely.
  • Please back up your answer with explanations and/or links, but please phrase them in layman terms. Literally everything I know about orbital mechanics is in the above.
  • $\begingroup$ Depending on the orbits of the moon and the planet slowing down would have a very limited effect on the moon if it is on a collision course, all it would do would be to slow the impact, not stop it, first you'd want to have the mages "push" Radial Out or Radial In (dependent on the orbital line), this would remove the collision course part of the orbit and make it a flyby, then yes this process above takes over and can cause the moon to enter into orbit, other youtubers on KSP asteroid capture worth watching are Matt Lowne, Scott Manley and Marcus House, they cover this sort of thing $\endgroup$ Commented Jul 25, 2018 at 7:17

3 Answers 3


You have only mentioned slowing the object, so I assume the mages can not increase the speed or energy of the object.

Assuming it is on a collision course before any intervention, you will need to slow the object while it is still at a large distance from the planet. This will cause the trajectory to cross the planet's orbit after the planet has passed.

At that point, some significant slowing is needed to bring the object into a closed orbit. Without further and immediate slowing, the object will be deflected by the planet but will head back into space.

It would be best for the object to pass well behind the planet in orbit. That will define the closest point in the orbit. Your mages will be able to lower the orbit, but not raise it. Unless they can absorb the exact amount of energy, the orbit will be highly eccentric.

By removing energy at the highest point, the orbit can be made to approach the planet more closely. By removing energy when the object is the closest, the mages can make the orbit more circular.

Yes. If you have mages who can slow an astronomical object currently on a collision course with the planet, they can capture the object and place it in an orbit at any desired height.

  • $\begingroup$ good catch on the "behind" approach. Assuming the planet orbits in a counter-clockwise direction, the moon would orbit in a clockwise direction. when an orbit is backwards from the orbit of the planet, it's called retrograde orbit. $\endgroup$
    – Stephan
    Commented Jul 24, 2018 at 18:34
  • $\begingroup$ @Stephan I hadn't thought about how orbit would be retrograde. If they saw an object that would pass far enough in from of them, and slow that object some, but not enough to collide with the planet, they could establish an orbit where the object was rotating properly for the planetary system. With magic, does there need to be somewhere for the momentum to go? Is conservation of energy, momentum, and angular momentum something mages handle in their work? $\endgroup$
    – cmm
    Commented Jul 24, 2018 at 18:44
  • $\begingroup$ The magic works exactly as if the mages were pulling on the moon opposite of its velocity. They are doing the opposite of pushing on it from the front (they are pulling on it from the back). I'm not sure if there's any net difference between the two methods, but that's how it works. $\endgroup$ Commented Jul 24, 2018 at 18:49
  • $\begingroup$ it kind of depends on what the OP means by "slow down". if we assume they can only decrease the moons orbital velocity (meaning relative to whatever body its in orbit around: first the sun, then the planet) then i think we're locked into retrograde no matter what. your suggestion would cause it to fall back relative to the planet, but really its now orbiting closer to the sun. $\endgroup$
    – Stephan
    Commented Jul 24, 2018 at 18:49
  • 1
    $\begingroup$ that's effectively what you're doing $\endgroup$
    – Stephan
    Commented Jul 24, 2018 at 18:51

Really there are two steps for capture.

  1. Maneuver for Intercept - Slowing the object down (if it's orbiting farther away from the sun than the planet) so that its orbit crosses the path of the planet at the desired periapsis (altitude of closest approach to the surface of the planet).
  2. Maneuver for Capture - Slowing the object down while at periapsis to bring its orbit around the planet. You stop slowing it when the apoapsis (altitude of farthest distance from its parent body) is at the desired altitude.

The main thing to remember is: when at periapsis, slowing down will bring your apoapsis closer to the planet. When at apoapsis, slowing down will bring your periapsis closer to the planet.

  • $\begingroup$ Would this create a (semi)circular orbit, or would I need to further tweak the speed somewhere? $\endgroup$ Commented Jul 24, 2018 at 18:14
  • $\begingroup$ as long as you dont overshoot during the intercept phase, you can be pretty close to circular when in the capture phase. if you slow it down too much during the first phase, youll end up with the moon coming too close to the planet, if you slow it down too much or not enough in the second phase, youll have the moon in an elliptical orbit. $\endgroup$
    – Stephan
    Commented Jul 24, 2018 at 18:18

I posted this as a comment but decided that your request for Layman's terms means I've made this as an answer: First off a simple glossary makes life a lot easier, just refer back to this whenever people get stuck

  • Apoapsis: Highest Altitude compared to the Parent Body it orbits
  • Periapsis: Lowest Altitude compared to the Parent Body it orbits

The below are directions of the orbital line (this is for a Anti-Clockwise Direction which is standard)

  • Prograde: Forward or in the direction the orbit already follows, basically speed up
  • Retrograde: backwards or against the direction the orbit already follows, bscailly slow down
  • Radial In: Left in relation to the direction the orbit already follows
  • Radial Out Right in relation to the direction the orbit already follows
  • Normal: Up in relation to the direction the orbit already follows
  • Anti-Normal: Down in relation to the direction the orbit already follows

The final part is the all important orbital mechanic of the question, if you think of the orbit as a circle around central point. at the Top of the Circle is point A, and the bottom of the Circle is point B, if you slow down at A, this doesn't change the orbital line at A, it reduces the altitude of point B, on the other side of the orbit.

So for your answer

if the Moon and the Planet were both orbiting the Sun and on a collision course due to impact in an more than about 6 months time, then it stands to reason that you and the moon would most likely be somewhere in the region of the Point A, (but only if the moon was not within the sphere of influence of the planets gravity), part of the Orbit whereas the collision would be at Point B of orbit, so that means that any retrograde force applied now, would stop this impact from happening, and can be made to simply capture the moon.

However if the collision is taking place in less than that time, or the moon is already inside the Planets gravity Well or sphere of influence, then a retrograde force would actually only make the collision even more certain, if this is the case you would most likely need to push Radial In or Radial Out the left or right to stop the impact from happening and then once the Moon was at its Periapsis, you would want to slow it down because this would lower its Apoapsis, Again slowly down Point A to reduce the altitude of Point B. and this would bring the moon's orbit entirely into the Planets Gravity Well, , if the moon was travelling too fast then it would fling back out into a Solar Orbit rather than staying Orbiting the Planet.

Aerobraking is a way to reduce the amount of fuel and energy needed to slow an object or craft. if you don't use Aerobraking to do this then you need to burn your engines and fuel constantly to slow down. this is not required for your moon, but it is an option (as you said ignoring the massive ramifications of that happening)

The next and final part is if you have used aerobraking that means your Periapsis is very low, and leaving it there would mean that the moon continued to slow on each orbit and eventually it would crash into the planet, so once you've got the Apoapsis where you want it, the you need to speed up the moon at the Apoapsis to raise the Periapsis, again Speed up Point A to Raise Point B.

Hope this helps. if you wanted to see this in action, then i recommend Kerbal Space Program, (the game in your second link) it has an entire tutorial section on just this

  • $\begingroup$ I don't quite follow why having the moon collide in under 6 months makes the collision certain. If it's caught by the planet's gravity, yes, but what if it isn't, and will just arrive in under 6 months? Why won't slowing it down do anything? $\endgroup$ Commented Jul 25, 2018 at 14:57
  • $\begingroup$ @ThomasMyron, what was meant by that was that due to the way orbits work, they are already orbiting the parent body, ie the sun, if the planet, assuming similar to earth: 12 month orbit. and the collision course is going to occur in 6 months, the earth and again assuming the moon would be on the other side of its orbit compared to that of the collision, Point A and Point B again, this would mean that a retrograde force would stop the collision. but at any other position in its orbit, then retrograde forces become less effective at averting the collision itself $\endgroup$ Commented Jul 26, 2018 at 6:12
  • $\begingroup$ @ThomasMyron, being that your magic system is unexplained, i assumed that the moons orbit is similar to earth's otherwise the magic would have to reach insanely far and powerfully to exert a force on something so far away, if the moon was always nearby then although the distances are still huge, it seems more practical, therefore the 6 month comment, placing both bodies at Point A, and the collision at Point B. sorry for the double comment $\endgroup$ Commented Jul 26, 2018 at 6:16
  • $\begingroup$ The 'moon's' orbit is more like a comet, but I think I see what you're saying. $\endgroup$ Commented Jul 26, 2018 at 15:19

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