# Orbital mechanics of Dragon's Egg

In the novel Dragon's Egg, the human crew use one asteroid to swing other asteroids in place to counter the gravity of the neutron star. I understood that it was similar to a gravity sling shot, but I wasn't able to fully get how the crew were able to move the smaller asteroids in place using the big one. Can anyone explain that further?

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Please note that Dragon's egg is a "hard sci-fi" book, which in theory should be consistent with known laws of physics (so assume them), including GR. – Sklivvz Dec 11 '10 at 15:32
maybe whoever can answer this could also explain how six asteroids, orbiting their common center of gravity, while collectively orbiting a neutron star, could possibly be stable with the kind of tidal forces that would be present. – JustJeff Dec 11 '10 at 17:01
I've read the book's plot on the wikipage, and I just realize that a Star Trek Voyager episode, "Blink of an Eye", basically stole the plot from Dragon's Egg. – Raskolnikov Dec 11 '10 at 18:45
I do not know about the answer but if you ever come across a guy who can calculate such stuff, ask him to contact an authority with the solution of n-body problem. – Cem Dec 12 '10 at 1:04
@Cemm: There are actually probes that "correct" the orbits keeping them stable... :-) – Sklivvz Dec 12 '10 at 4:23

## 3 Answers

I know nothing of this book, but I do know a little about N-body gravitational interactions. When N >= 3, you can do just about anything you want with a little propulsion, but it may take a very long time. This has been proposed by NASA (good approximate for hard SF) as a way of sending probes to the outer solar system: http://en.wikipedia.org/wiki/Interplanetary_Transport_Network

As far as the stability questions, it is known that you can put 7 or 8 equal-mass objects on a stable circular orbit around their center of mass: http://adsabs.harvard.edu/abs/1988A%26A...205..309S

After introducing an external mass and its accompanying tidal forces, I imagine there is still some stability regime, which could be pretty close to the NS, especially if the "asteroids" have WD density, their constellation could be on the order of 1km, while the NS radius is probably closer to 15km.

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When a small spacecraft slingshots around a large moon or planet, it can greatly increase its speed at the expense of very slightly decreasing the speed of the larger body. Given a method by which to move a larger body, you could very simply cause it to move smaller bodies by traveling near enough to attract them. More complicated would be moving it in such a way that the smaller body was given a specific increase in absolute velocity, which would be the "reverse slingshot".

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Thanks for the tip. – Bala Dec 14 '10 at 4:40

Not an answer but to give context:

There are eight asteroids: 2 large and 6 smaller ones. All the asteroids have been collapsed by injecting their core with magnetic monopoles that makes them collapse to white dwarf density. The large ones were originally 250km in diameter and collapsed to 100m.

The larger asteroids are on highly elliptical orbits.

http://books.google.co.uk/books?id=YataAAAAMAAJ&dq=isbn:034543529X&q=six+asteroids#search_anchor

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@Noldorin: AFAIK magnetic monopoles are consistent with the known laws of physics. And some GUT theories (most of them?) actually predict them. So let's wait and see :-) – Marek Dec 11 '10 at 22:28
@Noldorin: The zero RHS of Gauss's law is not a requirement in the theory, but simply an expression of the non-observation of magnetic monopoles to-date (notably various textbooks I've read exhibit the "with magnetic monopoles" form of Maxwell's Equations in a footnote or appendix). IMO positing physics allowed but not yet observed is fair game in that kind of fiction. – dmckee Dec 11 '10 at 23:11
@Noldorin♦: What a weird remark! So you suggest that there can't exist any macroscopic effect in nature predicted by QFT if it's not consistent with Maxwell's equations? I hold a completely opposite view: Maxwell's equations hold only in a very narrow parameter space. If you move outside of that space they don't need to (and they don't) hold anymore. Portion of that parameter space might be physics of monopoles. If they exist then Maxwell's equations simply don't describe our world completely (which we know anyway). – Marek Dec 11 '10 at 23:33
@Noldorin: Maxwellian electromagnetism does not rule out magnetic monopoles: observe one tomorrow and the theory adjusts smoothly to accommodate it. Heck, in this case the math is already worked out. – dmckee Dec 11 '10 at 23:50
Why would a monopole make an asteroid collapse down to white dwarf density? It might paramagnetically polarize the asteroid, but the monopole charge would have to be pretty large to destabilize the entire asteroid. And even if that happened, why would the exclusion pressure of electrons be enough to stabilize it, rather than causing the collapse to continue? – Jerry Schirmer Dec 12 '10 at 18:22