Which orbit change takes less energy?

Question

A recent question about changing asteroids' orbits is in line with my own thoughts lately. It seems to me that with enough advanced warning, an ion thruster could be landed on an asteroid and used to divert its orbit enough to take it out of harm's way.

Thinking of the reason that Pluto will never collide with Neptune, I thought that possibly inclining an asteroid's orbit might be easier than raising its apehelion above 1au.

Unfortunately, I don't know all the formulas involved so it's not a simple answer for me. Assuming diverting orbits of Neo's as a given, which would be better - increasing their energy to stay outside Earth's orbit, or inclining them off of the ecliptic so that there is no intersection.

Whoops - I meant Perihelion, not Apahelion. Sorry about the confusion on that point.

Answer 1

Orbital plane changes require much more energy than changes within the orbit's existing plane.
If the asteroid's trajectory is accurately determined before it reaches its aphelion or perihelion then only a relatively moderate energy application is needed to effect significant changes when performed at those points.

Answer 2

There are no plans to either change inclination or perihelion distance, as both those changes would take FAR too much energy. The plan is to speed up or slow down the asteroid by a very small amount so that in 20 or 30 years when it is expected to cross Earths orbit at the precise moment that Earth is there, it will instead cross either just after or before the Earth passes that point. Now of course changing the speed along the orbit will naturally change either the aphelion or perihelion distance, but not by much. Not enough to prevent the orbits crossing. In effect, we are only avoiding that one collision, not making the orbit so different that no collision is ever possible ever again. That would take far too much energy. A speed change of 1/20 of a mile per hour would be enough to avoid a collision that is expected 20 years in the future. Changing the orbital plane or aphelion or perihelion would require a velocity change of hundreds or even thousands of miles per hour.

Answer 3

You don't have to push so hard as to change the inclination of the orbit much. All you have to do is to divert that asteroid once by a few thousand kilometers. The near earth flyby will alter its orbital velocity around the sun further and make sure that it won't come back for a very long time. Certainly longer than humankind needs.

For all you folks who believe that we absolutely need technology to divert asteroids, please keep this in mind:

Those things come here to visit once in a geological epoch. Less, actually. Not one human in the history of mankind has been harmed by one and it is very unlikely that one ever will.

However... and this is important to understand, the first nation to develop the technology to divert a large asteroid has automatically the technology in hand to divert a small one. Small enough to hit one country or a continent and completely wipe it out, yet leave the attacker mostly unharmed. Not only that, but the technology wouldn't leave any radiation, either. So you get the effect of a thousand nuclear explosions and none of the mess of global nuclear war. Cute, isn't it?

Just something to keep in mind for you who think that asteroid diverting technology is all for the general good. It isn't. Its military abuse potential is million times higher than its potential to save mankind from an event that is likely to never occur naturally because there are many more suitable small rocks to throw out there than big ones that need diversion.

Answer 4

Inclining the orbit to the ecliptic results in having 2 points of intersection between the orbits. Better I think to decrease the orbital speed and let it fall into the sun.