How to intercept an Asteroid in Kerbin's SOI?

[Tassyr]

So I've figured out- roughly- how to intercept an asteroid that's out of Kerbin's SOI, but it takes a very long time. I decided I'd try to capture a little A class that was dinking about, just ready to pass close enough to actually register as being 'orbiting' (Sort of) Kerbin.

And I can't figure out how to do it. I can figure how to intercept something (at least in theory) That's in a circular orbit, but this parabolic orbit defeats me. It even defeated mechjeb, which I turned to for help. Anyone got any advice?

[SnappingTurtle]

Launch into the same inclination as the asteroid. Then push your apoapsis out to the asteroid's periapsis. When you get there raise your periapsis until the period of your orbit is such that when you come back around you and the asteroid are in the same place.

[Tassyr]

Launching into the thing's inclination might be my problem, I think- it's "rotating" in the opposite direction of Kerbin.

[LameLefty]

Launching into the thing's inclination might be my problem, I think- it's "rotating" in the opposite direction of Kerbin.

It's not. I've captured multiple asteroids passing through Kerbin's SOI. Here's the trick: track it until it enters Kerbin's SOI. Once it's inside the SOI, launch into the plane of the object's orbit. During your first orbit around Kerbin, fine-tune and correct as necessary at the appropriate node (ascending or descending).

Now comes the tricky part (and do NOT trust MechJeb to do it; it just can't and not sure if Sarbian and the other current MJ developers consider it a priority). Plot a maneuver node manually and just play with it; add or remove prograde velocity, move it around your orbit around Kerbin, etc. It really helps to use a mod with a maneuver node fine-tuning editor where you can tweak by small amounts (down to even a tenth of a meter/sec) and move the planned maneuver node forward or backward in time by one second increments. Small differences in this first maneuver make a huge difference later. It's probably not possible to get a very close approach with the first burn but don't worry too much yet. Basically, play around until you get an intercept (marked by the two white markers on your orbit and the target orbit). Then tweak the node in very small amounts to get it as close as possible.

Remember this is not a one-step process; once you've got the maneuver node close, just go ahead and make the burn. Now for step two: while en route, plot a SECOND maneuver to fine-tune the approach. MJ actually works well for this part - tell MJ to fine tune your approach to 100m or so and it should work fine. Then do your approach, rendezvous and docking ("Claw-ing?") as usual.

Of course, once you've done that you're now effectively on the same hyperbolic trajectory as your target. So either have enough dV in reserve to capture the object into Kerbin orbit or be able to ket go of the asteroid and brake yourself if you intend to return.

Good luck!

Edited May 7, 2014 by LameLefty

[Kasuha]

Put your ship on launchpad.

Go to map view.

Select the asteroid as target.

Rotate map view until you see the asteroid's trajectory as a straight line crossing Kerbin. Not horizontal, but straight.

Zoom in to see Kerbin as big as possible, but not lose view of the asteroid's trajectory. It should stay straight.

Time warp until your ship on launchpad is on the line. There are at least two such points.

If the asteroid goes around Kerbin prograde, you need to launch eastwards, if it goes retrograde, you need to launch westwards. If the asteroid is not in the SOI yet you may have enough time to launch prograde and invert your orbit later but let's skip that for now.

Launch at one of the two points described above, and launch towards either target prograde or target retrograde, depending on which direction you need launch (east or west). Perform gravity turn in the direction to/away from target, that will put you on correctly inclined orbit.

When in orbit, put a maneuver to a spot opposite to the asteroid's location, pull prograde to make it highly elliptic and try to get a close encounter.

The rest is like if you're making rendezvous in orbit.

[cantab]

Yeah, launching into a retrograde orbit simply means launching west (or northwest or southwest etc)