Conditions for "Fly by"

[Ocanada]

Flying - apoapsis in the atmosphere, periapsis in the atmosphere

Subobrital - apoapsis in the space, periapsis in the atmosphere

Orbital - apoapsis in the space, periapsis in the space (may something else?)

What about fly by

Edited October 27, 2017 by Ocanada

[Pand5461]

Periapsis in space, eccentricity > 1 (maybe apoapsis beyond SOI limit would also suffice).

[MajorMushroom]

Basically, fly near it (close enough to get an encounter showing up on map) so that you enter the objects sphere of influence, however the apoapsis has to be out of its SOI

Apoapsis out of SOI Periapsis inside SOI

[Zeiss Ikon]

Worth noting that for contract fulfillment purposes, you'll automatically fulfill a flyby requirement if you go into an orbit around the designated body.  So, if you have a tourist who wants to Flyby the Mun, and also Orbit around the Mun, if you keep your contract status display open as you warp outward toward Mun, you'll see the Flyby clause get the green check (seemingly, as soon as you enter SOI), even if you subsequently brake into an orbit (and no longer have an escape trajectory from the body's SOI).

Flyby, for contracts, seems to just mean enter the SOI.  On my very first flyby contract, there was a fulfillment hint: "To fulfill this, just fly anywhere near the Mun."  Sounds suspiciously like entering the Mun's SOI to me.

[OhioBob]

I would say a flyby is anything that enters a body's SOI, passes through periapsis, and the exits the SOI.  The vessel's trajectory in relation to the celestial body is hyperbolic, meaning it has an eccentricity >1, a semimajor axis <0, and no apoapsis.  Only a closed elliptical orbit can have an apoapsis; a hyperbolic orbit is an open or unbounded orbit.  You might see an apoapsis outside the SOI, but this is the apoapsis of the vessel's orbit around the sun (or around the planet if we're flying by a moon).  When inside the SOI we see the hyperbolic geocentric orbit, and when outside we see the elliptical heliocentric orbit.

Because of the way KSP does patched conics, there could be the unlikely scenario in which a vessel passes through a body's sphere of influence while not actually being on a hyperbolic trajectory.  That is, it could have an eccentricity <1 with an apoapsis so far away that it's outside the SOI.  However, for any kind of normal interplanetary encounter, the vessel will be carrying enough excess velocity that the chances of this happening is nil.

As far as contracts are concerned, I think we just need to enter a body's sphere of influence.
 

Edited October 28, 2017 by OhioBob

[Kryxal]

As the game defines things for contracts, flyby simply means your periapsis is sufficiently high.  To get a flyby of Kerbol, you only need to leave Kerbin's SoI.

Edited October 27, 2017 by Kryxal