Furthest distance between two planets?

[Taki117]

I'm planning a Jool tour, (Not quite a Grand tour, don't plan on landing on/returning from Laythe or Tylo) and was planning on putting a communications network in orbit around Jool (Thanks to RemoteTech) but the dishes that I know are going to work are far too large for the satellite I am designing.

This leads to the question how do I determine the farthest distance between two planets? Is it a simple matter of subtracting ones Apoapsis from the others Periapsis, or is it more complicated than that?

Edited January 30, 2014 by Taki117

[Bobe]

You can determine the largest distance between any two bodies orbiting a common body by combining their semi-major axes. The semi-major axis is the greatest radius in a body's orbit.

However, it's a bit more complicated to determine when the two bodies will come into alignment to achieve that distance.

[Taki117]

You can determine the largest distance between any two bodies orbiting a common body by combining their semi-major axes. The semi-major axis is the greatest radius in a body's orbit.

However, it's a bit more complicated to determine when the two bodies will come into alignment to achieve that distance.

I don't really care when it will happen, I just need to know if the maximum distance between Kerbin and Jool is greater than the range of the next smallest dish antenna (I know one will do the job, but I want to know if the smaller one will also do the same job)

[Bobe]

I don't really care when it will happen, I just need to know if the maximum distance between Kerbin and Jool is greater than the range of the next smallest dish antenna (I know one will do the job, but I want to know if the smaller one will also do the same job)

I was mistaken, you don't use the semi-major axis, you indeed just add Jool's apoapsis to Kerbin's apoapsis, which comes out to be 85,812,078 km.

[Taki117]

I was mistaken, you don't use the semi-major axis, you indeed just add Jool's apoapsis to Kerbin's apoapsis, which comes out to be 85,812,078 km.

Don't you mean Subtracting Jools Apoapsis from Kerbins Periapsis? Making it 58,612,398,131m?

[Bobe]

Don't you mean Subtracting Jools Apoapsis from Kerbins Periapsis? Making it 58,612,398,131m?

The greatest possible distance between Jool and Kerbin is the result of combining each planet's greatest distance from Kerbol. Since Kerbin's orbit is perfectly circular, we can assume a point where Jool is at its apoapsis and Kerbin is on the opposite side of Kerbol. At this point, the distance between Jool and Kerbin is Jool's apoapsis plus Kerbin's apoapsis.

[wasmic]

Don't you mean Subtracting Jools Apoapsis from Kerbins Periapsis? Making it 58,612,398,131m?

No, you'll have to add the apoapses. In a case where Kerbin and Jool are both on their apoapses and direcly opposite each other in their orbit of Kerbol, the distance would be Kerbin's apoapsis + Kerbol's diameter + Jool's apoapsis.

EDIT: Ninja'd.

[Taki117]

The greatest possible distance between Jool and Kerbin is the result of combining each planet's greatest distance from Kerbol. Since Kerbin's orbit is perfectly circular, we can assume a point where Jool is at its apoapsis and Kerbin is on the opposite side of Kerbol. At this point, the distance between Jool and Kerbin is Jool's apoapsis plus Kerbin's apoapsis.[/quote

Ah, ok. This makes sense then. In which case it looks like I'm going to have to use the larger dish anyway, thank you all!

[SRV Ron]

There will be a period of several days when communication will be cut off by the sun at the point of maximum separation.

[KerbMav]

There will be a period of several days when communication will be cut off by the sun at the point of maximum separation.

Reason enough to get a comm-relay in orbit around every planet with enough dishes to target each other - plus 1 or 2 for the future? - and shorter ranged/wide-area dishes to target the orbited planet and its moons.

[Kasuha]

I would just put two satellites in Sun orbit with the same orbital period as Kerbin about the same trajectory, one ahead and one behind Kerbin. If you want to keep it looking realistic, place them where L4 and L5 points would be. Then you'll always have visibility to all planets.