How is the lower orbit of each planet determined?

[tim233]

There is a mission to fly by the sun but i do not know how to achieve the most fuel savings

[James Kerman]

Welcome to the forum @tim233! 

Your question has been moved from KSP Discussion to gameplay questions.

[Leganeski]

If you need to do something while in "space just above the sun", then that means below an altitude of 1,000,000 km.

[king of nowhere]

6 hours ago, tim233 said:

There is a mission to fly by the sun but i do not know how to achieve the most fuel savings

if your mission is to achieve a "fly-by", then you only need to exit the sphere of influence of kerbin.

[darthgently]

6 hours ago, tim233 said:

There is a mission to fly by the sun but i do not know how to achieve the most fuel savings

After some trial and error, my method of getting to solar orbit and back with the least DV is that you want your solar orbit velocity to be close to Kerbin's solar orbit velocity upon exiting Kerbin's SOI.  This will minimize DV to return back into Kerbin's SOI. 

Don't forget to linger in Solar orbit long enough to satisfy the contract.  Usually 10 game seconds or so