# How does KSP simulate objects out of focus?

I've attempted to perform a controlled aerobraking maneuver against Kerbin upper atmosphere (>55km) when returning from a moon and decided to speed things up with fast-forwarding from the Tracking Station. And when I had my ship circle a few rotations in the 'aerobraking' orbit, I observed that the orbit did not change, despite it should've done so.

I figure that the game does not simulate objects far from player focus properly. Is there a description what and how far from player goes out of proper simulation? A few suspected examples that I've come up with:

• Eccentric sub-atmospheric orbits
• Objects on collision course with one another (interaction/destruction/passing through)
• Science stations in orbit and on land of distant bodies (science generation/energy consumption)

My KSP is the most latest current version - 1.0.4

KSP models the physics of all craft that are within 2.25km (thereabouts) of the currently active craft.

That means that if you fastforward from the tracking station, NO craft are getting physics calculations.

What happens to craft who get no physics calculations? They are calculated on rails. Meaning their orbit is assumed to be fixed, much like how the planetary orbits always work.

On rails calculation makes two exceptions:

• Suborbital trajectories (things falling to the ground) will destroy the object correctly.
• Orbits with a periapsis below an arbitrary "atmospheric height" will be assumed to aerobrake enough to become a suborbital trajectory and be destroyed.

My guess in your case would be that the 55,000m altitude you're at now is still higher than Kerbin's "atmospheric height" setting. Meaning there's not anough atmosphere for the on-rails system to assume your orbit is decaying.

Solution: Time warp while said vessel is your active vessel. Then proper physics will be calculated.

Edit
I can't say for sure, but my guess would put Kerbin's atmospheric height at 30 or 40km, given how that's where aerobraking is enough to kill most orbits in a single "dip" into the atmosphere.

Edit 2
As commented by CyanAngel, the deletion of a craft (when outside of physics) occurs when a craft gets below 23km altitude on Kerbin.

• Thanks. This explains a lot. I suppose science labs won't consume electricity, but will make science? And maybe you could advise what will happen to on-collision course crafts? Commented Sep 7, 2015 at 10:49
• Collisions are physics, which don't get modeled during on-rails. Picture on-rails as just an unchanged orbital path. I'm not sure about science lab generation, but I figure this is a time-driven gain, not a physics-driven one. I do believe power usage is modeled somewhat, as, I used to have RT2 and had satellites going down due to being eclipsed by a planet. That could've been RT2 specific, though. Commented Sep 7, 2015 at 11:00
• Kerbin's atmosphere height is 68km. If you're after the vessel deletion threshold it's when they pass under 23km on Kerbin when outside of physics range. Otherwise spot on answer. Commented Sep 8, 2015 at 10:48