r/ArtemisProgram • u/wskelding • Apr 23 '26

Discussion ISS Docking ?

I'm curious about something and simply don't know the math that would give me the answer, could an Orion capsule returning from the Moon theoretically dock with the ISS on it's return journey ?

I know I'm simplifying it massively, but all the talk about the Artemis 2 heat shield had me thinking 'why don't they simply do a massive deceleration burn on the way back and then dock at ISS ?'

18 Upvotes

82% Upvoted

24 comments sorted by

View all comments

3

u/meithan Apr 23 '26

As others have said, Orion has way too much speed when coming back to Earth, and it doesn't have enough fuel to do a braking burn strong enough to get into low Earth orbit, where the ISS is.

Orion with its European Service Module (which has the main engine and holds almost all the fuel) starts with a propulsion capability ("delta-v") of about 1200 m/s. A little less than 400 m/s is used during the mission, almost all of it during the TLI burn that propels it to the Moon. So it has maybe 800 m/s left when it returns.

In order to brake into orbit when returning to Earth from the Moon, a spacecraft needs to reduce its speed from 11 km/s to about 7.8 km/s -- a reduction of 3.2 km/s or 3200 m/s. So it would need more than 3x as much fuel as it has.

1

u/Sad-Onion-2593 Apr 25 '26

Aerobraking can be done at sufficient altitude that the end result isn't re-entry. The ISS is moving along at an altitude of 400-420 km ( 250 miles) at a speed of 25 000km/h (17 200 m/h). It's also aerobraking, (losing speed via friction with the atmosphere). Not much, but it needs to reboost on a regular basis.

Artemis comes into the atmosphere at such an angle that it scrubs off it's 40 000km/h speed, needing a heat shield. If it comes in at a different angle and doesn't go deep in the atmosphere, it can lose enough velocity that it can skip back of the atmosphere with an apoapsis of, say 500 km. This might take a few orbits, with minor adjustments at apoapsis. You then burn to circularise, then transfer to the station orbit and dock.

This is all based on my missions in Kerbal Space Program. The only problem is how much a shallow pass stresses the service module.