# Effects On ISS Orbital Mechanics Resulting From Added Mass Of Dragon Capsule Visit

Does the ISS require any type of orbital mechanics management once the Dragon capsule was confirmed with a positive airlock seal, and the mass of the spacecraft, 2 additional astronauts, and supplies and cargo are added to the ISS's total mass?

Seems that this might aggravate whatever the stable orbital decay rate happens to be when nominal.

• You probably should reword to "What adjustments must the ISS take every time a capsule docks or undocks?" It's hardly just a Crew Dragon issue :-) . But to answer your last sentence: heck no! Since both the ISS and the capsule are moving with the same vector velocity, the fact that they connect is irrelevant. Jun 11, 2020 at 11:39
• Other than adding a tiny bit of drag I don't see how it would impact the station.
– GdD
Jun 11, 2020 at 13:41
• I was under the impression that once any mass is added to a given orbital mechanics equation, the outcome of that orbital track is going to occur. Jun 11, 2020 at 13:53
• Also the Shuttle was WAY bigger, massier, and had way more surface area to add drag when it was docked. Jun 11, 2020 at 15:42
• @BigNutz the trick there is that in the maneuvers before docking, the capsule matches the ISS orbit exactly. So it technically does not matter if they're connected or simply flying right next to each other. Jun 11, 2020 at 16:37

The orbital mechanics of satellites are independent from the mass of the satellite. As long as the sats mass is tiny compared to the mass of Earth.

The total mass of the ISS is much larger than the mass of the dragon capsule itself, the same is true for the volume and surface of both. So the atmospheric drag of both changes only very little after docking.

• Only mostly true. The decay rate depends on mass, altitude, and ballistic coefficient. Jun 11, 2020 at 14:42
• @JohnHoltz But the classic orbital mechanics do not depend on the atmospheric influence.
– Uwe
Jun 11, 2020 at 14:48
• @JohnHoltz what % increase in total ISS mass does a Dragon capsule represent? Jun 11, 2020 at 14:53
• @OrganicMarble and more important, what's the delta ballistic coefficient Jun 11, 2020 at 18:01
• The ISS is about 400,000 kg. Dragon is about 4000 kg (dry) plus up to 6000 kg of payload. That makes a loaded Dragon roughly 2.5% the mass of the ISS. Jun 11, 2020 at 19:40

It should make the ISS orbit decay a bit slower

Yes, it does add a bit more drag surface, thus increasing the total drag force on the ISS.
But it also increases the mass of the ISS, and does so by a bigger factor than the change in drag. (loaded capsules are rather dense, the ISS is very low average density due to huge air regions)
Thus the resulting deceleration of the ISS due to drag will be a bit less.

So yes, it does affect the need for orbit reboost of the ISS.
It very slightly decreases the reboost need.