I’m just watching the Iridium-6/GRACE-FO launch from SpaceX.

Before the launch they showed a video of the Iridium satellite network and in this mission they deployed 5 additional satellites into LEO.

They are at 500km altitude and moving at 27000km/h.

They deploy one satellite from the second stage and then wait 100 seconds for each subsequent satellite deployment.

But... surely each satellite is still moving at that release speed and in the same direction. So are all of these satellites going to be orbiting in a tiny group all nestled together? That doesn’t seem right...?

How does this deployment pattern end up with a spread out network of satellites?

I guess they don’t have enough propulsion on board to significantly alter their orbit? Or do they?

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    $\begingroup$ Similar questions here and here. $\endgroup$ – Chris May 22 '18 at 21:44
  • $\begingroup$ If you play KSP and want to do a fun mission, try to make a launch vehicle with 2 separate satellites at the tip of it. Once you hit ~500km orbit, detach one satellite, and immediately perform a retrograde burn to get its perapsis to 100km. Switch back to your "satellite deployer" and wait until you're at the exact opposite point and release the second one, also burning down to 100km on the perapsis. You'll see why this is done :). $\endgroup$ – Magic Octopus Urn Jul 23 '18 at 13:41

While significant orbital changes generally require a lot of propellant, it takes very little propulsion to move to a different relative position along the same orbital path. To advance ahead of your sibling satellite, you burn retrograde (counterintuitively) briefly, which lowers the opposite side of your orbit, making the orbital ellipse smaller and faster. One complete orbit after the burn, you're back where you made the burn, but you've gotten there faster than your sibling did. You then perform a burn opposite to the one you started with, that is, prograde, and you're back on the same path, just further ahead of where you would have been.

If you want to be further ahead still, you just wait several orbits before making the counter burn. You can make the initial burn arbitrarily small if you're willing to wait a long time to take your final relative position.

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    $\begingroup$ Nice explanation on the retrograde/prograde burns! Now I imagine satellites passing each other using "the inside line" as if they were on Daytona 500 :)) $\endgroup$ – BlueCoder May 23 '18 at 9:26
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    $\begingroup$ Thanks very much! Makes perfect sense :-) I actually saw a video from Scott Manley just the other day explaining the retrograde thrust to get ahead. $\endgroup$ – Fogmeister May 23 '18 at 11:46

Actually, the Iridium satellites don't stay in their insertion orbit, but travel to a higher altitude in several steps. I have posted an answer to a similar question here.


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