9
$\begingroup$

Almost all satellites in LEO have been launched to the east, to take advantage of the Earth's surface rotational speed of about 5% of required orbital speed (Israel being a small exception for geopolitical reasons). Vandenberg and Satish Dhawan in India launch polar satellites to the south out of geographical reasons. But from Kodak and Plesetsk they are launched to the North instead. I don't know about inland Baikonur and Chinese space ports.

Does it matter substantially for the space debris risk that polar orbits are opposite? Since it results in an impact speed of about 18 km/s, which should be much higher than two roughly equatorial prograde objects colliding.

$\endgroup$
  • 2
    $\begingroup$ The proposed Shuttle flights from Vandenberg would have done the same; this led to some interesting down-range abort landing sites like Easter Island. $\endgroup$ – Organic Marble Oct 18 '16 at 17:17
  • 5
    $\begingroup$ here is a though: launch a satelite to the North. wait about 12 hours. Launch another satelite to the North. $\endgroup$ – njzk2 Oct 18 '16 at 20:48
  • 1
    $\begingroup$ I'm sure geopolitical reasons are more important than physics ones at this point, but it seems to me that you'd want to launch towards the nearest pole (North or South) so you have the least amount of the sideways (towards the East) momentum to cancel out. What do I know though, I'm basing this off Kerbal Space Program. $\endgroup$ – Cody Oct 18 '16 at 23:28
  • $\begingroup$ You've got the same initial momentum either direction, surely? $\endgroup$ – Russell Borogove Oct 18 '16 at 23:37
  • 3
    $\begingroup$ If by "Kodak" you mean Kodiak, launches from there are not to the north. Launch azimuth from Kodiak can range from 110 degrees (east south east) to 220 degrees (roughly southwest). $\endgroup$ – David Hammen Oct 19 '16 at 12:57
24
$\begingroup$

There's no real distinction once you're in orbit; a northbound LEO orbit at a given longitude at a given time is a southbound orbit at nearly the opposite longitude 45 minutes later. The rotation of the Earth brings any chosen launch site under the ground track of every polar-orbit satellite, in both directions, daily.

Therefore, the choice of northerly or southerly launch usually comes down to geopolitical considerations, as noted. For the early part of ascent, over-ocean trajectories are better than over-land; flying over unpopulated areas is better than over populated ones; flying over ally nations is better than flying over rivals.

I believe Baikonur polar launches are to the north for these reasons.

I don't know whether more current polar satellites were originally launched to north or south, but as noted, once in orbit that distinction is not important.

$\endgroup$
  • $\begingroup$ What about the space debris from a frontal collision of two polar satellites in opposite directons? Would it be better if they all were launched in the same direction? $\endgroup$ – LocalFluff Oct 19 '16 at 5:37
  • 2
    $\begingroup$ @LocalFluff One, space is big. Even similar orbits still have plenty of space to avoid each other. Two, their trajectories are planned and synchronized with all the other satellites, so you don't really get even close calls. Three, the satellites have active propulsion - if they end up on a potentially dangerous trajectory, you just nudge them a tiny bit, and they will miss each other at large distance. And have a look at what polar satellites are used for :) $\endgroup$ – Luaan Oct 19 '16 at 8:34
  • $\begingroup$ @Luaan They are used for spy satellites, which might not have published orbits, do sudden trajectory changes and maybe even have stealth technology against radar detection. $\endgroup$ – LocalFluff Oct 19 '16 at 8:41
  • 3
    $\begingroup$ What do you mean by "same direction"? All inclined satellites alternate northbound legs with southbound. The only way to have them all going in the same direction would be to fly them all in the same plane, which would greatly increase the chances of collision. $\endgroup$ – Russell Borogove Oct 19 '16 at 9:32
  • $\begingroup$ I'm wondering about high-inclination sun-synchronous orbits, wouldn't one of them precess in the wrong direction? $\endgroup$ – uhoh Oct 19 '16 at 10:53
2
$\begingroup$

Once in orbit, they are the same effectively. Once a satellite is in orbit, it essentially stays fixed while the Earth rotates. When a satellite is heading north on one hemisphere, on the other hemisphere it will be going south. Wait about 12 hours, and the same satellite would then be going south over your location, while a point on the opposite side of the world it will be going north. (The exact timing depends on the inclination of the orbit)

Bottom line, the difference between launching north and launching south for a polar orbit is only the time of the launch, which will differ by about 12 hours.

$\endgroup$
  • $\begingroup$ Wait 12 hours, and the same satellite would then be going south over your location, that would only be true for sun-synchronous satellites, no? $\endgroup$ – gerrit Oct 20 '16 at 15:55
  • $\begingroup$ Exactly 12 hours, yes. Still, the same principal applies regardless. $\endgroup$ – PearsonArtPhoto Oct 20 '16 at 16:02
  • 1
    $\begingroup$ Actually, I think exactly 12 hours would only be true for a sun-synchronous satellite at the equator. $\endgroup$ – gerrit Oct 20 '16 at 16:10

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.