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The Channel News Asia article New Zealand launches into space race with 3D-printed rocket says:

Ships and planes need re-routing every time a rocket is launched, which limits opportunities in crowded U.S. skies, but New Zealand, a country of 4 million people in the South Pacific, has only Antarctica to its south. The country is also well-positioned to send satellites bound for a north-to-south orbit around the poles.

Why is Rocket Lab's New Zealand launch site good for polar orbits?

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    $\begingroup$ Not mentioned in the one answer so far: New Zealand is an island far removed from other land. The only bad launch angles from a range safety perspective are those that would take the launch vehicle over New Zealand itself. $\endgroup$ – David Hammen May 25 '17 at 12:04
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    $\begingroup$ I wonder what a north-to-south orbit around the poles means. Aren't all polar orbits both north-to-south and south-to-north at some point? $\endgroup$ – Steve May 25 '17 at 20:32
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    $\begingroup$ @Steve You've identified what it is that was probably bothering me about the wording, thank you! I've asked the follow-up question Is there a significant difference between launching a polar orbit mission south from New Zealand versus launching it north? $\endgroup$ – uhoh May 25 '17 at 22:25
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Launch sites far from the equator have to cancel out less of the Earth's rotation to get to a polar orbit, the Rocket Lab Launch Complex 1 at Mahia Peninsula is at 39 degrees south giving it an extra ~361 m/s, compared to ~465 m/s at the equator. This is ~104 m/s less velocity to cancel out compared to a launch site like the EAS launch site in Kourou, French Guiana. The fact that there is no one living north and south of the launch site means that they could launch both north and south to reach a polar orbit. The empty ocean to the east also means that they can target a large range of orbits without flying over inhabited land.

There are also convenient islands further north in the Pacific to place ground stations on to control the launcher on ascent and place weather stations on etc (mentioned in passing in this article on Electrons first test flight). I was not able to find a reference to which islands Rocket Labs used but island nations like Fiji and Samoa are quite close to a likely flight path.

New Zealand being a developed nation with good infrastructure and no imminent territorial conflicts is also a plus. This fact makes it easier to accommodate the people working on the project as well as easing the movement of materiel needed for the operation of the launch range.

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  • $\begingroup$ I'm curious what the latitude of the launch site is, and how much of an effect this makes on the Earth's rotation velocity. Which islands are you thinking of that could be the potential sites of ground stations for polar orbits? $\endgroup$ – uhoh May 25 '17 at 7:21
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    $\begingroup$ @uhoh: the delta-V benefit for polar orbits goes up as $(1-cos(latitude))*464m/s $. The rest is up to local environmental, economical, social and political factors. $\endgroup$ – SF. May 25 '17 at 9:17
  • $\begingroup$ @SF. I know. I'd like to see the author include it as part of the answer - this is their first answer here in SXSE so I'm adding a little helpful coaching. $\endgroup$ – uhoh May 25 '17 at 10:32
  • $\begingroup$ @lijat I'm curious what the latitude of the launch site is, and how much of an effect this makes on the Earth's rotation velocity. Do you think you could add something about that to your question? Also, which islands are you thinking of that could be the potential sites of ground stations for polar orbits? $\endgroup$ – uhoh May 25 '17 at 10:46
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    $\begingroup$ Thanks for the feedback, I will try to update the answer with them later tonight. $\endgroup$ – lijat May 25 '17 at 13:15

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