# Why do we launch rockets during the day?

The clips that I have seen of rockets launching all seem to be carried out during daytime. However, we learnt at school that rockets are fired closer to the equator and towards the east to take maximum advantage of Earth's rotational motion, getting a boost in speed of roughly $460m/s$ at the equator. If we would go to such lengths to gain a boost of speed of $460m/s$, why don't we take advantage of the fact that the earth orbits the sun at the massive speed of $30km/s$ to get an extra boost in velocity relative to other stellar objects? But to take advantage of both the rotational speed of the earth about its own axis and the rotational speed of the earth about the sun the rocket would have to be launched at nighttime:

as the only times when the boost from the purple rotation was in line with the boost from the red rotation is during the night-time. It seems that this would greatly reduce the fuel needed to reach distant objects (for instance Pluto: New Horizons seems to have launched during the daytime).

• Not really relevant, given Fibonatic's fantastic answer, but some missions did launch at night, notably Apollo 17. But this was a short term scheduling decision so as to be on the Moon at a certain time: I've never understood why. – Selene Routley Sep 8 '15 at 13:37
• There have been many nighttime launches over the years of both manned and unmanned missions. This page lists only the night time launches of the Space Shuttle: home.comcast.net/~username54321/starfield/nightlaunches.html – Todd Wilcox Sep 8 '15 at 13:45
• You get a 30km/s boost regardless of where you launch from. Imagine we launch a rocket during the day (so it's going at 29.54km/s relative to the sun) - if we wait for half an orbit, it'll be on the other side of the Earth, and will be going 30.46 km/s relative to the sun like you want. – user253751 Sep 9 '15 at 3:20

When launching into a low Earth orbit only your velocity relative to the Earth matters, as seen from the not-rotating reference frame of the Earth. Your velocity relative to the sun does not matter, because once you are in the orbit your velocity vector relative to the Earth will oscillate between pointing towards and away from the velocity vector of the Earth relative to the sun.

When performing an interplanetary transfer the Earth's velocity does matter. Usually such transfer is performed when in low Earth orbit. So if you want to travel to space outside Earth's orbit, then you want to leave Earth's "gravity" in the same direction as its velocity relative to the sun, also called prograde. But because the Earth will also slightly curve your escape trajectory you will have to burn while near trailing side of the Earth (where the sun is setting) such that you pass behind Earth's night side. The opposite is true when you want to go to space inside Earth's orbit.

• I think in your first sentence you forgot a. – doppelgreener Sep 8 '15 at 13:19
• I don't think performing an interplanetary transfer matters either. Spacecraft typically orbit several times before doing the escape burn, so it doesn't matter when the launch happens as long as it's prograde. – ithisa Sep 8 '15 at 15:08
• @user54609 I meant that it matters where you perform the transfer burn, when you are already in orbit. I thought that I implied that in my answer, if not then I will try to improve on that. – fibonatic Sep 8 '15 at 15:14
• This is quite right. I will just add that in some cases, like the New Horizons probe launch to Pluto, there was no low earth orbit, just a direct shot out of earth's sphere of influence. This was done for efficiency, and would require launching at the correct part of Earth's day. From Wikipedia: "[New horizons] is also the first spacecraft launched directly into a solar escape trajectory". en.wikipedia.org/wiki/New_Horizons#Launch – Dan Sep 8 '15 at 18:19
• As always, you can try this yourself in Kerbal Space Program. :) – Robin Ekman Sep 9 '15 at 8:55

Speculatively building on the answer given by @fibonatic here:

If you were instantly "far from earth", then your argument for launching at night makes sense for long distance travel. But if you consider that you are orbiting Earth for a short while as you gain altitude, you actually "sling shot" around the earth by starting on the day side - the gravitational pull from the earth accelerates you in the direction you ultimately want to go, and you actually end up overtaking the earth:

This means that the correct place to launch will be determined not just by your instantaneous velocity at the point of lift-off, but also by the rate of climb that you can sustain.

For missions beyond Earth orbit, it's very common to launch into a parking orbit around Earth before translunar or interplanetary injection.

Therefore, the time of launch doesn't constrain the departure angle from Earth; you can get any departure angle you want by waiting no more than 90 minutes. This allows more flexibility in launch time even if the window for the injection burn is very short.

That said, night launches aren't that uncommon for e.g. direct launches to geosynchronous orbit, although daytime launches seem generally preferred for practical, ground-logistics reasons.