# Which parameter causes two launch times in a launch day?

Launch happens when trajectory plane contains launch site, launch asymptote and center of Earth. On a launch day there will be two launch opportunities, one at morning and other at the night. Which launch parameter makes these two launch time separate?

• Welcome to Space Exploration! Could you please edit your question to be clearer what you're asking, perhaps adding some reference and/or example to it? Are you referring to duration of a launch window, or what factors can affect its frequency? – TildalWave Mar 10 '14 at 12:18
• Launch opportunities are not always in the mornings or the nights. Some launch windows do not include neither morning, nor the night (the launch windows can start and end during the same afternoon). See here some examples: history.nasa.gov/afj/launchwindow/lw1.html Could you please elaborate in more details what do you mean by the morning and night opportunities? – mpv Mar 10 '14 at 14:46
• I suspect this is about launches that have to rendezvous with an already orbiting object, such as the ISS. In that case you have 2 launch opportunities per day, 12 hours apart. – Hobbes Mar 10 '14 at 16:10
• @Hobbes - I suspect it's the other way around, that is when you aren't worried about some other object such as the ISS, the Moon, Mars, or beyond. If all you care about is the orbital plane then there are more or less two opportunities per day. As soon as you worry about reaching some other body, there might be long stretches where there are no cost-effective launch opportunities. – David Hammen Mar 10 '14 at 17:56
• That's true for the Moon etc, but as far as I can work out, not for LEO objects. For LEO, you just need to match the inclination and the longitude of the ascending node, and the longitude of the ascending node gives you 2 launch opportunities per day: Earth rotates underneath the orbiting object, so any given longitude on Earth crosses the orbital path twice a day. – Hobbes Mar 10 '14 at 18:14

## 1 Answer

There will be roughly two opportunities per day if

• The launch site's latitude is less than the orbital inclination,
• The launch is directly into the desired orbit, with orbit insertion happening when the launch vehicle reaches the desired altitude (i.e., no transfer orbit), and
• All you care about is being on the right orbital plane at the right altitude but not where the vehicle is in the orbit.

The orbital plane is roughly fixed in inertial space. (Note: This ignores precession due to the non-spherical nature of the Earth and due to external bodies. That's why I said "roughly.") The Earth's slightly less than 24 hour rotation rate (the sidereal day) means that the launch site will switch from being on one side of the orbital plane to the other side twice every sidereal day. Add in precession and this will change somewhat, but it's still roughly twice per day.

That last condition, all you care about is being on the right orbital plane at the right altitude but not where the vehicle is in the orbit, usually does not apply. Space vehicle operators tend to care a lot about where the vehicle will be on the orbital plane. For example, it's rather important if the vehicle is to rendezvous with some other object, if it is to be in a particular sun-synchronous orbit with a particular local sub-satellite point time, or if it is to be in just the right spot at geosynchronous altitude.

TL;DR version: Which parameter causes two launch times in a launch day? The answer is the Earth's ~24 hour rotation rate.

• I'm wondering if countries that care about their inhabitants will try not to launch over land, and so for more polar-ish orbits they may not have access to both the ascending and descending halves of the orbit? If so, wouldn't geography be a factor as well? – uhoh Apr 8 '19 at 3:08