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If viewed from the North Pole, the Earth spins counter-clockwise.

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Because of this, many satellites also orbit in the same direction as you can take advantage of the Earth's rotation and essentially receive a "boost".

However, what if the Earth spun in the opposite direction - clockwise? How would that affect space travel and exploration? I figure it won't change much for LEO as you can just simply point your rocket the other way. But what about for interplanetary spacecraft? For example, a spacecraft injecting into a Hohmann Transfer to Mars will execute a pro-grade burn above the side of the Earth that's facing away from the Sun (i.e. night side).

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But a spacecraft that's in LEO orbiting clockwise can't do that, because the Earth is revolving counter-clockwise around the Sun. Therefore the spacecraft first has to cancel the 30 km/s of velocity of the Earth's revolution, and then enter a clockwise Hohmann orbit.

Another way is that the spacecraft would just ignore the Earth's spin and just rough it out and enter a counter-clockwise orbit around Earth. This seems unlikely because in reality, it takes about 9.4 km/s of delta-v to enter LEO (counter-clockwise direction) and delta-v and fuel is very precious, especially in interplanetary missions.

A likely possibility is that spacecraft would instead execute their Hohmann injection burn on the day side of Earth. But I'm not sure of all the side effects that would create (perhaps a mini-side question).

Question: If the Earth spun clockwise, how would that affect Mankind's efforts of Space Exploration? What Space Exploration missions would have not been possible if the Earth spun clockwise?

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    $\begingroup$ Argentine will launch first satellite and send first human being to space but Chile will beat them on the way to the moon. SpaceX would probably be an Australian company. $\endgroup$ – user3528438 Apr 29 at 15:16
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    $\begingroup$ uhoh, Uwe: KSC would still be in Florida, but on its west (Gulf of Mexico) coast. $\endgroup$ – DrSheldon Apr 29 at 19:01
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    $\begingroup$ On the scale of an interplanetary Hohmann transfer, the difference between one one side of Earth or the other (since your orbit would be reversed), is reeeeaaaally tiny. Are you asking for any difference, or reasonably significant differences? $\endgroup$ – Flater Apr 30 at 10:43
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    $\begingroup$ Given the origin of the term "clockwise", it's likely that if the earth spun clockwise, it would be spinning "counterclockwise", as clocks would go the opposite direction as well. $\endgroup$ – Tristan Apr 30 at 15:35
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    $\begingroup$ The earth does spin clockwise. You're just looking at it from the wrong end. $\endgroup$ – candied_orange May 1 at 5:13
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Primarily, locations of spaceports would change. California, not Florida would host the NASA's main launch site. Russia would be in slightly better position, able to send rockets over the Black Sea, nicer inclinations than currently available from Baikonur - although Vostochny wouldn't happen or would be closer to Chita. ESA could forget about French Guiana, likely with a spaceport somewhere in Portugal. Israel would be quite happy, not needing to change anything but getting several hundred m/s of delta-V for free.

As for spaceflight, only the Moon and Earth-Moon Lagrangian points would get harder as a destination. In this case Apollo 11 and other lunar missions would indeed need the extra delta-V to launch into retrograde orbit and "tough it out".

LEO, GEO as well as Mars all the other planets would be no less accessible - you missed the point with 30km/s. Earth still circles the Sun in the same direction at 30km/s, independently of its axial spin, so LEO satellites circling Earth at 8km/s still move at 38km/s relative to the Sun when on one side of Earth, and at 22km/s when on the opposite, the sides just flip, currently the "fast" is the night side, and "slow" is the day side. Now they'd perform the departure burn for Mars and outer planets from the point closest to the Sun, and for Venus, Mercury and solar fly-bys, from the farthest point, crossing Earth orbit on their way in either direction, and changing their perihelion (for Mars Hohmann transfer) or aphelion (for Venus) by about 13,000-14,000km versus what it is currently.

And that means 13,000km (Earth diameter plus LEO altifude) modification on an orbit of 1AU apsis. Change of the opposite apsis by 14 thousand km while on apsis of 150 million km, costs a sneeze of RCS thrusters.

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    $\begingroup$ @DrSheldon and risk abort into Mexico? $\endgroup$ – SF. Apr 29 at 19:29
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    $\begingroup$ NASA HQ is in Washington, DC, and its location is not dependent on the rotation of the Earth, but on the center of gravity of the federal government. en.wikipedia.org/wiki/NASA_Headquarters $\endgroup$ – Organic Marble Apr 29 at 20:44
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    $\begingroup$ @Mazura 1: Israel currently launches rockets west, into retrograde orbits, so that they don't shoot them over countries that are hostile to them. This direction adds Earth rotation speed to delta-V required. If Earth was spinning in the opposite direction, the value would be subtracted. 2. Lagrangian points would travel in retrograde direction relative to Earth spin, and require the extra delta-V to reach, same as the Moon. 3. On the night side, in sun's frame of reference. See this. $\endgroup$ – SF. Apr 30 at 0:48
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    $\begingroup$ @OrganicMarble If the Earth spun the other way, Europe's climate would be very different, likely a lot more hostile, European countries would not have become superpowers colonising the world, the USA would be speaking Chinese with its capital at the west coast, as would the headquarters of NASA. I've not considered if plate tectonics would be impacted (probably not). But now we're in full Worldbuilding territory :) $\endgroup$ – gerrit Apr 30 at 9:01
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    $\begingroup$ @gerrit it is unlikely the evolution of life would have taken the same course; the scenario is pure fantasy as stated. $\endgroup$ – Organic Marble Apr 30 at 11:21
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Deep space missions would not change much. The spacecraft needs energy to climb out of the gravity well. It starts with kinetic energy (speed) but as it climbs higher it's exchanged for potential energy (height). The higher the orbit, the slower is motion of spacecraft relatively to the Earth. A LEO orbit needs about 7.9 km/s but at a GEO orbit satellite is moving at less than 3.1 km/s. No matter how you started (clockwise or counterclockwise) as you climb higher that speed would be converted in height that doesn't depend on direction of initial velocity. Leaving Earth gravity well completely means converting all the velocity to height [assuming that there was just enough energy to leave].

There still will be some (relatively minor) effects. For example you don't normally start a deep space mission with a kinetic energy that is just enough to get out of gravity well. Generally you'll add some extra delta-v to keep moving somewhere else once you leave Earth sphere of influence. You can add this energy in two stages, going to high orbit first and then starting engine again to start moving to other planet, but this is more complex and less efficient due to Oberth effect, so you will lose several hundreds of m/s delta-v. Plus it'll take too much time and space to climb out of Earth gravity well completely, so there will be some small, but non-zero leftover speed to cancel. Overall that would make deep space missions a bit more difficult, but would not lead to dramatic changes.

Then, there's actually no need to launch spacecrafts in prograde orbits. You can equally well launch these in retrograde or polar orbits. This would cost few hundreds of extra m/s delta-v, but you'll be spared from the need to attain 8 km/s speed just to revert it once you are in orbit. That trick would be likely used for Moon missions because unlike other planets it orbits within Earth gravity well at a speed of ~1 km/s and we'd like to match it.

Finally, I think that scenario that you were actually thinking about is an Earth itself moving in retrograde orbit (in opposite direction from other planets). That certainly would make space exploration of different planets a whole lot more difficult. But fortunately there's no way for Earth to move in retrograde orbit in all theories of planet system formation and I'm not even sure that such an orbit would be stable long-term.

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For example, a spacecraft injecting into a Hohmann Transfer to Mars will execute a pro-grade burn above the side of the Earth that's facing away from the Sun (i.e. night side). But a spacecraft that's in LEO orbiting clockwise can't do that, because the Earth is revolving counter-clockwise around the Sun.

Of course it can. It just needs to execute the prograde burn when it's on the side of the Earth that's facing towards the Sun, when its velocity relative to the Earth points in the same direction as the Earth's velocity relative to the Sun.

(Actually, for an optimal ejection, the prograde burn should occur a little before the point where the velocity vectors are aligned, since the spacecraft's path will continue to curve somewhat as it climbs out of the Earth's gravity well. The ejection burn will put the spacecraft on a hyperbolic trajectory relative to the Earth, and optimally the forward asymptote of this hyperbola should align with the Earth's velocity relative to the Sun. But in any case, all this is true regardless of whether the initial Earth orbit you're starting from is clockwise or counterclockwise. It even works the same with inclined or polar orbits, although in that case you'll also want to ensure that the axis of the initial orbit is perpendicular to the direction of the Earth's velocity around the Sun.)

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