This will be super basic, but necessary for all of our horsing around in space.

First and foremost: Space is huge. 

• There is enough room between Earth and the Moon to fit every major solar system body. 
  
  
  And that's just between us and our moon. Space is huge, even the "small" distances. I won't go into how long it would take us to get to different places because our heads can't quite wrap around those speeds or those distances, but I will give a comparison that we could talk about: Lightyears. A Lightyear is a distance - it's exactly as far as a photon can travel in one year. Photons travel at 3x10⁸ m/s - that's 300,000,000 meters every second. For comparison, a meter is roughly the length from the tip of your nose to the tip of your middle finger with your arm stretched as far as it can go to the side. A standard room measures less than 3 meters high. Using the picture above could help us try to figure out how fast that actually is.
  
  The New Horizons craft made it to the moon in the quickest time to date - travelling at 58,000 km/hr ((that's 16,111 m/s)) it took only 8 hours, 35 minutes to get to the moon. That's awesome - manned missions take days longer because they need to slow down before they get there, unlike Horizons who just kept on going (towards Pluto). However... that impressive speed is nothing compared to the speed of light - it's only 0.00005% light speed. Earth's fastest craft took 8hr35min to get to the moon. Light takes 1.3 seconds to cover that same distance. That's 1.3 seconds to travel past the bulk of all major solar system bodies, like you see in the picture above. That's insane.
  
  And that is a relatively short distance there. 1.3 s to get from Earth to Moon... what about other places in our solar system?
  
  It takes light 8.3 minutes (around 499 seconds) to get from the surface of the sun to Earth.
  It takes light 43.2 minutes to get from the surface of the sun to Jupiter, which is 5x the distance from the sun to Earth.
  It takes light 4.1 hours to get from Sun to Neptune, which is 30x the distance from sun to Earth.
  
  This is all still within our solar system. What about to the closest star system outside our own? 
  
  The closest star to the Sun (Sol) is Proxima Centauri, and it takes four years for light to travel from the surface of our sun to the star Proxima Centauri. Four. Years. And that's traveling at the maximum speed allowable (as light makes its medium as it travels, it cannot be slowed down, as other objects forced to travel through other mediums must). Taking that back into perspective, if our Sun were a large grapefruit sitting in the middle of Washington DC, Proxima Centauri would be a cherry (hey, it's little!) sitting in California. Earth, btw, would be the size of the head of a pin, sitting only 50 ft from the grapefruit sun. 
  
  So space is huge, and it's mostly empty. 
  Want to try something fun and interactive? Play around with this: a scaled model of just our solar system, where the moon is just one pixel on your screen: Pixel Map of the Solar System. The creator went through and plotted the planets, proper size and scale, on there, and it can really help drive home the idea that space is huge and mostly empty, lol.
  

What's makes up Space? 

• The Universe
  The Universe is everything. Everything that ever was an will be. It's infinitely large (we can't prove otherwise) and super old - it's like the whole container for all of this stuff. 
  When someone uses the words "Another Universe" or "Alternate Universe", what they typically mean is an alternate timeline, or an alternate reality - not the next grouping of stars to the left. 
  
  
  
  
• Galaxies
  A Galaxy is an extremely large grouping of Stars.
  We are part of the Milky Way Galaxy; our sun is one of over 200-400 billion stars that make up the Milky Way. In size, the Milky Way is between 100,000 and 180,000 light years - it would take that many years for light to travel from one side to the other. It's huge. The Sun is located about 30,000 light years from the Galactic Center, and it takes the sun about 240 million years to make it once around the Galactic Center. *In Sailor Moon, the Galactic Center is where the Galaxy Cauldron is located; in Sailor Moon Stars, every senshi mentioned, every planet, every setting and every story came from within the Milky Way galaxy. It's so large that she did not need to include any other Galaxies, as, in our galaxy alone, there are hundreds of billions of stars, each with their own planets - it's simply not necessary to involve other galaxies, especially considering the distances to other galaxies.
  
  
  
  
  
• The Local Group
  The Local Group is the space immediately surrounding our galaxy - it's a small, "close", galaxy cluster - you can think of them kind of like the cities in a state. The biggest and baddest of our local group is the Andromeda Galaxy, and then us, the Milky Way galaxy. We're both badasses and tear other galaxies apart, but Andromeda is coming for us and we're going to be s.o.l. when it gets here. 
  
  Why? Because both the Milky Way and the Andromeda Galaxy are Spiral Arm galaxies, and the pressure waves that make those beautiful spiral arms are able to tear apart the galaxies they travel close to, tearing stars away from that galaxy and mixing it with their own. The core of those galaxies will occasionally survive and end up orbiting the cannibal galaxy as a globular cluster. One day (say... in about 3.75 billion years, around when the sun starts to turn into a red giant), our two galaxies will collide and merge together.
  
  The Andromeda Galaxy contains around one trillion stars (double that of the Milky Way), and is around 220,000 light years across, and it is around 2.5 million lightyears from Earth.
  
  Fun fact! The closest galaxy to the Milky Way, (at 100,000 light years, smaller than the Milky Way's diameter!) is a dwarf galaxy "in" the Sagittarius constellation (by "in" i mean we see it in that constellation, but all the stars of Sagittarius are part of the Milky Way, not this dwarf galaxy, and the dwarf galaxy is most certainly not "in" the Milky Way) and it is currently being devoured by the Milky Way. When we're done with it, the next nearest galaxy to us will be the Large Magellanic Cloud (a mere 160,000 light years away)
  
  
  
  
• Stellar Neighbourhoods
  Stellar Neighbourhoods are tiny parts of a galaxy - think like neighbourhoods within a city. Several stars in a stellar neighbourhood formed from the same nebular cloud, which is kinda neat because they'd all be around the same age and formed from the same stuff. 
  
  
  
  
• Star Systems
  A Star System is composed of a star (or two stars or three stars depending on how that nebular cloud split) and the planets / space debris that orbit it. Our star system is the Solar System, named so because our sun's proper name is Sol. Proxima Centaru's star system name would be "The Proxima Centauri System"; Sirius's star system would be the "Sirius System", or maybe a possessive form like the "Siri System". Anyway, these are where planets are located and are named after the star that provides that system with light and energy.
  
  
  
  
• Planetary Systems
  A Planetary System is composed of a planet and the debris/moons that orbit it. For example, the Jupiter System, or Jovian System, would be the planet Jupiter and all 63+ moons. The Saturnian System would be Saturn and all 62 of its moons. The Venusian System would just be Venus, and so probably wouldn't have "system" included in its name at all. The Earth, being the weirdo that it is with it's huge moon, is actually not the "Earth System" or the "Terran System", but the "Earth-Moon System" as our moon is so large comparatively. 
  
  
• Your Stellar Address
  So, when stating where you (or your senshi) is from, the proper way is similar to postal addresses - start specific, and then get broad. For example, the address of Neo Queen Serenity would be:
  
  Neo Queen Serenity
  Crystal Tokyo
  Earth
  Earth-Moon System
  Solar System
  Milky Way
  Local Group
  It doesn't flow, but that's whatever. Let's try with someone else - how about... someone living on Titan?
  
  So-and-So
  Titan
  Saturnian System
  Solar System
  Milky Way
  Local Group
  
  

This is one of those topics I'll probably go back and edit as time goes on, but I'm going to start by keeping things simple.

As you know from the first post, space is huge and mostly empty. ... I've been sitting here for a while trying to figure out what I want to talk about and what needs mentioning here, so, again, I'm going to keep this really basic, but if you have questions about anything here or anything related to this topic, please reply to this thread or send me a PM and I'll endeavor to answer it and include it in this post!

First, What is space like?
The basics:

• It's cold.
  In science we use the Absolute Temperature scale, or the Kelvin scale. It uses degrees that are the same size as Celsius degrees (which are smaller in scale than Fahrenheit degrees) but rather than using the freezing point of water for the 0 mark, it uses the freezing point of everything as the zero mark. What do I mean by that? Well, when water freezes what happens is that the temperature of the substance lowers to a point that the bonds between the water molecules strengthen, and rather than continuing as a liquid, the stronger bonds turn it into a solid. All temperature is doing, though, is giving us an easy reading of the kinetic energy of a substance - or how quickly the molecules of that substance are moving or vibrating. Faster vibrations mean more energy, thus higher temperature; lower temperatures mean slower vibrations and less energy. When water freezes it's because the liquid has lost kinetic energy to the point the the particles cannot move away and form themselves into crystalline structures -- this would be 0° Celsius. As the temperature goes down, the kinetic energy of the molecules decrease, and the vibrations of the particles get slower and slower and slower.... until they stop moving altogether. And when I say stop moving, I mean every part of that molecule. Even the electrons. The point where even the electrons stop moving is the same for all substances, and we call that "freezing point" absolute zero, and is 0 on the Kelvin Scale. It's around -273°C. ((so room temperature on the Kelvin scale is around 293 K))
  
  Space is empty, right? So there are no particles moving around, thus there is no kinetic energy and, tadaa! Cold. But, despite what you may think, it's still not absolute zero out there. This is because, despite what most everyone has ever said, it is not completely empty. We've got sparse particles out there - traces of gas and dust that hasn't been pulled into a larger body, even incredibly huge fields of the stuff (Nebulae!) that make up the Interstellar Medium (ISM) - and while inside a larger cloud of ISM the temperature can range from 10 - 50 K, in regular, mostly-pretty-darn-empty space, the average temperature is 2-3 K. ... That's cold.
  
  
• There is no pressure.
  Continuing from the first post and the one just above (not even empty space is empty what is going on), despite having those tiny particles of gas and dust, space is definitely empty to us. No air, no air pressure, and it's the latter that really gets you. We're used to constant pressure all around us, pushing at us from all sides, helping us keep our shape as we have adapted to this planet. In space there is no such pressure to help us along.
  
  Pressure is a lot more important than what a lot of people believe - it's pressure that causes liquids to boil, not temperature - when the internal kinetic energy within a liquid reaches a point that the internal pressure of that liquid is equal to the pressure outside the liquid (in the air) the liquid boils; on planets like Mars, where there is very little air pressure, water would boil at 10° C instead of 100°C. Imagine space, where there is no air pressure. How easy would it be for a substance to reach a pressure equal to the pressure of space? Tadaaa it already surpassed it, and so any liquid present (hahah blood) would boil away. 
  
  
• Radiation sucks.
  This is one of the many obstacles in our way for dreams of exploring space. Cosmic Radiation sucks. Our planet has a really cool magnetic field surrounding it, and a pretty awesome atmosphere - both work together to protect the surface from harmful radiation. In space, outside our magnetosphere, this isn't the case. Objects are bombarded with high energy high frequency high velocity particles that love to interact with whatever they come in contact with. Like your skin. 
  

As a lot of people like to ask/think about this question, I'll go ahead and answer it here. What happens to the unprotected human body in space? I could write it all myself, but instead I'm going to source this CNet article and just rewrite it here for you guys, bolding stuff I think is fun. 

What happens to the unprotected human body in space? wrote:

It's a recurring horror in sci-fi: the hull is pierced, a human is trapped without equipment in an airlock about to open, a door needs to be opened in order to expel something undesirable. With no air and almost zero pressure, the human body isn't going to last long without some form of protection.

But what does happen, exactly? Do your eyes explode outward while your blood evaporates? Well, no. The truth is both less dramatic and far more fascinating -- as we have discovered through accidents in space and in test chambers, and animal experimentation in the 1960s.

The first thing you would notice is the lack of air. You wouldn't lose consciousness straight away; it might take up to 15 seconds as your body uses up the remaining oxygen reserves from your bloodstream, and -- if you don't hold your breath -- you could perhaps survive for as long as two minutes without permanent injury.

If you do hold your breath, the loss of external pressure would cause the gas inside your lungs to expand, which will rupture the lungs and release air into the circulatory system. The first thing to do if you ever find yourself suddenly expelled into the vacuum of space is exhale.

The other things, you can't really do much about. After about 10 seconds or so, your skin and the tissue underneath will begin to swell as the water in your body starts to vaporize in the absence of atmospheric pressure. You won't balloon to the point of exploding, though, since human skin is strong enough to keep from bursting; and, if you're brought back to atmospheric pressure, your skin and tissue will return to normal.

It also won't affect your blood, since your circulatory system is able to keep your blood pressure regulated, unless you go into shock. The moisture on your tongue may begin to boil, though, as reported by Jim LeBlanc, who was exposed to near vacuum in a test chamber in 1965. LeBlanc's suit sprung a leak, and he remained conscious for about 14 seconds; his last sensation was bubbling on his tongue (he was safely revived, as the researchers began repressurising the chamber almost immediately -- after about 15 seconds).

Because you will be exposed to unfiltered cosmic radiation, you can expect some nasty sunburn, and you'll probably also get a case of decompression sickness.You would not, however, freeze straight away, despite the extremely cold temperatures; heat does not leave the body quickly enough for you to freeze before you suffocate, due to the lack of both convection and conduction.

If you do die in space, your body will not decompose in the normal way, since there is no oxygen. If you were near a source of heat, your body would mummify; if you were not, it would freeze. If your body was sealed in a space suit, it would decompose, but only for as long as the oxygen lasted. Whichever the condition, though, your body would last for a very, very long time without air to facilitate weathering and degradation. Your corpse could drift in the vast expanse of space for millions of years.

So... it's pretty important that we don't boldly go unprotected into space.

Unless you're a senshi

How do things move in space?
In order to move, we have to use and abuse the Laws of Motion. There are three of these, undoubtedly taught to you sometime in your school career based on how important they are, but that was probably a long time ago and worded in a ??? way so let's go at this again, Kyra-the-physics-teacher style:

1. First Law of Motion: the Law of Inertia
   
   An object in motion (moving with constant velocity (or at rest)) will continue in motion (with that constant velocity (or stay at rest)) until acted upon by an unbalanced force.
   This whole notion is pretty much what inertia is and what it does. Inertia is a property of Matter (thanks Bill!); everything that has mass, has inertia. Inertia is resistance to change. The more mass you have, the more you are able to resist change! And if nothing is forcing you to change, you're going to keep doing what you're doing - that's all the first law says!
   
   -If something is sitting there it's not going to spontaneously get up and move. Mass is lazy - unless forced, it's not going to start moving.
   
   -If an object is moving at a constant velocity (wherein either no forces are acting on it or all forces acting on it are balanced by a force equal in size and acting in the opposite direction), it's not going to change unless you force it to! It's not going to speed up, or slow down, or change direction unless something forces it to.
   
   --That last one is what people don't buy in to, but I need to make it clear, so here we go. Have you ever tried to stop a moving object? Did it ever help you stop itself? No. It works against you. It's resisting the change! Why things constantly seem to naturally slow down on Earth is because things on Earth are typically always touching the ground when they are moving. Gravity, it's a thing. Why things slow down is not because they naturally want to do so, but because of friction. Friction is a force that resists motion, because friction is a microbond made between objects - it sticks them together, and when something is sliding or moving past, friction makes them stick together and then the object has to use up its own energy to break those bonds and keep moving. Losing energy = losing speed = slowing down. It's friction's fault, not what the object has a tendency to do!
   
   Space travel is all based on this First Law of Motion. Once you get a ship moving in space, it's going to keep moving because nothing is out there to slow it down or make it change directions (...unless it's close to a planet and Gravity acts as that unbalanced force. I mean we can even use gravity as a free speed boost using a the slingshot method - unbalanced force yaaay). 
   
   
2. Second Law of Motion: Law of Constant Acceleration
   
   When an unbalanced force acts on an object, its acceleration will be inversely proportional to its mass.
   This, again, is all about inertia! The more mass, the more resistance to motion. If an unbalanced force acts on an object to get that object's motion to change, the more mass it has, the slower it will change - the smaller its acceleration.
   
   --Therefore, if we're talking spaceships, a tiny space ship without a lot of mass (or inertia) will be easier to accelerate (speed up, slow down, and change direction) than a huge hulking space ship with a lot of mass (and thus inertia).
   
   
3. Third Law of Motion: Law of Action/Reaction Forces
   
   For every action force applied to the first object, there is an equal and opposite reaction force applied at the same time to the second object.
   The old "I touch you, you touch me" law. If I kick a desk, the desk "kicks" me back with exactly the same amount of force, pointing in the exact opposite direction. This is how anything changes. It's always in effect - you walking is explained by this law - as you push your foot backwards against the earth and shift your weight forward, the earth pushes forward on your foot and your motion changes. 
   
   --Same thing for space travel. We push something backward to move the ship forward. The better the push backward (the bigger the force) the bigger the push forward, thus the higher the acceleration (the faster the gain in speed). ...What this means, though, is that if we wanted to change our motion, we'd have to expel something. On earth we can move through friction beneath our feet, water around us or air behind wings, but space has no medium within it - no water, no air, no ground - so the only way to move forward is to shoot something backwards.
   

Unless you're a senshi.

How fast can we move in space?
A little of this was mentioned in the first post about how quickly our fastest ship can travel, but, still, given the size and scale of the universe and our reliance on fuel, it would be a huge undertaking to go far.
unless you're a senshi.

---

Senshi don't care. Senshi gon' get there. BY THE WAY What Kakyuu seems to have done was to lead them all out of the plane of the Milky Way, flying "straight up" if you want to think of it in those terms, so that they can look down upon the middle of the Galaxy so clearly. SMART MOVE. Now when they fly towards it they won't have to mess with other systems or debris! Straight shot to Sagittarius Zero Star! ((I mean why fly through the fog if you could get above or below it?))


Sailor Senshi have none of this to worry about. Thanks to Naoko's chapters in Stars wherein Usagi travels to the center of our galaxy, we know that none of the problems presented above negatively affect our senshi when they are traveling between planets. From this image, and others within that chapter, we can deduce:

1. Senshi are not bothered by the cold
   
2. Senshi are not bothered by the lack of pressure
   
3. Senshi are not bothered by cosmic rays
   

Let's chalk that up to their Sailor Crystals; as we see that it's not just the princesses that can fly, but also the other three, we'll assume all Sailor Senshi have the ability for interstellar travel once they gain their Sailor Wings. BUT WHAT ELSE CAN WE GET FROM THIS PICTURE? headcanon+science warning:

1. They're talking. Now, as you are aware, there is no air filling up space. No water, no ground, no nothing, so sound waves cannot perpetuate in space. ((which is why no one can hear you scream - the vibrations made by your vocal cords are unable to travel from your vocal cords through particles and into my ear to make my eardrum vibrate at those frequencies, so i can't 'hear' you))
   
   --BUT WHAT DOES THAT MEAN THEN?! HOW CAN THEY TALK?! I'm thinking it's something akin to telepathy. Something short distance - they move their mouths to form the words and may even move their vocal cords because it's comfortable to them, but what they say would never reach the ears of anyone around them. I'm thinking that the thoughts they meant to project as words are telepathically communicated to the people around them via resonances with their Sailor Crystals. It's like having a heart-to-heart chat in space; so long as your heart has a Sailor Crystal to interpret the signals, you'll hear and understand what the other senshi was trying to say. 
   
   
2. They have wings that they use to fly through space. Those wings are inaction poses, as they are pictured elsewhere in that chapter, which suggests they are actively pumping and flapping those wings to move around in space. On Earth that would be normal, as the wings push back on the air and so the air propels the body forward, but in space? No air. No reason to flap those wings. UNLESS this is how they travel. Rather than throwing something backwards to push them forwards, what if their wings created the very medium they use to travel, like a photon? 
   
   Photons are packets of light that behave as both a particle and a wave. The wave part is the interesting part, and the very reason why light moves at the absolute fastest speed - rather than relying on a medium (solid, liquid, gas) for travel, light creates its own medium as it goes! It's like if you were dependent on a sidewalk in front of your feet in order to run as fast as you can, the sidewalk would be the necessary medium for you. So long as you had a sidewalk, you were good to go! Run your heart out! But... what if the sidewalk wasn't flat and smooth, but bumpy and course? it'd affect your running, right? What if there were no more sidewalk? You couldn't run. But let's say you are just that awesome and every time your running foot left the ground, sidewalk formed beneath you. You'd create your own medium - nothing could slow you down, you weren't dependent on your environment anymore - you made what you need! That's what light does - it makes its own medium. 
   
   I'm proposing that that is what Senshi Wings do, too - with every push forward, the senshi wings create their own medium and propel the senshi forward, faster than light, because...
   
   
3. They travel insanely fast. Like wow. Okay so remember that first post? Remember how Earth is 30,000 light years away from the center of the galaxy? Sailor Moon and crew made it there lickity split. I mean we have to assume it's lickity split because it certainly didn't take them over 30,000 years like those fastest moving particles did, and when they returned to Earth the time stream corrected itself, so it certainly didn't push Small Lady's future 60+ millennia into the future. ((21st century earth? it'd be 81st century earth at the earliest if the senshi could only travel at light speed)).
   
   So Light Speed < Flight Speed for Sailor Senshi. How fast can they travel? I don't know, Naoko didn't give exact times  BUT we know its faster than light speed, so it would take well less than four years to travel to our closest neighbour star, hooray. I personally like to imagine that 30,000 light year travel time to be on the order of days to make my head happy and to provide time for fun interstellar shenanigans. I mean not in that case, but in other cases. It also keeps the whole galaxy a little wider and less easy for someone to conquer in minutes.
   

So... that's space flight! I'm totally open to questions.