I worked with a friend on what basically amounted to story-telling in a shared world, but which out of necessity also involved quite a lot of world exploration and some building, because the characters we worked with only interacted to a limited degree. What we did amounted mostly to "killing time", but resulted in a fairly hefty collaborative short story (21K words, 200+ hours document editing time, both according to OpenOffice) which still had some room for further plot development within the originally envisioned framework when we decided to call it quits.
The biggest issue, I thought, was our slightly disparate views of what the world in which the story was set was like. It was never actually a problem, but did involve a lot of discussions outside of the story line development to hash out both details as well as parts of the larger picture. Since we both wanted to keep the story consistent in-universe, there was also a lot of detail editing going on, not just to recently written parts but sometimes to parts that had gone untouched for several rounds of going back and forth. (We basically each wrote as much as we felt comfortable with, then either left the other person's characters in a spot to work themselves out of, or until we ran out of creative juices. Naturally, this sometimes involved making the other person's characters do or say things, where we took a best guess and the "owner" of that character would rework it. Sometimes that affected the next part of the action, requiring multiple rounds of editing. Each one of us also worked on story pieces set in the same world that were not collaborative, sparking further discussions about what the world was like.)
Based on this experience, I'd say that the answer depends a lot on what kind of shared world you have in mind to create, but they both depend on an accurate description of the world being available to other authors.
If your idea is collaborative writing, be prepared for the fact that others are going to want to do things in your world that you might not have thought of, or allowed for your characters to do. Know how you plan to handle this when it comes up: is it your world all the way, or are others allowed to introduce other elements? The third option would be collaborative world-building, which I don't think is what you have in mind.
If your idea is to simply drop a world in other authors' laps then you have to come up with a very detailed description of the world. The description has to basically answer every question people might have about the world, in enough detail that they can actually work it out for themselves, unless you're willing to actually field questions and give thoughtful, consistent answers to them. This basically means to first create a world and describe it, and then place your own story in it using only the description of your world, so that you can be certain that you are not leaving out anything that is essential to telling a/the story.
The fact that you are including space travel makes it easier to allow other authors to introduce other elements, since those can easily be made specific to a planet or a solar system. (Interstellar travel would pretty obviously necessitate allowing for some form of faster-than-light travel.) The basic laws of physics would have to remain the same, however, if you want your universe to make any sense at all. For example, if you come up with a reason why space travel is limited to, say, a hundred times the speed of light, then you can't suddenly have a species gallivanting off making a weekend shopping trip to the next galaxy.