Travel at faster than the speed of light certainly can have dramatic implications that are difficult to understand, such as time travel.
When Wizards of the Coast returned spelljammer to the gaming multiverse, they removed the phlogiston from the system. The phlogiston was an integral part of the original spelljammer setting, so removing it was, and still is, controversial. Many of the old school spelljammer supporters weren’t happy with this, while others didn’t really care. Personally, I thought that totally removing it caused more harm than good, and that a lot of gaming potential would be lost.
The Astral Sea was introduced as a replacement for the phlogiston with the fifth edition release. Personally, I thought the astral sea made interstellar travel “too easy,” but I also understand how the phlogiston was viewed as “too complicated.”
I’m not going to take a side on this one, because I’m one of those people who think that both systems can be used side by side. With just a little tweaking, it can be and instead of or. Below is my way of addressing this. This is mostly a look at game mechanics and the “magical physics” that exist within the spelljammer system.
The basic concept is that the astral sea and the phlogiston are both core properties of the prime material plane, and that they co-exist. Think of them as different “layers” of the spelljammer universe:
- Wildspace is effectively infinite, much like outer space in our real universe. This includes the physical worlds that most adventures take place in and on. It’s the physical universe that we can all see and tough. If you want to travel from one star system to another, you should plan to spend several decades traversing open space (which is logistically impossible), or, you can use one of two means of interstellar travel. The speed of light is as much of an obstacle in this fantasy universe as it is in ours. Fortunately, that pesky speed limit is easier to circumvent in this fantasy universe than it is in our real one.
- The astral sea can be viewed as a mesh that lays on top of wildspace. In areas with complex gravity fields, like a star system, with planets, planetoids, and hoards of other objects with binary gravity planes, the prime material and astral planes partially overlap. They don’t overlap to the point where one can effortless toggle between the two, but with some care, a traveler on the prime material plane can use the astral plane to greatly reduce travel time. There are some local exceptions to this, naturally, but those are just that: exceptions.
- The Phlogiston, that endless sea of flammable plasma from the old system, is a property of the prime material plane. It is part of the infrastructure (for lack of a better term) that keeps the prime material plane bound together, and is key to keeping the universe working. If the astral sea is viewed as something draped on top of wildspace, think of the phlogiston as being underneath. It could be compared it to the backing of a carpet, or the unprinted side of wallpaper.
Returning to a more human scale, spelljamming makes use of all three layers, Travel in wildspace is well established, and remains unchanged from the original system introduced in second edition Dungeons and Dragons. When traversing wildspace, a spelljamming vessel will move incredibly fast once it is beyond the influence of a gravity field that is larger than itself.
The original system put wildspace, or interplanetary travel, at 100 million miles per day. [Or, if you prefer other measuring systems, 160934400 kilometers per day, or slightly more than one astronomical unit (AU) per day.] Consider this a fixed, physical constant, like the speed of sound or the speed of light. When magical energy is converted into physical energy (a job performed by a helm), that is the optimum speed that material objects can move before whatever physical laws are in place start messing with everything.
In this milieu, interplanetary travel is largely unchanged from the original edition. Interstellar travel has two different, but ultimately related, options. When your vessel reaches a certain distance from a star system primary, usually a star or group of stars, you enter the “transversal zone.” This is typically a few days travel beyond the last major body in the star system. It can vary, but typically falls within 1-4 (1d4) days of travel. (Here also, local exceptions do exist.) In the case of our solar system, it would be 1d4 days travel beyond the orbit of Neptune. Once within in the transversal zone, a magical spell is used to transverse into interstellar travel. Note that some worlds may lie in or beyond this zone. Again using our solar system, Pluto, Eris, and the rest of the Kuyper gang would be in and beyond this zone. So if you are planning to visit one of these worlds, do not switch to interstellar travel!
But if interstellar travel is your intent, then once within the transversal zone you have two options.
- The Astral Sea. The vessel can take advantage of the overlap between the astral and material planes to accelerate far beyond the speed of light, and travel to another star system in a matter of days or weeks, as opposed to decades. A magic spell, Astral Transition, is performed by a spellcaster on the vessel (not the healmsmen, obviously). This spell will take a the ship’s existing spelljamming field and attune it to the regional overlap with the astral plane. This allows the vessel to “surf” the overlapping planes and travel faster than light. Note that the vessel is still partially on the prime material plane, so events and objects in wildspace may theoretically effect it..
- The Phlogiston. In this context, the phlogiston is analogous to hyperspace. The spell Create Portal will create an opening into the phlogiston from the prime material plane (think of the jump gates from Babylon-5), through which a ship can travel, and then use the natural currents of the phlogiston to reach other star systems.
If traveling in the phlogiston is analogous to hyperspace, think of the astral sea as being akin to using a warp field. The biggest difference between the two is how a vessel is perceived by onlookers. A ship traveling on the astral sea is almost completely on the astral plane, but the key word is “almost.” The ship is still visible to onlookers, though it may not be recognizable as such because of its extreme speed. More importantly, conditions in wildspace, like a space storm, uncharted debris field, or even another vessel, can still effect it. In the phlogiston, on the other hand, once a vessel traverses a portal it is isolated from the rest of the prime material plane, and will remain so until it crosses another portal. When the travelers reach their target system, a spell caster must either cast Create Portal to gate back from the phlogiston to wildspace, or, cast Astral Transition to fully phase back into wildspace. Hopefully the crew will be observant enough to know which spell to use.
One of the best explanations of this difference I’ve ever seen came from GURPS space. To summarize, a vessel traveling by warp has a special energy field around it that allows it to travel faster than light. The vessel is still aware of space around it, and can still interact with it. On the other hand, a vessel in hyperspace has entered a parallel dimension from normal reality, and will be out of contact with the regular universe until it portals back.
Why have two systems? There are some places that the phlogiston can reach that the astral sea can not, and vice versa.
For example, the astral plane is highly tied to sentient thought, and this effects how it can be applied. Generally, voyages on the astral sea are restricted to five light years or less, apparently because of general limitations on telepathy, psionics, and sentient thought. This usually isn’t a problem for interstellar travel, as there are enough star systems about to make leap-frogging between systems fairly straightforward. As with long ocean voyages, a long voyage on the astral sea can be broken into a series of short voyages.
But this does present an issue. If a star system doesn’t have sentient thought present, which would be the case for an uninhabited or unpopulated system, the astral sea won’t have a connection. The planes will still overlap, but the absence of higher sentient thought will make it nearly impossible for a helmsman to tune in to whatever worlds the vacant system may have. An special example is the Darkspace system from the classic module Crystal Spheres. That system is inhabited entirely by undead, most of which aren’t truly sentient. There are sentient liches and vampires present, but they are too few in number to allow approach from the astral sea. That system almost requires approach by the phlogiston.
There is also the issue of range. Astral sea routes are at most five light years in length. This isn’t a hard rule, and local or regional exceptions could theoretically exist, but so far no such exceptions have been discovered. If a target system is further that five light years away, and there isn’t an intervening system along the route, then the only viable option is the phlogiston.
Range doesn’t present this problem in the phlogiston. The distance a vessel can travel in the phlogiston has more to do with how many resources it can carry, and how long the crew can function normally. It also tends to result in faster travel times than the astral sea. This this isn’t a major issue for short voyages, but on long voyages the difference becomes noticeable.
Because the phlogiston is a separate dimension (“layer”) of the prime material plane, it is nearly impossible to track a vessel that is using the phlogiston. For this reason has frequently been used by pirates and criminals. But there is a flip side to this. The largest disadvantage of the phlogiston is that it is inherently more hazardous. The chances of an encounter in the phlogiston is twice that of the astral sea, and many of those encounters will be hostile. Simple observation has shown that there are far more creatures and phenomena in the phlogiston than in the astral sea. So any voyage in the phlogiston is more likely to encounter difficulty than a similar voyage in the astral sea, even allowing for the shorter travel time.
There is also the issue of getting lost. In the phlogiston, getting thrown off course can be catastrophic. One could end up in an area where a friendly system can’t be found, or worse, an area where there aren’t any systems at all. In the astral sea, one can typically transverse to wildspace at any time, get their bearings, adjust course, then resume. In extreme cases the astral sea has the possibility of time dilation. This is usually just a nuisance for the crew, with the effect only covering a few hours or days, but it’s still something best avoided. But in the phlogiston, one needs to be near a system to open a portal. If if there aren’t any systems nearby, or the system is hostile, then things become very difficult very quickly.
There is one more thing that needs to be explained, and that is the notion of a “crystal sphere.” In this milieu, the phrase is a misnomer. The phrases “crystal sphere” and “star system” are functionally interchangeable. Both refer to the collective gravity fields of a group of stars, planets, or other complex system of objects. The historical origin of the term came from how a star system appears on approach. In the phlogiston, it really does look like a large, physical, dark colored sphere made of crystal. Many of the earliest star charts date back to the ancient Juna, and evidence suggests that they did not use the astral sea. (Why this is so is still being examined.) If the dominant method of interstellar travel at this time was the phlogiston, it is likely that the name “crystal sphere” somehow “stuck” in mainstream parlance, and remains in use to this day.
Oddly enough, In the astral sea a star system also takes on a sphere-like appearance from a distance, usually silver-blue in color with swirling clouds of vapor. This may be another reason for the term’s persistence. Either way, the name “crystal sphere” refers to the shadow, or reflection of a star system within the phlogiston or astral sea.
Truth be told, the notion of star systems being housed inside of giant, hollow crystal balls was the one part of the original cosmology that I could never fully except. I worked with it, but never liked it. But ultimately, that is a “me” problem. This hybrid system is how I resolved it.
So there is it. Many have attempted to find ways of including the phlogiston without messing up the astral sea. This was my attempt. Comments welcome.
Image credit: The Pillars of Creation, part of the Eagle Nebulae, as photographed by the James Webb Radio Telescope, in 2022. This image was shared on www.bbc.com, referenced on September 29, 2026.
quotefancy.com, viewed October 11, 2026.
This article started as a post that Richard made on piazza.org.uk in August of 2026. It has been edited extensively since then.
Jackson, Steve and William A. Barton. GURPS Space: Roleplaying in the worlds of tomorrow. Steve Jackson Games, third edition. 1999. The explanation of hyperspace versus warp space occurs on pages 29 through 31.