Space Terms 5: Some Jargon, Some Relativity
Austin Morris, Director of Engineering
7 minute read
Today we begin our journey on what is currently the last planned installment in the Space Terms series. In this entry we will look at how objects in space move through their orbits and how those orbits can be changed by executing different spacecraft maneuvers. We will also get into some of the specific terms that cross my mind and desk daily as we theorize and work with others in planning the future of humanity in space.
Prograde & Retrograde
As detailed in 6DoF in Space Terms: An Intermediate Guide to Jargon, there are three translational axes, and in orbit, each has a name that is relative to your trajectory (I know, relativity again). An easier way to imagine this is to pretend that your object is a rocket, with a nose on one end and an engine on the other. Let’s also pretend this rocket is orbiting around the equator, at a 0° inclination (also described in previous installments of Space Terms). If you point your rocket so that the nose is pointing the direction you are going, the same way as Earth’s rotation in this case, and you turn your engine on, this is called a Prograde burn. If instead you were to point your nose toward where you just were and your engine in the direction you are traveling, this would be a Retrograde burn, effectively slowing your velocity relative to the Earth. This is easily thought of like putting a car into either Drive or Reverse.
Normal & Anti-Normal
Having nothing to do with the actual abnormality of this discussion, if you were to point the nose of the aforementioned example rocket perpendicular to the plane of your orbital trajectory (think as if you were pointing parallel to the direction of North in this example) this would be along the Normal vector. Yet again, the opposite direction of the example rocket (parallel to South in this example) is an Anti-Normal translation. Back to the car analogy, this would be like driving at speed and sharply drifting to turn to the left or right, which may be very cool but is maybe not the most effective or efficient method, just like in space.
Radial & Anti-Radial
Continuing the example from Prograde and Retrograde, if you pointed the nose of the example rocket straight at the center of the Earth and turned your engine on, this would cause Radial (or Radial-in) movement. If you instead turned your engine toward Earth and your nose away, this is now an Anti-Radial (or Radial-out) maneuver. For the car analogy there is no analogy. The car would be driving straight down or straight up and either way this is not considered normal driving. Please consult either a mechanic or a physicist if your vehicle operates like this.
ADR
Active Debris Removal, or ADR, is the application that KMI is actively pursuing. This entails the creation of a spacecraft that is capable of rendezvousing with active debris pieces in orbit and removing them (I wonder why it’s named ADR). This is a problem that we have discussed at great length in other segments, and will no doubt continue to discuss, as it is our major focus. As such, I’ll avoid cluttering this column up much further with detail about ADR.
MEV
Mission Extension Vehicles, or MEV, are an application of technology similar to that which KMI is pursuing for the purposes of ADR. Mission extension, in this context, is the prolonging of a satellite or spacecraft’s life by interacting with it for a number of purposes. This could include boosting it up to a higher altitude if it has run out of fuel and is falling, or this could be moving it to another orbital path or inclination, or any number of other options. The future of this technology is actively being worked on and demonstrated by several companies and ensures that the missions of yesterday we still rely on today might continue to be successful tomorrow and long after.
OSAM
This one is going to be a bit long, because I am getting idealistic. Prepare yourselves. On-orbit Servicing, Assembly, and Manufacturing is the lengthy title given to the process described in the name itself. This is where I believe the real future of humanity’s expansion into space lies. By servicing craft already in orbit we can prolong their usefulness, or even bring them back into service after having ended their lives prematurely. By assembling and manufacturing things in orbit we can bypass many issues that arise when trying to build and launch large structures from our gravity-bound facilities on Earth.
If something is built in microgravity, and is never needed to survive the challenges that can be encountered by high gravity and atmosphere, this allows a whole new world of design and manufacturing possibilities. Building a car or a house that can survive the conditions it will experience in its lifetime is something that humans have gotten quite good at. Likewise, we are fairly good at design and construction of submarines and ships. Yet amphibious cars and houses don’t seem to be flooding the market.
The reason is that, yes, there is probably little desire for it, but also that designing something capable of withstanding two hugely different environments can be a Herculean task. Yet that is our only option for spacefaring vessels at the moment. Once we can unburden our designs from the shackles of gravity and atmosphere, I believe we will see dramatic increases in the scope of stations and spacecraft that we can construct, enabling expansion of humanity further into the universe than ever before.
We could have a spacecraft that is a massive cube and it won’t suffer from terrible drag because it never passes through an atmosphere. We should of course not do that, because it would likely be an ugly, monolithic spaceship reminiscent of the Borg or the Vogons and I for one encourage smooth edges as an aesthetic choice, not just a practical one.
Relativity and Non-relative Measures
Relative measurements are those that only make sense based on perspective, such as an object’s velocity or direction. If you are standing at a train station and a train comes by at 25 miles per hour, it looks like it passes you at 25 miles per hour. If you are in a different train traveling in the same direction at 10 miles per hour, then the passing train looks like it is only moving 15 miles per hour. Tada, relativity.
Non-relative measurements are those that are true independent of perspective. For instance, if you were to look at Earth orbiting around the sun from any perspective, it would always measure the same maximum distance from the sun. Likewise, the speed of light in a vacuum is a universal constant and is not adjusted relative to your velocity. If you are driving your car in a vacuum at half the speed of light (by the way, don’t do that) and then turn on your headlights, the light emitted from them won’t travel at 1.5 times the speed of light (your speed plus the light speed). It will still only travel at its maximum speed, as it cannot go faster than that. Time fluctuations are another, hairier animal altogether, and fall under the umbrella of Special Relativity, which I am not going to dive into at the moment as I’m no Einstein. However, if you enjoy the subject of relativity, stay tuned for a future installment in Relativity and Non-relative Measures by KMI Director of Technology Adam Kall.
I have enjoyed writing this series, at the very least for the purposes of putting down all my thoughts into explanations to review for my own reference when needed. It is my sincere hope that through these past five installments I have taught some, and maybe even occasionally entertained. I very much look forward to writing on other topics in a much more conceptual capacity, rather than these deep dives that, while informative, can cause one to get lost in the page (either as the reader or as the writer). That being said, this is not the definite end of the Space Terms series, as there will always be more to learn, and I will be happy to share the things that I learn as I continue to do so. Until then however, let’s look toward the future and all the excitement to come. Stay up to date with KMI as we continue on our journey and mission of keeping space clear for all.
Recommended column to read next: Space Terms 4: What’s your Inclination?