Sciency Words: Orbital Vocabulary

Mark BallThere’s been a lot of discussion lately about how scientifically accurate works of science fiction should be. It’s an endless and ongoing debate, and a question to which there is perhaps no answer. But if there’s one thing we can all agree on it’s that we should at least try to make the science in our stories seem plausible. Arming ourselves with a full scientific vocabulary is key to this effort.

To that end, I’ve decided to begin sharing a series of weekly articles by writer James Pailly. The series aims to embiggen our vocabularies and first appeared on the Planet Pailly blog – we’ll be borrowing the articles from there.

We’ll start by looking at a few backdated articles from 2014 before catching up to James’ more recent output. Enjoy.

Today’s post is part of a special series called Sciency Words. Every Thursday, we take a look at a new and interesting scientific term (or in today’s case, four terms) to help us all expand our scientific vocabularies together.

Orbital Vocabulary

Space travel really isn’t complicated once you understand the terminology. Words like left, right, up, and down don’t mean much in zero gravity, nor do words like forward or backward. If you’re aboard a ship that uses centrifugal force to simulate gravity, even the terms port and starboard might cause confusion. So in order to navigate in space, we need to use a whole new vocabulary.

I recently started playing Kerbal Space Program as a way to learn more about space travel. Thanks to the game and various game F.A.Q.s I’ve found online, I’ve picked up four new sciency words every space navigator needs to know.

  • Apoapsis: the highest point in your orbital path.
  • Periapsis: the lowest point in your orbital path.
  • Retrograde: if you fire your rockets in the opposite direction to your movement, you’re firing them retrograde. This will cause you to slow down.
  • Prograde: if you fire your rockets in the same direction as your movement, this is prograde, and it will cause you to speed up.

Kerbal Space Program

When you want to move your spaceship to a higher orbit, fire your rockets prograde. If you want to lower your orbit, fire your rockets retrograde. According to my research and my experience in Kerbal Space Program, lowering your orbit from your apoapsis (highest point) is the most fuel-efficient option. The same is true for increasing your orbit from your periapsis (lowest point).

These four terms are still new to me. I’ve only been playing Kerbal for a little while now. So if anything I’ve written here is mistaken, either in regards to Kerbal Space Program or real life, please let me know in the comments below.

Written by James Pailly.

To read the full ‘Sciency Words‘ series, visit the Planet Pailly blog.


  • John H Reiher Jr

    I’ve learned a lot about orbital mechanics from Kerbal Space Program. With some of the mods out there, you can reproduce most torch drives and see how long it takes to travel to other worlds.

    As for words, having an understanding of how those words are applied is just as important as the word itself. Otherwise you’re just spout’n jargon and technobabble.

    Also, we worry about km/s not mph when in orbit. Another nice tidbit is that for every 3 km/s of velocity gives your ship the kinetic energy equivalent to it’s mass in TNT. This is why kinetic kill weapons are more devastating than any nuclear warhead. If you can get your KKW up to hundreds of km/s, it is most definitely doing nuclear weapons grade damage.

    As an example, the ISS orbits the Earth at around 7.8 km/s. So if something is in retrograde orbit impacts it, it will do 2.6 times its mass in TNT’s worth of damage. This is why the ISS has Whipple shields up to slow down or stop anything from hitting the station directly.

    In any case, I wholeheartedly applaud this new series! Moar! Moar!

    • James Pailly

      I haven’t heard of Whipple shields before. That’s a very interesting new term. Thank you!

      • Looks like you’ve got a lot of new suggestions for future sciency words articles. You’d best get to work 🙂

    • That’s why my “Terrans” use Deuterium Slug cannons. The kinetic energy of an object traveling at very high speeds is tremendous.( Technically if it would travel at anything close to light speed near infinite you have the ultimate weapon as it would impact with almost infinite energy , but that would require near infinite energy to propel it)
      Or let’s say your very fast space ship hits a micro meteor, than thing might go straight through your ship.

      A good book to read is The Physics of Star Trek by Lawrence Krauss..

      • John H Reiher Jr

        I’d just use iron slugs. Mass is mass. At the very least, your slugs will have a top velocity of 6 km/s for a railgun, the limitation is the melting point of the rails and the projectile.

        Coilguns can get a higher velocity, but that requires a very long “barrel”, tons of power and switching system that’s fast enough to accelerate the round. But they get really long. Really long.

        Obligatory Atomic Rockets Link:–Kinetic_Kill_Weapons

        • The main reason for using deuterium is its availability, water and its components are abundant and the space ships (Union) use water and hydrogen as a fuel source.
          The society most of my stories take place are a tad beyond the “Atomic Rocket” stage and have control over gravitation. Meaning the acceleration of a projectile is not caused by magnetic forces.
          Also the average size of a Union battleship is in the 3000 meter range and large cannons or coil guns are not a problem.
          Of course any projectile based weapon is technically very inadequate in a space battle where targets move at considerable speeds and distances are measured in thousands of kilometers.
          I know you might accuse me of Handwavium and call the tech of my Universe “soft” . I do have a degree in physics and all technology used by the “Union” is based on postulated theories.
          I once calculated the explosive yield for Photon torpedoes waaay back in 1993 and used it in a “Gopher net ” based ST RPG – (under a pseudonym) it is still on the net in many forms and several “authors” claim it as their work – Not that I mind, I am saying this that I actually use math and physics , calculations and equations before I “invent” or “adapt” a technology to my Universe.
          Here are a few obligatory links to the GalNet Wiki of mine:

  • You can check on what a Whipple shield is on Wiki of course or you could (shameless self promotion) go to:

  • Paulo R. Mendes

    Try to say all this four words quickly one after the other and you will never able to use your tongue again… 🙁

  • Paulo R. Mendes


  • DT Krippene

    Always fascinating, factual, and entertaining, James. Keep it coming.

  • Here are a few more:

    Apolune: That point in a lunar orbit which is farthest from the Moon.

    Argument: Angular distance.

    Argument of periapsis: In an orbit, the angular distance between the point of periapsis and the ascending node.

    Ascending node: The point at which an orbiting object or spacecraft, traveling from south to north, crosses the plane of the equator.

    Attenuation: The decrease of a propagating physical quantity, such as a radio signal, with increasing distance from the source, or from some obstruction.

    Attitude: Orientation of a space vehicle as determined by the relationship between its axes and some reference plane, e.g. the horizon.

    Azimuth: The angular position of an object measured in the observer’s horizontal plane, usually from north through east. Bearing or direction in the horizontal plane. As one of the coordinates expressing celestial location, it is sometimes used in tracking spacecraft.

    Barycenter: The common center of mass about which two or more bodies revolve.

    Bow shock wave: The compressed wave that forms in front of a spacecraft or satellite as it moves rapidly through Earth’s atmosphere; more generally, any such wave that forms between an object and a fluid medium.

    Cholorella: A genus of unicellular green algae, proposed for converting carbon dioxide into oxygen for use in spacecraft.

    Chromosphere: A reddish layer in the Sun’s atmosphere, the transition between the photosphere and the corona.

    Cislunar: Relating to the space between the Earth and the orbit of the Moon.

    • James Pailly

      I for one would love to hear more about how to get to Alpha Centauri. I have a somewhat abstract understanding of how transfer orbits work within the Solar System, but I’m kinda clueless about the mechanics of travel between stars.

      • Wonderful question and here is my answer.

        To answer this I decided to use my Wiki as it allows me to include graphics and pictures.

        This is going to be a long answer in several parts.

        • James Pailly

          I knew fuel would be a problem, but I didn’t know how much of a problem it would be. I suppose you’d also have to take extra so you could make minor course adjustments on the way, which brings us to…

          Navigation. I just kind of assumed computers could handle that. But of course, even with the benefits of modern computers, space probes within the Solar System require minor course adjustments. I guess this is much trickier than I expected.

      • Of course you could also take a Trans Matter step to SOL -Hub and get yourself a ticket to Centauri Gate ( Ticket is around 30 credits) Since the Centauri Triple Star system was the first Extra solar system colonized it is now a major Union core system and connected to the Space train network. Ticket is a little more (60 Cr) but you be there in about 10 minutes. If you got the credits, you could hire a charter (for around 200 credits)
        Should you be around after 5040 (OTT) you can of course use the LRTM (Long Range transmatter tunnel) – travel time less than a second.
        In case you hate travelling – hook yourself into a Virtu Couch, make a GalNet connection and sent your hard light avatar to Alpha Centauri. Not that there is much to see for a Human Terran like you. I suggest Para Para if you like Sun and beaches or perhaps something more exotic like Gore II ?

      • Was my answer adequate or do you have additional questions?

  • Kirov

    While on the topic of scientific terminology, I’d like to point out the difference between geosynchronous and geostationary orbits. It really bothers me when people use the terms interchangeably, saying one when they mean another.

    Geosynchronous Orbit – An orbit whose period (time it takes to complete one orbit) matches the rotation of the Earth. An orbit which goes from the South pole to the North pole in half a day would be a geosynchronous orbit.

    Geostationary Orbit – An very specific type of geosynchronous orbit which also happens to be equatorial, so that the body in orbit remains “stationary” above the same point of the Earth. This is often a very useful and commonly used orbit.

    Also, even though the terms can be used universally, the prefix geo- does technically refer to Earth. An aerosynchronous orbit would the equivalent of a geosynchronous orbit over Mars.

    • It seems I am the only one talking here but I can’t resist (John might understand) But you barely scratched the subject of Orbits:

      Here are a few less known ones:

      Horseshoe orbit: An orbit that appears to a ground observer to be orbiting a certain planet but is actually in co-orbit with the planet. See asteroids 3753 Cruithne and 2002 AA29.

      Exo-orbit: A maneuver where a spacecraft achieves an orbit that is unstable due to atmospheric drag.

      Prograde orbit: An orbit with an inclination of less than 90°. Or rather, an orbit that is in the same direction as the rotation of the primary.

      Retrograde orbit: An orbit with an inclination of more than 90°. Or rather, an orbit counter to the direction of rotation of the planet. Apart from those in Sun-synchronous orbit, few satellites are launched into retrograde orbit because the quantity of fuel required to launch them is much greater than for a prograde orbit. This is because when the rocket starts out on the ground, it already has an eastward component of velocity equal to the rotational velocity of the planet at its launch latitude. A gravity assist around the moon can reduce the fuel premium. Retrograde orbits are often used in anti-satellite warfare.

      Box orbit: An orbit in a triaxial elliptical galaxy that fills in a roughly box-shaped region.

      Pyramid orbit: An orbit near a massive black hole at the center of a triaxial galaxy. The orbit can be described as a Keplerian ellipse that precesses about the black hole in two orthogonal directions, due to torques from the triaxial galaxy. The eccentricity of the ellipse reaches unity at the four corners of the pyramid, allowing the star on the orbit to come very close to the black hole.

      Tube orbit: An orbit near a massive black hole at the center of an axisymmetric galaxy. Similar to a pyramid orbit, except that one component of the orbital angular momentum is conserved; as a result, the eccentricity never reaches unity.

      I got many more here:

      • James Pailly

        Hang on… I’m taking notes…

  • Andrea Mirò Silvestri

    Hey, I loved this topic!
    By the way, you say that writing left, right, up, down, forward, backword doesn’t make sense when you’re in space and I agree with that, but if I’m out of the mass lock of a celestial object and moving out of an orbit I can’t use the words you gave us since they’re related just to orbits. In that case what words should I use?
    I hope I’ve been clear ^^

    • I think those terms are still relevant, so long as they’re used relative to the position of the ship. But terms like port, starboard, aft, ventral, and dorsal might sound a little better.
      Saying that a ship moved or fired “ventrally” is sure to make you seem clever 🙂

      • Andrea Mirò Silvestri

        Yeah of course, I was just being lazy xD
        Thank you for your reply =)