Space Terms: A Beginner's Guide to Jargon
Austin Morris, Director of Engineering
6 minute read
If you’re reading this, you should know that I am going to miss something. This article is going to cover a lot, but it is pretty unlikely that it’ll cover everything. However, I’m hoping that this guide will be a good starting point for anyone who is trying to understand the concept of space and orbit but is getting buried in the specialized terms and acronyms and needs a little bit of help wading through the weeds.
Anyone who has ever spent time in the corporate world, working in retail, working with the military, or even amongst academia can attest that there is an overabundance of acronyms and terms that are not unlike the Isla de Muerta from Pirates of the Caribbean: they can only be understood by those who already know what they mean. This is the first installment in what I expect will become a long series of some of the most commonly used (and infrequently explained) terms and acronyms that I have come across in my short time in the aerospace industry. Follow KMI and we’ll continue to learn together.
The terms covered here are listed in an order of understanding, not alphabetical.
Orbit
Orbit is the curved trajectory that an object follows as it travels around the gravitational pull of another object, such as the Earth around the Sun or the Moon around the Earth. Objects remain in orbit when this trajectory is stable and does not intersect the ground.
Orbital Period
An Orbital Period is the amount of time it takes for an object to travel through one full cycle of its orbit and return to its original location. For example, the orbital period of the Earth around the Sun is how we measure one calendar year.
Orbital Path
This is a less clearly defined term than many to come, but it is typically used to denote the exact altitude and inclination of a particular orbit or group of orbits.
Altitude
Altitude is the height of an object above a reference height. When talking about Earth orbit, we typically use sea level as the reference height.
Attitude
Not to be confused with Altitude, Attitude refers to a satellite’s personal opinions towards being thrown around the Earth at 17,000 mph, as well as to the orientation of an object. For example, when an airplane is flying along, perpendicular to the ground, it has zero Pitch. During takeoff, with the nose pointed upward, this is positive pitch, and if the nose points downward this is negative pitch. The same is possible with Roll, which refers to either the right wing angled upward and the left down, or vice versa, and also with Yaw, which refers to turning the nose to the left and the tail to the right, or vice versa.
Inclination
Inclination refers to the tilt of an object’s orbit. If a satellite travels in the same plane and direction as the Earth’s equator, that is a 0° inclination. If an object travels directly over the poles, in a polar orbit, this is a 90° inclination. If a satellite travels in the same plane as the Earth’s equator but in the opposite direction, that is a 180° inclination, and so on.
LEO
Low Earth Orbit. Orbital space around the Earth from 100 km altitude to 2,000 km, containing most Earth observation and imaging satellites, some communications satellites, and the International Space Station.
MEO
Medium Earth Orbit. Orbital space around the Earth from 2,000 km altitude to 35,786 km (there’s a reason it’s so specific, just wait until GSO), containing mostly communications and navigation satellites including the GPS, GLONASS, and Galileo constellations.
GSO
Geosynchronous Orbit. Orbit around Earth with an orbital period matching Earth’s rotation on its axis (one day). A circular geosynchronous orbit maintains a constant altitude of 35,786 km. GSO contains many communications, broadcast, and weather tracking/imaging satellites.
GEO
Geostationary Orbit or Geosynchronous Equatorial Orbit. The terms Geostationary and Geosynchronous are frequently used interchangeably, though they technically have slightly different definitions. GEO is a special case of circular Geosynchronous Orbit in the plane of Earth’s equator, which is unique because it stays effectively motionless relative to the point on the ground directly beneath it. GEO is used mostly for communications, meteorology, and navigation.
Eccentricity
Eccentricity is a measure of how circular or not circular (typically elliptical) an orbit is. An eccentricity of 0 is a perfect circle around a planet, while an eccentricity of 1 is a perfect line through the center of the planet, and arguably not a very good orbit. An eccentricity of more than 1 results in a line wider than a circle, which is also not a very good orbit, but for different reasons. Please don’t ask about negative eccentricity.
Apoapsis
Many orbits are not perfectly circular (some are elliptical and not at all circular), and tend to waver up and down in altitude by some amount. Apoapsis is the highest altitude point of an orbit, when the object is furthest from the object that it is orbiting. When the object is the earth, the term can be further specified to Apogee. Other objects have their own suffixes for these terms as well.
Periapsis
In contrast to Apoapsis, the Periapsis is the lowest altitude point of an orbit, when the object is closest to the object that it is orbiting. If an orbiting object has a Periapsis at or below ground level, it is either passing through a canyon or will no longer be an orbiting object. Likewise, this term can be further specified as Perigee when referring to the Earth.
Tumble
Here’s a short and easy one to give you a break: Tumble is the colloquial term for rotation in one, two, or all three of the rotational axes as described in Attitude. A child who claims they can go down the stairs three at a time is an example of a pre-tumble.
Debris
Space Debris, Orbital Debris, etc. generally refers to any human-made object in orbit that is not functional or controllable and can potentially cause damage to other objects by collision. Many pieces of debris are expected to be tumbling some amount in some number of rotational axes.
Launch Vehicle
Launch vehicles are the mode of transportation by which a payload reaches orbit. Most people think of rockets, which are indeed launch vehicles, although they can also be balloons or spaceplanes as well, or a combination thereof (balloon-launched rockets, plane-launched rockets, rocket-launched balloons...actually, probably scratch that last one).
Satellite
Satellites are objects that orbit other objects in space. They are typically described as either natural satellites (such as meteoroids, rock chunks, and the Moon) or as artificial satellites (such as those made by humans and placed intentionally, except in rare cases, into orbit ever since Sputnik 1).
Spacecraft
Spacecraft are a more specific type of artificial satellite that is designed and intended to fly in space and provide some purpose, ranging from navigation to communications, or Earth observation to even space debris removal (*wink wink*).
As mentioned at the beginning, this is by no means a comprehensive guide to all terminology, and may not necessarily contain the best-written explanations of each of the terms thus far. This industry, as any other, has many levels and layers of complexity, which means that a full and in-depth explanation of the terms and jargon requires much more time (and a larger existing basis of understanding) than this column and writer is ready for at the moment.
I like to think that I have always been and will always be a student in some capacity. As we continue to learn and gain knowledge, it seems the only prudent thing to do is to share that knowledge to make it easier for others to learn than it was for me. It has always been a core belief of mine that those who learn have the duty to become the teachers that future students grow beyond. It is my hope that those of you reading this will eventually (if you haven’t already) grow beyond my knowledge and advance the future of humanity in ways that I cannot even imagine. Keep checking in on KMI and you will learn with and from us. Follow us on our journey, join us, communicate with us, learn alongside us.
Recommended column to read next: Space Terms 5: Some Jargon, Some Relativity