Words have meaning: Satellite, Spacecraft, Observatory, Payload

Ever look up in the sky and see a star moving slowly across the sky, passing the other stars around it? What you are seeing is a satellite, an object orbiting another object, reflecting sunlight off its surface.

In this case, the satellite is man-made. However, the moon is also a satellite, just like that little dot you saw. It is an object orbiting the Earth. The Earth itself is a satellite, since it orbits the Sun. And there are man-made satellites that also orbit Mars (e.g. Mars Odyssey), Saturn (Cassini), the Sun (SOHO) and many other solar system bodies. Even those observatories that are no longer functioning (like Mars Global Surveyor) are still satellites, they still are in orbit. I find that many people think of satellites only as man-made objects, even though nature made them first. And we tend to forget that the word satellite has little meaning without knowing what body the object is orbiting.

If you watch NASA TV (and who doesn’t, right?) and listen to those engineers and scientists talking about their satellite it can seem like they are using a lot of different words for what seems like a simple subject. However, in the space industry there are certain very specific terms that we use that have specific meanings. So while it may seem that we are spouting lots of words to just sound extra-super-duper-smart, in reality we are using those terms in order to be precise. Take, for example, the terms “spacecraft,” “observatory,” and “payload.”

A spacecraft is a support structure. It contains the hardware necessary to power, command, and control the “payload” that is connected to it. Often the term “spacecraft bus” is used to indicate the fact that this structure is the support for the payload. When the spacecraft and payload have been mated together, the whole unit (at least for a science mission) is the “observatory.”

Hardware that has been connected to support hardware is a “payload.” The instruments (scientific sensors) are the spacecraft’s payload. (For commercial satellites, the payload may be an antenna/transmitter for relaying cell phone signals, or digital TV.) The observatory is the launch vehicle’s payload. A spacecraft bus without its payload is like a person’s brain with only rudimentary sensory input. It can function, and even receive and respond to commands, but it has a hard time interacting with the world around it and will not be very productive. Likewise, it is unusual to expend a launch vehicle without a payload (though this situation does happen sometimes in testing a new launcher).

So, the next time you see that small speck of light moving among the stars, remember: that observatory in space, that satellite of the Earth, is a complex system made up of multiple functional parts, each with their own precise terms. And it has gone through many phases to become what may seem to you to be a simple “satellite.”


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