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According to this CNET article,

The last time a human broke free from Earth's orbit was the Apollo 17 mission in 1972.

I thought that the moon orbited the Earth, and that visits to it were still within Earth's orbit, as with anywhere else in Earth's gravity well that's not going fast enough to depart. Is CNET just wrong, or am I missing something?

On a search of this site, I only found this suggesting that higher than 200 miles might be considered "beyond Earth's orbit," but that seems to be a pretty unreliable source with nothing to back it up.

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closed as primarily opinion-based by David Hammen, Hohmannfan, Rikki-Tikki-Tavi, Nathan Tuggy, ForgeMonkey Dec 13 '16 at 9:40

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ Would you say that the ISS is orbiting the Sun? $\endgroup$ – chirlu Dec 11 '16 at 18:28
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    $\begingroup$ I would not say it had broken free from solar orbit. Am I wrong on that? $\endgroup$ – WBT Dec 11 '16 at 18:38
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    $\begingroup$ If one is in orbit around the Moon, do you consider that to be in Earth orbit? $\endgroup$ – Organic Marble Dec 11 '16 at 18:53
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    $\begingroup$ I'm voting to close this question as off-topic because the OP is picking a ridiculously small nit. And asking an opinion-based question, to boot. $\endgroup$ – David Hammen Dec 12 '16 at 9:44
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    $\begingroup$ It might have been more aptly phrased as: "the last time a human traveled beyond low Earth orbit...", since the Moon is in Earth orbit, the Apollo lunar transfer trajectory was still a closed orbit bound to the Earth-Moon system, and the greatest altitude any human has attained since the Apollo program falls within the definition of "low Earth orbit" (not more than 2,000 km). $\endgroup$ – Anthony X Dec 12 '16 at 20:43
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The CNET article is wrong, I think what it is trying to say is the last time humans left Low Earth Orbit (LEO) was in Apollo 17, which is completely true. Apollo spacecraft did not accelerate enough to leave earth's gravity well, they accelerated enough to get to the point that the moon's gravity well became stronger than the Earth's.

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I think you are right to be troubled by the phrase "beyond Earth's orbit" because it's a poor choice of words that muddies what's actually a slightly complicated situation when your looking at three bodies - in this case the Sun, the Moon, and the Earth.

So lets forget the "beyond" part for a moment because that implies a certain distance or at least a certain clearly delineated boundary. Even if you treat the Sun Earth Moon system as a 2D plane, that boundary would probably have to be drawn in four dimensions - at each point x,y there can be some velocity vectors vx, vy that will keep a spacecraft going around the Earth, while another vx, vy might put it free of the Earth to orbit the sun, and another vx, vy that will put it in at least a temporary orbit around the Moon. In the real 3D space it's a boundary in six dimensions in real space. In a CR3BP - a mathematical idealized system with perfectly circular orbits, it's fewer than six, but that's not the real world in a number of ways.

So I would propose you just ignore the literal interpretation of "beyond" as simply greater than a certain distance. Think of it as no longer orbiting just the Earth alone. Orbiting a body can be though of as simply "goes around and around" the body, and so you are right, a spacecraft orbiting the moon is also orbiting the Earth, and is also orbiting the Sun, and is also orbiting the center of the galaxy.

And there are fairly complicated shaped orbits that look like a hybrid between an Earth orbit and a Lunar orbit, or even an orbit which literally stays between the Earth and the Moon and those orbits get named after dead mathematicians, or athletic feats like (just for example) "Lyapunov" and "back-flip or Lunar Cycler" orbit.

But when a spacecraft starts circling the moon, going around and around the moon, it is really common for people to say that it is no longer orbiting the Earth. It certainly is also in orbit around the Earth, but usually people will say it's "primary" is now the Moon since that's the body it is now most closely associated with now.

Experienced "rocket scientists" know what each other mean when they say it's not in orbit around the Earth any more, and in the back of their mind they still know it is indeed orbiting the Earth.

So to answer your question: There is no simple, absolute, closed boundary in 3D (x, y, z) space where you can point and just say anything on one side is definitely in Earth orbit, and anything on the other side is definitely not in Earth orbit.

Let me know if this helps!

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  • $\begingroup$ The Voyager space probes are not in Earth orbit now, but you can't tell when and where they left Earth orbit. The same is true for New Horizons and other space probes very far away from Earth. $\endgroup$ – Uwe Aug 28 at 15:07

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