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What is the furthest Earth orbiting satellite?
What is its speed and purpose?

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    $\begingroup$ Pretty sure it's the moon... :P $\endgroup$ – Mehrdad May 11 '18 at 8:44
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    $\begingroup$ How are you defining "highest orbit"? Highest apoapse? Highest periapse? Highest mean orbital altitude? $\endgroup$ – Baldrickk May 11 '18 at 10:43
  • $\begingroup$ So highest apoapse then. It's good to be clear :) $\endgroup$ – Baldrickk May 11 '18 at 10:58
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    $\begingroup$ It is not the moon. $\endgroup$ – Muze May 12 '18 at 16:48
  • $\begingroup$ It was the moon for the original question ;). $\endgroup$ – Magic Octopus Urn Apr 17 at 17:50
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I don't have a list to find the highest, but I suspect that Spektr-R RadioAstron (used for long baseline radio interferometry) is one of the highest altitudes that isn't associated with a Lagrange orbit.

The moon interacts with its orbit, so the apogee changes over time. According to the User Handbook

the apogee distance will vary from 286,938 to 371,233 km

Spektr-R RadioAstron photo

It has a very high eccentricity, so I believe there are others with a greater semi-major axis. I'm not sure which measure the question might want for "farthest out"

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    $\begingroup$ Spektr-R is quite ahead of its time! There will be much more radio astronomy done from Spacecraft some day. $\endgroup$ – uhoh May 11 '18 at 7:45
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I found the following "far out" spacecraft:

  • TESS (Transiting Exoplanet Survey Satellite) recently launched, not in final orbit yet
  • Spektr-R
  • IBEX or Interstellar Boundary Explorer
  • Geotail

Here are there IDs:

TESS       43435    2018-038A
Spektr-R   37755    2011-037A
IBEX       33401    2008-051A
Geotail    22049    1992-044A

I downloaded both TLEs and orbit data from JPL Horizons in order to piece together this qualitative data.

The problem is that for such high orbits, the gravity of the Sun and the Moon can push them around significantly so their obits change over time, sometimes by quite a bit!

The all-time winner (of the four that I found) seems to be Geotail. Using historical TLEs shows Geotail's maximum semi-major axis of about 280,000 km or about 44 Earth radii, and a maximum apoapsis of orver 500,000 km or about 81 Earth radii. However, according to Wikipedia, Geotail's maximum distance from the Earth reached 210 earth radii, or over 1,300,000 km from the Earth! This would not likely have been long term stable, and so after sampling the tail of the magnetosphere out there it was ramped down closer to Earth.

I have two plots for TESS, both current data from TLEs and future data (the big DOT) after it will use a close swing-by maneuver with the Moon and then another propulsive maneuver in order to reach it's half lunar month orbit. Once that happens, TESS will be the longest period artificial satellite around the Earth, at least one with a fairly stable orbit and whose information is available publicly.

TESS has this orbit in order to spend most if its time staring at nearby stars looking for exoplanets, then it makes a close pass by Earth to download data, once every two weeks.

You can read more about how TESS' orbit works in this answer to the question TESS orbit and moon resonance.

I've put a plot of TESS' calculated orbit from Horizons below. The green, tightly repeating orbit is the Moon's. The red orbit, inclined, evolving, changing orbit is for TESS only for a few years currently in the Horizon's simulation. It's almost a miracle that it can remain so close to it's orbit. Well, it's "just F=ma" (roughly), but it's still beautiful!

TESS might represent the highest non-Lagrangian Geocentric orbit that is stable over decades. It was carefully designed to have half the period of the Moon in order to cancel out perturbing effects. It is called a 2:1 resonant orbit. For orbits higher than that, lunar perturbations may become problematic.

Earth-Moon Lagrange orbits are Geocentric orbits that are in 1:1 resonance with the Moon (they are not lunar orbits). They will have their own stability issues and require station keeping. A particularly stable Geocentric orbit which is associated with the Earth-Moon Lagrange points is the Near Rectilinear Halo Orbit. Read more about it in the questions and their answers:

The Halo orbit associated with the Earth-Moon Lagrange L1 and L2 points are probably the highest Geocentric orbits that are also usefully stable. Instead of being perturbed by the Moon's gravity, they remain in resonance with it and use it to provide additional stability. However, they still require station keeping.

enter image description here

below: I've put a plot of TESS' calculated orbit from Horizons below. The green, tightly repeating orbit is the Moon's. The red orbit, inclined, evolving, changing orbit is for TESS only for a few years currently in the Horizon's simulation.

TESS' orbit from Horizons

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  • $\begingroup$ What orbits is in your last picture? $\endgroup$ – Muze May 11 '18 at 9:59
  • $\begingroup$ It's TESS and the Moon around the Earth. I described it earlier but I'll move it closer. $\endgroup$ – uhoh May 11 '18 at 10:00
  • $\begingroup$ Thanks I think that answers my other question as well. The green one is like the one I drew. $\endgroup$ – Muze May 11 '18 at 10:05
  • $\begingroup$ @Muze I updated the drawing. The Moon's orbit looked elliptical because the scales were not equal. Once I required X, Y, and Z scales to be equal, you can see the Moon's orbit is now a circle. $\endgroup$ – uhoh May 11 '18 at 10:10
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    $\begingroup$ In the last plot, the moon's orbit seems to be much more repeatable in the bottom left corner, and much more variable elsewhere. Is this a genuine effect, or an artifact of the simulation? $\endgroup$ – Martin Bonner May 11 '18 at 11:41
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It’s a bit of a cheat, but a satellite at L4 or L5 is roughly 100 million miles from Earth, quite stable, and has a one year period to its motion around Earth. That’s seems to be the farthest stable & orbit-like setup.

Whether one considers L4 and L5 “orbiting the Earth” is a question for another day...

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    $\begingroup$ No, today is a good day. These are not true orbits around the Earth. They are definitely Heliocentric orbits that happen to be in loose 1:1 resonance with the Earth. If the Earth suddenly disappeared, they would continue to move in nearly the same orbit around the Sun, with only a small change in orbital parameters, few percent at most. Venus "goes around" the Earth every 584 days, but we don't say it "orbits" the Earth. $\endgroup$ – uhoh May 11 '18 at 4:51
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    $\begingroup$ @uhoh The distance and angle to Venus shows complex motion, including retrograde motion. On the other hand, viewed from Earth, the celestial position of L4 moves more regularly than the Moon. Yes, the force of the sun on an L4 object is larger than the force of the Earth, but that’s also true of the Moon. So while I agree that the Earth isn’t at the focus of a Keplerian ellipse for L4, there certainly are reasons to think of then as an answer to the question. YMMV. $\endgroup$ – Bob Jacobsen May 11 '18 at 5:34
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    $\begingroup$ If the Sun disappears, the Moon stays in orbit around the Earth very nicely. If the Sun disappears, an object at Sun-Earth L4 or L5 will not stay in any kind of orbit with the Earth and will just fly away as if it had never really known the Earth on a first-name basis. These Earth-Trojan orbits are so strongly perturbed by Venus that they aren't very real at all in fact. $\endgroup$ – uhoh May 11 '18 at 7:42
  • $\begingroup$ I consider your answer to be fully within the "spirit" of the question. The OP did ask for an Earth orientated answer. A Lagrange point (some at least) seems to me reasonable option for a very large, very slow "orbit around the Earth" from any practical point of view. Calling it heliocentric, although true, does not invalidate this answer and it's probably more of a nomenclature issue than an objective space faring concept. $\endgroup$ – armatita May 11 '18 at 12:15
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    $\begingroup$ I agree with uhoh here. Whether a Sun-Earth libration orbit is an Earth-centered orbit is precisely relevant to the question at hand, and in any reasonable projection it's obvious that such an orbit is not Earth-centered. See e.g. en.wikipedia.org/wiki/2010_TK7 It is not a nomenclature issue, it's fundamental. $\endgroup$ – Erin Anne May 11 '18 at 18:52

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