# What is the highest non-equatorial earth orbit used?

When looking at the operational altitude of satellites, it seems that the highest one is GEO (36000km), which is an equatorial orbit. Non-equatorial orbits are less high (from LEO to 20000km for GPS).

Is there an operational satellite orbiting Earth in a higher non-equatorial orbit than the GPS satellites? If so, what is its altitude?

• The graveyard orbit is always above it, but since you refer to "operational" satellites this doesn't apply. Oct 7, 2017 at 10:22
• Question. If there were to be an operational satellite farther from GPS but in a non-equatorial orbit, in what orbit would it be? Isn't there a reason why all GEO satellites are above the equator? Oct 7, 2017 at 10:39
• @Matthew: The reason is so they aren't just geosynchronous, but geostationary; non-equatorial geosynchronous orbits describe a figure eight on the ground track, which makes antenna pointing more complicated. A non-geosynchronous orbit (higher or lower) doesn't care, since antenna pointing is guaranteed to be complicated anyway. Oct 7, 2017 at 16:05

## 2 Answers

Spektr-R and IBEX are both dancing with the Moon. They are both in high Earth orbits that are strongly affected by perturbations from the Moon and possibly the Sun. According to Wikipedia:

In June 2011 IBEX shifted to a new orbit that raised its perigee to more than 30,000 kilometres (19,000 mi). The new orbit avoids taking the spacecraft too close to the Moon, whose gravity can negatively affect IBEX's orbit. Now the spacecraft uses less fuel to maintain a stable orbit, increasing its useful lifespan to more than 40 years.

You can see by the plots below that both Spektr-R and IBEX have semi-major axes that vary between about 170,000 and 200,000 kilometers. But while IBEX's eccentricity varies between about 0.4 and 0.65, Spektr-R's eccentricity varies betwen about 0.6 and 0.95! Recently it seems to be the case that IBEX reaches a higher apoapsis than Spektr-R, and IBEX has historically had the higher semi-major axis and longer period of the two. Recently however (2016), Spektr-R's semi-major axis has spiked from about 170,000 to 200,000.

Due to a reduction in TLEs for both spacecraft starting in 2017, it's going to be a difficult job to say which is in the "higher" orbit definitively in the future.

Orbital parameters deduced from TLE dumps:

The highest that I can find record of ever was the AIMP-D, which "perigee varied between 6 and 44 earth radii". Currently it's perigee is about 265,000 km.

The furthest operation is probably Spektr-R, with a perigee of 10,000 km and an apogee of 390,000 km initially, and I now understand it to have a perigee of 64,000 km, due to gravitational interaction with the Moon.

• Concerning your now deleted comment below about DSCOVR, wouldn't a satellite orbit Earth at, or very near, Lagrange distance (~4 Lunar distances or ~1% of an AU), if it were in a polar orbit? If the satellite's orbital plane keeps facing the Sun, it would never get near any Lagrange point. Oct 8, 2017 at 5:51
• @uhoh It depends on definitions , and I assume there are no classified payloads that are further out. I used the perigee to make my determinations. Oct 8, 2017 at 10:24
• The 64,000 km was a perigee, I've made that clearer. Oct 8, 2017 at 14:56
• OK I see, ya the perigee is all over the place - maybe it will hit the atmosphere some day? It looks like it's dropped as low as about 1,100 km altitude.
– uhoh
Oct 8, 2017 at 15:05
• I imagine that they keep the perigee at some minimum. When we lose contact with it, anything could happen, including hitting the Earth's atmosphere. Oct 8, 2017 at 15:18