mjhjmtu

I was perusing wikipedia and saw this awesome infographic:

Original location of infograhpic: Wikipedia

Then it hit me, in all the time I've been browsing space articles and such...

I've never heard of any artificial satellite in a High Earth Orbit.

There are 2 satellites in HEO as of right now...

I love being wrong many, many times... There's 15 according to @CoAstroGeek

My questions are:

• Is this type of orbit just not very useful, or is it just hard to get above 35,000KM above the earths surface in a stable, circular orbit?
  
  
  • Is it a 'more work than its worth' type of situation?
  
  

Specifically, I'm not asking about interstellar staging, etc... kind of want this question focused on current-day technology and what's already there or what's planned to be there. Also, to clarify, I'm talking about significantly beyond GEO/GSO, I want to know what uses an HEO could be used for. Most articles that I find touch heavily on GEO/GSO and state it as the boundary for HEO, but don't talk about HEO.

Upon researching a bit more I've found 2 satellites that fit this bill:

• 
  https://en.wikipedia.org/wiki/Vela_(satellite)
  
  
  • Launched very early on (1967).
  
  
  • 1/3 the distance to the moon.
  
  
  • In a near-circular, low eccentricity orbit.
  
  
  • Probably not the best example for a satellite requiring HEO.
  
  


• 
  https://en.wikipedia.org/wiki/Interstellar_Boundary_Explorer
  
  
  • Launched October 2008.
  
  
  • Possibly a better example of a satellite requiring HEO.
  
  
  • I would love extrapolation on why this satellite functions better in HEO.
  
  

Though the pages don't really state how HEO is required for these to function to any degree.

There are a few other VELA and Explorer satellites, 2 SOLRAD missions and the Japanese GEOTAIL mission from 1992:

| CatalogNum | SatName | IntlDes | Country | Launch | Site | Inclination | Apogee | Perigee | Period |
+------------+----------------------+-----------+---------+------------+-------+-------------+--------+---------+---------+
| 2258 | EXPLORER 33 (AIMP-D) | 1966-058A | US | 1966-07-01 | AFETR | 24.14 | 480762 | 265679 | 38792.4 |
| 6197 | EXPLORER 47 (IMP-7) | 1972-073A | US | 1972-09-23 | AFETR | 13.97 | 232222 | 204011 | 17642.9 |
| 6893 | EXPLORER 50 (IMP-8) | 1973-078A | US | 1973-10-26 | AFETR | 50.55 | 253706 | 181001 | 17553 |
| 8748 | SOLRAD 11A | 1976-023C | US | 1976-03-15 | AFETR | 27.33 | 119054 | 117941 | 7319.39 |
| 8749 | SOLRAD 11B | 1976-023D | US | 1976-03-15 | AFETR | 27.33 | 119521 | 117905 | 7338.38 |
| 2766 | OPS 6679 (VELA 8) | 1967-040B | US | 1967-04-28 | AFETR | 37.17 | 123849 | 99054 | 6708.75 |
| 3955 | OPS 6911 (VELA 10) | 1969-046E | US | 1969-05-23 | AFETR | 61.05 | 150634 | 72080 | 6700.68 |
| 1459 | OPS 6564 (VELA 6) | 1965-058B | US | 1965-07-20 | AFETR | 13.12 | 167834 | 55198 | 6714.3 |
| 2765 | OPS 6638 (VELA 7) | 1967-040A | US | 1967-04-28 | AFETR | 9.58 | 167787 | 55166 | 6710.88 |
| 1458 | OPS 6577 (VELA 5) | 1965-058A | US | 1965-07-20 | AFETR | 22.61 | 171865 | 51411 | 6724.69 |
| 22049 | GEOTAIL | 1992-044A | JPN | 1992-07-24 | AFETR | 12.19 | 190602 | 50635 | 7506.67 |
| 836 | OPS 3662 (VELA 3) | 1964-040A | US | 1964-07-17 | AFETR | 75.71 | 161013 | 45586 | 6024.72 |
| 3954 | OPS 6909 (VELA 9) | 1969-046D | US | 1969-05-23 | AFETR | 51.79 | 178487 | 44067 | 6693.89 |
| 33401 | IBEX | 2008-051A | US | 2008-10-19 | WRAS | 30.22 | 333432 | 43051 | 14240.9 |
| 674 | VELA 2 | 1963-039A | US | 1963-10-17 | AFETR | 26.69 | 177085 | 40568 | 6485.83 |
+------------+----------------------+-----------+---------+------------+-------+-------------+--------+---------+---------+

Be careful in using the acronym HEO - in some communities this means Highly Eccentric Orbit

Borrowed from this answer to the question What artificial satellite has the farthest orbit around the Earth?. You can read more about their orbits in that answer.

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:

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

Here is a quick breakdown of these four "highest of the highs" candidates. There are going to be others as well. It's a diverse group of reasons, there is not going to be one single reason beyond getting far from Earth.

TESS

TESS needs to be far from the Earth most of the time to keep the Earth "out of the picture". From Wikipedia:

In order to obtain unobstructed imagery of both the northern and southern hemispheres of the sky, TESS will utilize a 2:1 lunar resonant orbit called P/2, an orbit that has never been used before (although IBEX uses a similar P/3 orbit)

IBEX

From Wikipedia:

This very high orbit allows the IBEX satellite to move out of the Earth's magnetosphere when making scientific observations. This extreme altitude is critical due to the amount of charged-particle interference that would occur while taking measurements within the magnetosphere. When within the magnetosphere of the Earth (70,000 km or 43,000 mi), the satellite also performs other functions, including telemetry downlinks.

Geotail

From Wikipedia:

The primary purpose of this mission is to study the structure and dynamics of the tail region of the magnetosphere with a comprehensive set of scientific instruments. For this purpose, the orbit has been designed to cover the magnetotail over a wide range of distances: 8 R⊕ to 210 R⊕ from the earth. This orbit also allows us to study the boundary region of the magnetosphere as it skims the magnetopause at perigees. In the first two years the double lunar swing-by technique was used to keep apogees in the distant magnetotail. The apogee was lowered down to 50 R⊕ in mid November 1994 and then to 30 R⊕ in February 1995 in order to study substorm processes in the near-Earth tail region. The present orbit is 9 R⊕ × 30 R⊕ with inclination of -7° to the ecliptic plane."

Spektr-R

From Wikipedia:

Spektr-R's orbit allows it to get as far from Earth as possible in order to produce a long baseline for Very Long Baseline Interferometry (VLBI)

Spektr-R[6] (or RadioAstron) is a Russian scientific satellite with a 10 m (33 ft) radio telescope on board. It was launched on 18 July 2011,[7] by Zenit-3F launcher, from Baikonur Cosmodrome to perform research on the structure and dynamics of radio sources within and beyond our galaxy. Together with some of the largest ground-based radio telescopes, this telescope forms interferometric baselines extending up to 350,000 km (220,000 mi).

One of the biggest ones is TESS, which is using a specific orbit to avoid gravitational interactions with the moon to observe deep space, specifically to look for planets. A few other things that can be done are observing the magnetic field of the Earth, which can be from a very high distance. They could be used to get a fuller picture of the Earth for events that don't require high resolution, such as monitoring nuclear weapons testing.

You could go visit the various Earth-Moon Lagrange points.