Overcrowding Not a Problem in Vast Medium Earth Orbit
PRAGUE, Czech Republic — Medium Earth orbit, which later this decade is likely to be populated with 105 operational navigation satellites that will be disposed of and replaced on a regular basis, does not merit the special-status treatment accorded low Earth and geostationary orbit, NASA’s chief orbital-debris scientist said.
While these four navigation constellations — the U.S. GPS, Russia’s Glonass, Europe’s Galileo and China’s Beidou/Compass — are almost certain to be replaced, medium Earth orbit (MEO) is such a large region it does not need the same treatment as the more-populated lower and higher orbits, according to Nicholas L. Johnson. Johnson said he was expressing a personal opinion that is not necessarily NASA’s official position.
In a Sept. 29 presentation here during the 61st International Astronautical Congress, Johnson nonetheless urged nations with navigation systems to coordinate their efforts to reduce the already-minute chance of satellite collisions, and to promote proper disposal of satellites being retired.
One idea that has been floated — assigning each constellation a specific operating locale and specified satellite-disposal orbits — runs head-on into the United Nations’ Outer Space Treaty, which classifies the space environment as a global commons.
“Outer space, including the moon and other celestial bodies, is not subject to national appropriation by claim of sovereignty, by means of use or occupation, or by any other means,” the treaty reads in part.
Medium Earth orbit is currently defined as the region between 2,000 and 36,000 kilometers in altitude. In terms of volume, it is 150 times the combined size of low Earth orbit and geostationary orbit, both of which have been subject to special measures called for by a group of spacefaring nations. Only 5 percent of today’s operational satellites are stationed in medium Earth orbit.
“It’s safer to be in MEO than in the other two,” Johnson said.
That is not likely to change anytime soon, although sponsors of the four navigation systems will be retiring satellites and launching new ones at a rate of between seven and nine per year assuming a 12- to 15-year operational life for each spacecraft.
The only orbital debris issue in medium Earth orbit so far has been the leftovers of the U.S. Air Force’s Project West Ford of the early 1960s, which was designed to launch several hundred million copper needles, each about 1.8 centimeters long, into a 3,700-kilometer orbit. Fanning out over the orbital arc, the needles would create a rudimentary reflector antenna that would survive jamming attempts.
The needles were supposed to burn up on re-entry into the Earth’s atmosphere after three years, but a deployment failure caused some to remain in orbit much longer. Johnson said NASA’s latest assessment is that two-thirds of the West Ford debris has been removed by natural forces.
The navigation satellite constellations operate in a much higher section of medium Earth orbit between 19,100 and 23,200 kilometers. The only disposal option is to boost the spacecraft into a higher orbit, as operators of telecommunications satellites in geostationary orbit are asked to do by nonbinding international agreements.
Managers of the U.S. GPS and European Galileo systems say they will abide by promises to remove their spacecraft from the operating orbits by placing them at higher altitudes, although there appear to be no formal agreements on what constitutes a “graveyard” in medium Earth orbit.
Russia’s Glonass system, at 19,100 kilometers in altitude, would need to boost its spent satellites by more than 4,000 kilometers to clear Europe’s Galileo constellation, operating at 23,200 kilometers, although the two systems’ satellites are at different inclinations relative to the equator.
Performing a maneuver that is recommended for low Earth orbit — that is, pushing the satellite into a slightly lower orbit so that it is pulled into the Earth’s atmosphere within 25 years — is not feasible in medium Earth orbit because of the amount of energy it would require of each satellite’s propulsion system.
Medium Earth Orbit at a Glance
- The MEO region is currently used primarily by global navigation satellite constellations:
Space system Mean altitude Inclination Nominal operational Spacecraft launched
(kilometers) (degrees) spacecraft from 2000 to 2009
Glonass 19,100 65 24 36
GPS 20,200 55 24 18
Beidou/Compass 21,500 56 27 1
Galileo/GIOVE 23,200 56 30 2
- A MEO region communications network (ICO: 10,400 kilometers) was designed in the 1990s, but only one spacecraft has been successfully deployed (2001).
- From 2000 to 2009, on average, only about two spacecraft were launched annually into elliptical orbits passing through the MEO region.
- Many more objects (for example, rocket bodies and mission-related debris) are left in geosynchronous transfer orbits each year, passing through the MEO region.