Graveyard Orbit

A Graveyard Orbit (Junk Orbit, Disposal Orbit) is an orbit that lies significantly away from common operational orbits, where spacecraft can be placed at the end of their operational life. Most commonly, it refers to a supersynchronous orbit that lies significantly above synchronous orbit. Satellites are moved  into such orbits to reduce the probability of colliding with operational spacecraft or generating space debris. A spacecraft moved to a graveyard orbit will typically be passivated.

A graveyard orbit is used when the change in velocity required to perform a de-orbit maneuver is too large. De-orbiting a geostationary satellite requires a delta-v of about 15,000 meters per second (4,900 ft/s), whereas re-orbiting it to a graveyard orbit only requires about 11 meters per second (36 ft/s).

For satellites in geostationary orbit and geosynchronous orbits, the graveyard orbit is a few hundred kilometers above the operational orbit. The transfer to a graveyard orbit above geostationary orbit requires the same amount of fuel that  a satellite needs for about three months of stationkeeping. It also requires a reliable altitude control during the transfer maneuver. While most satellite operators try to perform such a maneuver at the end of their satellites' operational lives, through 2005 only about one-third succeeded. However, as of 2011, most recently decommissioned geosynchronous spacecraft were said to have been moved to a graveyard orbit. 

According to the Inter-Agency Space Debris Coordination Committee (IADC) the minimum perigee altitude, `DeltaH`, above the geostationary orbit is:
`DeltaH = 235+(1000C_RA/m)`, where:

• `C_R` = solar radiation pressure (typically between 1.2 and 1.5 N/m^2 or Pa)
• `A` = aspect ratio
• `m` = mass

References

Wikipedia (https://en.wikipedia.org/wiki/Graveyard_orbit)