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How stable are graveyard orbits? How big of an issue is orbital decay?
As the wikipedia article states, the graveyard orbit is higher than the synchronos orbit from which the satellites are disposed of. A lower graveyard orbit would make little sense, since you need to cross it when bringing new satellites into position. However, orbital decay might lead to dead satellites entering the "good" synchronous orbit again. I guess this is accounted for, has somebody an estimate?
The answer to your question depends on which part of "stable" you are interested in. The height of the orbit does not change much, atmospheric drag can be neglected almost completely. The orbital lifetime is in the order of many many thousand, possibly millions of years.
On the other hand, the precise orbit is not stable but varies over time. Gravitational influence of the Sun and Moon cause a change of inclination by almost one degree per year - see Why is the ribbon of decommissioned geosynchronous satellites skewed?. If you want to dive into the dynamics of these orbits, here is a nice PDF with details.
Another major influence on the orbit is the radiation pressure from the Sun. As the orientation of the decommissioned satellites is not fixed, the pressure can be estimated to be equal in all directions (averaged over time). Hence, it does not lead to orbital decay, but orbital perturbations only. For this reason, the height of the graveyard orbit is adapted to the surface-to-mass ratio of the satellite: The higher the expected influence of radiation pressure, the higher the desired orbit.
In summary, a typical graveyard orbit is about 300 km above GEO, consisting of
Orbital decay due to atmospheric drag is not an issue at geosynchronous altitude. According to NOAA, drag is a factor worth considering for altitudes below 2000 km. Drag is not constant:
The drag force on satellites increases during times when the Sun is active. When the Sun adds extra energy the atmosphere the low density layers of air at LEO altitudes rise and are replaced by higher density layers that were previously at lower altitudes. As a result, the spacecraft now flies through the higher density layer and experiences a stronger drag force. When the Sun is quiet, satellites in LEO have to boost their orbits about four times per year to make up for atmospheric drag. When solar activity is at its greatest over the 11-year solar cycle, satellites may have to be maneuvered every 2-3 weeks to maintain their orbit 1.
In addition to these long-term changes in upper atmospheric temperature and density caused by the solar cycle, interactions between the solar wind and the Earth’s magnetic field during geomagnetic storms can produce large short-term increases in upper atmosphere temperature and density, increasing drag on satellites and changing their orbits. The North American Aerospace Defense Command (NORAD) has to re-identify hundreds of objects and record their new orbits after a large solar storm event (Figure 2). During the March 1989 storm event, for example, the NASA's Solar Maximum Mission (SMM) spacecraft was reported to have "dropped as if it hit a brick wall" due to the increased atmospheric drag.
The NRLMSISE-00 atmosphere model uses actual drag data from satellites.
