# Second derivative of mean motion as indicator of deorbit

I noticed that the second derivative of mean motion (SDMM) is non-zero for only a small percentage of the NORAD catalog. All those objects I checked have reentered since the TLEs I was using were made.

Is the SDMM a reliable indicator of impending deorbit? What else can the SDMM tell me?

• Could you perhabs briefly describe what SDMM (and for this purpose also MM) is? Or even give the (reference to the) definition? Commented May 13, 2020 at 19:16
• SDMM is in the NORAD TLEs for all satellites. See en.wikipedia.org/wiki/Mean_motion for a description of mean motion. The SDMM is approximately the acceleration of the object Commented May 13, 2020 at 20:54
• @Craeft There should be a better way to say that. An orbit is always accelerating towards the body it orbits, otherwise the motion would be a straight line. Drag continuously lowers LEO's and therefore continuously increases their mean motions in orbits per day, The second derivative of the mean motion is the rate that this continuous increase is itself increasing.
– uhoh
Commented May 16, 2020 at 22:00
• As far as "Is the SDMM a reliable indicator..." Depending on their source, individual TLEs are not particularly reliable for anything. People who need reliable data on spacecraft trajectories and future projections thereof generally subscribe to more advanced sources of state vectors and orbit solutions. So an answer to your question would have two parts, 1) the relationship between actual acceleration of mean motion and altitude and drag, and 2) the reliability of the SDMM parameter in any given publicly available TLE.
– uhoh
Commented May 16, 2020 at 22:05