The structure of space: orbital families

click diagram to enlarge

Asteroid observers are well acquainted with the kind of diagram above: a plot of the semi-major axis of the orbit against orbital inclination. Doing this for asteroids allows to discern resonances, and clusters visible in such a diagram point to related objects with a shared origin (asteroid 'families').

The diagram above is however not showing asteroids in heliocentric orbits, but is a similar diagram showing orbits for all 18439 well-tracked artificial objects (satellites, rocket stages and debris) in orbit around our Earth. A number of clusters can be seen: the distribution of the objects in a-i space (*) is not random but structured.

The structure corresponds to satellites with a specific purpose (and the related rocket stages and debris), or from a specific family. Some functions of satellites demand a specific type of orbit distinguishable in a-i space.

Well recognizable clusters for example in the plot above, are Geosynchronous satellites; and satellites in HEO ('Molniya') orbit. These are often communication or SIGINT satellites. NAVSTAR navigation satellites (GPS) form a recognizable cluster too.

Two loose clusters of objects can be seen that correspond to Geostationary Transfer Orbits (GTO). These are the rocket stages left from launches into Geostationary orbit. They move in eccentric orbits with low inclination. Two groups can be discerned: those launched from Kourou in French Guyana by ESA, and those launched from Cape Canaveral by NASA and NRO. The fact that these two groups group and distinguish in inclination, is because the inclination of GTO launches correlates to the latitude of the launch site.

Some clusters are debris clusters which are the result of the breakup of objects (usually exploding rocket stages) in space: two of these are indicated in the plot above.

Interesting is also the cluster that represents Earth Observation satellites in sun-synchronous Polar orbit. Let us look at this part of the plot in more detail:

click diagram to enlarge

Sun-synchronous objects are objects in orbits designed to have a rate of RAAN (node) precession that matches the precession of the sun in Right Ascension. This is beneficial to optical remote sensing observations of the earth, as it means the orbital plane moves along with the shift in Right Ascension of the sun, thus ensuring that images are made around the same solar time each day, which aids shadow analysis.

The objects in this cluster display a clear obliquely slanted trend in a-i space. This is because the sunsynchronous character of an orbit is a function of semi-major axis, eccentricity and orbital inclination. Hence, a specific orbital inclination is necessary for each orbital altitude, causing the slant in the distribution in the plot above.

[EDIT 19 oct 2019, 21:55 UT]

In the diagram below, the black line is the theoretical trend in a-i space for a circular sun-synchronous orbit. For more elliptical orbits, the slant of the line is slightly different:

click diagram to enlarge

I am not entirely sure what is behind the noticable gap visible in the distribution around inclination 101 degrees. The upper sub-cluster around 102 degrees inclination contains a number of meteorological satellites, plus debris from associated, broken up rocket stages, so it might be a sub-cluster representing a specific family of satellites

A couple of other object 'families' can be seen in this detail diagram as well, as distinct clusters. There is another breakup event visible (Kosmos 1275, a Soviet navigation satellite that disintegrated in orbit some 50 days after launch), as well as two payload families, including the Iridium satellites. The Westford Needles are tiny metal rods that are the result of a weird,  ill-conceived and eventually abandoned communication experiment during 1961 and 1963 (read more here).


* note: a-i means: semi major axis (a) versus orbital inclination (i)