Catalog of Space Debris Clouds - Plot Descriptions
KEY - Blue: initial elements; Green: current elements if still up; Black: last elements if reentered.
For each debris cloud four sets of plots are provided, described below.
We define the following concepts:
- Initial orbit - the earliest available orbital elements for an object.
Note that debris objects are often cataloged years after the event, and so
because of atmospheric drag this `initial orbit' may be significantly
different from the true initial orbit of the object immediately after
the debris event. But, it's the best we can do!
- Final orbit - the last orbital data for an object prior to its reentry
or prior to when it became lost.
- Current orbit - the most recent orbital data at the time the catalog was updated,
for objects still in orbit.
- Height - the orbital height of a debris object in km, meaning
the mean of its perigee and apogee, i.e. (A+P)/2 or, equivalently, equal
to the semi-major-axis minus the Earth equatorial radius, (a-Re).
- Inc - the orbital inclination in degrees relative to the Earth equator of date.
- Spread - the value S = (A-P)/2 = ae, half the difference between apogee and perigee
heights. I use S in km rather than the dimensionless eccentricity e because
I have more of a sense of what a given value means.
- Node - the right ascension of the ascending node of the orbit in degrees.
The orbital node precesses with time, so comparing node values for objects measured
at different times isn't useful. Initial orbit node values are propagated
back to the event time using a simple first order J2 correction and current/final
values are propagated to the present (i.e. epoch of catalog update).
- Apse - I use this term to refer to either the perigee or apogee height. For example,
an object in a 200 x 500 km orbit has two apse values, 200 and 500.
In a debris cloud whose parent was in a circular orbit at height h,
the initial orbits of the debris objects usually
have either their apogee or their perigee equal to h. I make a histogram
of these apogee and perigee values (lumped together) and find the mode of that
histogram (in km). This ApseMode value is often a good estimate of the height at
whch the debris event occurred.