2015 Apr 26
Sunday, Day 116
Kwangmyongsong 3-2 - Orbit Evolution
NOTE - if you are looking for information concerning North Korea's missile threats during 2013 April, there are some basic notes here.
In the first few weeks after launch, monitoring of the basic orbit parameters may reveal some new facts about the payload, the rocket and the fragments that accompany it in orbit.
This plots the nodal orbital period (equator crossing to equator crossing) of the four objects so far catalogued. At the start, things are a little confused as SpaceTrack tries to refine its orbital element measurements and differentiate between the four objects, but then life settles down.
Unusually, the third stage rocket body is in a higher orbit than the satellite, suggesting there was a separation manoeuvre by the rocket.
Rate of Decay
The lines on this plot indicate how each object is reacting to air drag. The rate of decay is proportionate to the mass/cross-section ratio that a satellite presents to the upper atmosphere. Denser objects are much more resistant to the thin trace of air, where orbits of low mass/area objects decay much more quickly.
One thing obvious from the start is that decay rates for the two debris items are very close to being the same value. It probably indicates that they are a pair of similar items.
NOTE - the occasional 'spike' in a line is due to small errors in SpaceTrack's data and should be ignored.
Apogee and Perigee
The greatest effect of air drag on an elliptical orbit is that the slightly higher amount of drag at perigee tends to reduce apogee while having very little effect on the height of perigee itself. Any changes in the orbit will reflect more obviously in the apogee value.
Perigee values of the objects are tightly bunched as a result of the separating from each other as the Unha 3 entered orbit. Of the four, the object 2012-072D/39029 was first catalogued about 24 hours after launch, where SpaceTrack had the other three listed right from the start. Actual initial values for both apogee and perigee can be seen in the launch summary further down the page.
Summary of the Launch
A point to note about the two debris items is that SpaceTrack has catalogued them as items that came from the Unha 3 launch vehicle rather than the satellite. They have the same perigee as the rocket, suggesting that they came off at satellite separation. Their inclinations differ from that of the rocket by ±0.04° respectively suggesting they might have been ejected symmetrically sideways but their apogees are not distributed evenly either side of the rocket.
They may be clamping bands that held the satellite to the rocket body, or they may be another part of the separation mechanism. The fact that their orbits have similar decay rates reinforces the idea of them being a pair.
Charts on this page are produced using JpGraph.
Page date: 2012 Dec 15
Updated: 2013 Feb 28
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