Rise or Fall
2013 May 20
Monday, Day 140
Phobos-Grunt - Orbit Watch
From launch, Phobos-Grunt was either venting gases or using its thrusters to attain a particular orientation. Either way, the thrust produced had an effect on the orbit. It can be seen in the plots of orbit parameters below.
Whatever the event(s), the thrusting ceased around Nov 20 and evolution of the orbit settled down, showing the usual signs of decay through air drag. The possible reasons and a discusssion of the changes can be found in the archive of the Seesat-L satellite oberver group. There is a link in the left-hand menu.
The Δv imparted by the episode was about 2m/s in total and, as evidenced by perigee rising but apogee falling very much as expected, the thrust was generally being applied near apogee when Phobos-Grunt was in sunlight. This fits nicely with the Russian statement that Phobos-Grunt was in "emergency mode" and systems were switched off upon entering the Earth's shadow.
On November 29, Spacetrack catalogued a piece of debris (2011-065G/30740) that had detached from Phobos-Grunt, it was listed as having re-entered the same day. The following day, a second debris object (2011-065H/30747) appeared also with a high rate of orbital decay. 2011-065H had no element sets after December 1 but it was another five days before Spacetrack issued a decay notice dated December 2.
They are plotted on the Orbital Period chart. 2011-65G is the very short line/dot near the top - there were two very similar element sets issued for precisely the same epoch. Several element sets were issued for 2011-065H. The density of both was low, hence the rapid decay of their orbits. Extrapolation indicates that 2011-65G separated November 29 around 03:00 UTC and 2011-065H departed amost exactly 24 hours later on November 30, also around 03:00 UTC.
The possibility exists that there may have been other debris that was more difficult to detect, or re-entered very quickly, and so failed to enter the public catalogue because NORAD was unable to get enough orbital data.
Looking back at the way the two pieces departed, 2011-065G seems have been ejected somewhat more explosively than first thought. 2011-65H just seems to have drifted away.
Orbits heights of the two fragments are shown in the table above. Although 2011-065H is lower than Phobos-Grunt, it is consistent with having decayed through air drag in the twelve hours or so between separating and being catalogued. The orbit of 2011-065G is significantly different from Phobos-Grunt. It is a different shape and the inclination is different. Even without knowing precisely what happened, it is possible to estimate the minimum Δv involved.
In order to move between the two orbits there was an in-plane velocity change in the region of 14 metres per second. The 0°.12 change in inclination would have required at least 15 m/s of velocity at right angles to the orbit. Adding them together results in a minimum separation speed of 20 m/s. This is more of an explosive velocity.
In summary, it suggests an low-pressure explosive event (ruptured tank or battery?) but nothing violent enough to cause Phobos-Grunt to tumble in a way that could not be compensated by its attitude control system. As it departed, 2011-065G may have dislodged something else that became the second fragment when it eventually drifted away one day later.
Observation to to the end showed that Phobos-Grunt was keeping itself steadily oriented, and up to the beginning of January was probably pointing towards the Sun as it would be expected to do if it were en-route to Mars. It certainly ws not "tumbling" as erroneously reported by the Planetary Society in a story on its Home web page.
Some observers reported a periodic variation and others that it had a steady appearance and there were occasional, but predictable, flares from flat surfaces - indicating stability. Phobos-Grunt was an elongated shape with solar panels so variation in brightness was to be expected as the viewing aspect changed. Russia was been trying to fire the engine to raise the orbit. This is not something that could have been contemplated if unpredictable tumbling was a fact.
Jan 4, Thierry Legault published a very distinctive image of Phobos-Grunt. Together with images obtained 24 hours earlier, it showed Phobos-Grunt stable, and moving in what Ted Molczan described as "shuttlecock mode". The major mass of the propellant tanks at one end of the craft was leading, with the solar panels acting like 'feathers' to keep it stable through aerodynamic drag.
Perigee and Apogee
A point to note is that during the early-day period when Phobos-Grunt appeared to be operating its thrusters, the obvious change was the increasing height of perigee while apogee continued to decline as would be expected with orbital decay through atmospheric drag - see the notes on Orbit Evolution, above.
Perigee is traced in blue and apogee in red.
Rate of Decay
This is the value 'ndot2' from the Twoline Orbital Elements sets. The unsettled nature of the first few days' readings is down to thruster operation. The effect ceased around Nov 22.
After that, the occasional spike indicates an error or an uncertainty in the Spacetrack data. Ignore them, and the general trend is obvious, including a slow-down for several days from Dec 10. It possibly results from Phobos-Grunt orienting itself on the Sun and presenting a smaller cross section to the direction of travel..
Argument of Perigee
Towards the end, the Argument of Perigee started to drift from the smooth line that it had been following. The reason was that the apogee and perigee figures derived from the Twoline Orbital Element (TLE) sets relate to a standard model that assumes the orbit focus is the centre of a perfectly spherical globe. It is the reason that orbit data issued by Russia seem to differ from what is published elsewhere. Russia always quotes maximum and minimum heights above the Earth's true surface.
Because the Earth is pear-shaped, the actual location of the focus of the orbital ellipse is about 14 kilometres south of the equator, roughly on the Earth's north-south axis. At the 51° inclination of Phobos-Grunt's orbit, its true height above the Earth's surface (and, more-importantly, the upper atmosphere) while at its most northerly point is about 10km less than the TLE values would suggest, and the opposite is true in the southern hemisphere. As a result, once the difference in apogee and perigee height drops below 20 km, TLE-derived figures can start to lose their meaning.
In an extreme case, a TLE could indicate that the orbit is nearly circular with perigee at the most southerly point. The reality is that the satellite actually dips deeper into the atmosphere near northern apex of the orbit. As a result, changes in Eccentricity and Argument of Perigee can start to look random rather than following the smooth graph plot of an eccentric orbit.
This is the reason that comparisons should only be made between figures derived from the same sources or that are known to use the same orbital model.
Charts on this page are produced using JpGraph.
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