Sunday, November 03, 2013

GOCE below 200 km - now one week or less from reentry

Over the past 12 days I have been covering the demise of GOCE, ESA's Gravity Field and Steady-State Ocean Circulation Explorer. Following the cut-off of its ion engine on October 21st, it is coming down, at an increasing speed. It is heading towards an uncontrolled re-entry later this month, perhaps even within a week from now.

click diagram to enlarge

The diagram above shows the increasing rate at which the orbital altitude is dropping since October 21st. As I write this, the drop rate has increased to about 4 km/day (from initially about 1.5 km/day on October 21st) and that rate is increasing exponentially, as can be seen in the diagram.

The average orbit of GOCE, at 227.5 km altitude before the engine cut off, has now dropped to below 200 km altitude. Perigee is already below 195 km. The Mean Motion (the number of orbital revolutions per day) of the satellite is increasing fast.

While it is still too early to provide really meaningful predictions, I currently have a nominal re-entry prediction for a window of a few days centered on November 10.7. It hence now seems possible that GOCE is within a week of plunging into the atmosphere.

The diagram below shows how predictions of the re-entry window have been evolving the past days (the grey line is the nominal prediction value, i.e. the center of the uncertainty window, and the dashed lines show the window of uncertainty on the prediction. The top three panels give observed and predicted solar flux; drag terms; and the orbital drop rate trend).

These predictions were made using two satellite orbital decay programs written by Alan Pickup, called SatAna and SatEvo. The program called SatAna analyses the orbital evolution over a number of recent orbit determinations and (taking into account the average solar flux for that period) fits a decay term that should be a bit more realistic than the decay terms from one single orbit only (which vary widely, see the second subwindow with B* values in the diagram above). The SatAna result and the predicted average solar flux for the next few days is next fed into SatEvo, a program that evolves the orbit into the future, up to re-entry.

The calculated moment of re-entry becomes more accurate based on orbital elements closer to decay. Currently, some 7 days before the prognosed moment of re-entry, the uncertainty window is still very large (several days), so giving an exact time at this moment is still meaningless. It is also still completely impossible to say where it will come down.

A lot can still happen the coming few days, that can drastically alter the picture. I have written about this before: e.g., if GOCE for some reason loses its current drag-reducing attitude (flight orientation) or is starting to shed bits and pieces at some point, the drag it experiences will significantly change, and with that the re-entry predictions will significantly change. Differences between the predicted solar activity for the upcoming days and real solar activity values the coming days, likewise can change the re-entry time.

(note: I thank Alan Pickup for making available SatAna and SatEvo)

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