Tuesday 5 November 2013

GOCE update (5 Nov 2013)

GOCE, ESA's Gravity Field and Steady-State Ocean Circulation Explorer, is still steadily coming down (see previous posts). This morning, the average orbital altitude had dropped below 191.5 km (from originally 227.5 km before Oct 21) and perigee is already down to 188 km. The orbital altitude has dropped 36 km in 15 days and it is currently coming down at a rate of 4.5 - 5 km per day, and increasing.

The nominal center of the re-entry forecast window, a period of several days, is still hovering near November 10-11. My latest forecast (issue date Nov 5.22 UT) has it nominally at Nov 10.97, +/- 1.2 days. This is the approximate date the satellite would re-enter, if it remains in its current drag-reducing attitude (orientation).

The latter caveat is an important point to note in this case and its implication should not be ignored. The special aerodynamic design of GOCE means that (much more so than for other satellites) there is a large difference in the amount of drag it is experiencing in drag-reducing attitude compared to what it will experience when that attitude is lost. If the attitude control mechanism (magnetic torques) fails somewhere during the coming days and the drag reducing attitude is lost as a result, GOCE will come down much earlier than the current forecast suggests. That is one reason to be very cautious with GOCE re-entry predictions, certainly at this point in time.

It is currently impossible to say if, and if so when, such a loss of attitude will happen. But if it does, the current re-entry forecast will be turned completely obsolete.

Meanwhile, let us not forget that GOCE is still functioning! As it spirals down to its doom faster and faster, it continues to gather valuable data on the Earth's gravitational field.

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