Thursday 31 October 2013

About half-way: another short update on GOCE, 10 days after its engine cut off

Ten days ago the ion engine of GOCE, the European Space Agency's 1-tonne Gravity field and steady-state Ocean Circulation Explorer scientific satellite (2009-013A), cut off after the satellite ran out of fuel.

Originally orbiting at an average orbital altitude of 227.5 km, it is since coming down (see earlier posts here and here and the first two diagrams below). Its perigee is now below 205 km and it is currently coming down at a rate of 2.7 km/day. That rate is increasing (see third diagram below): it is already a factor 3 larger than it was on October 21st just after the ion engine of GOCE cut off.

click diagrams to enlarge

As I wrote earlier, for various reasons it is still too early to provide reliable re-entry estimates. I nevertheless estimate that GOCE is now about half-way, in terms of days not altitude, from inception of its fall to re-entry.

After the latest orbital updates, the nominal re-entry date appears to slowly creep to an earlier date. Currently I have re-entry forecast for a several-day window around November 10.

That date can still (dramatically) shift if anything happens to GOCE, for example if it loses its current drag-reducing attitude (flight orientation - see also a previous post) or if we see a drastic change in solar activity.

The Sun has been very active the past few days, spewing several CME's. This solar activity has an influence on the density of the upper atmosphere, and this in turn has an influence on the magnitude of the atmospheric drag on GOCE, influencing how fast its orbit evolves.

I do expect the prognosed re-entry date to slowly creep to an earlier date over the coming few days. It therefore does seem that GOCE has less than two weeks, and possibly closer to a week, of lifetime left.

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