Tuesday, October 22, 2013

[Updated diagrams] GOCE is falling!

[diagrams updated 23 Oct 2013, 9:15 UT] 

GOCE, The European Space Agency's 1-tonne slick Gravity field and steady-state Ocean Circulation Explorer scientific satellite (2009-013A), is now truely coming down.

During the night of October 17-18, fuel reserves became so low that the pressure in GOCE's ion engine fuel system dropped below a critical 2.5 bar.  Next, between October 21.12 and 21.54, the ion engine stopped functioning, and as a result GOCE is now clearly losing altitude.

click diagrams [updated 23 Oct 9:15 UT]  to enlarge 

The first orbital determinations after the engine cut-off on October 21 are still inaccurate and as a result they are fluctuating, as the observational arc is still very short. But in the diagrams above, it can be clearly seen that the Mean Motion (the number of orbital revolutions per day that the satellite makes, i.e. how many times it circles the earth each day) jumps to much higher values. More orbital revolutions per day means that the orbit is getting smaller. The orbit getting smaller means the satellite is coming down.

This can be seen in the second diagram too. The apogee (the highest point in GOCE's slightly elliptical orbit) is steadily coming down since yesterday. The perigee (the lowest point in GOCE's slightly elliptical orbit) is dropping too.

GOCE's ion engine, when still working, provided a force countering the drag that the satellite experienced from the outer layers of the atmosphere in its low ~225 km orbit. As a result the drag parameter Bstar fluctuated around zero. When the ion engine cut out, the satellite suddenly experienced the full force of atmospheric drag. This can be seen in the lowermost diagram, which shows that the drag parameter Bstar made a strong jump to high positive values. The drag slows down the satellite, and as a result it drops in orbital altitude.

Over the coming days GOCE will rapidly lose altitude. So shortly after ion engine cut-off it is still too early to provide an accurate prediction about when it will truely re-enter and largely burn up: but as a ballpark figure this will happen somewhere between 2 to 3 weeks from now, somewhere during the first two weeks of November. In the days before re-entry, I will update re-entry forecasts on this blog.

Most of GOCE's one-tonne mass will burn up on re-entry, but some 250 kg (in many small fragments) might survive re-entry. At this point, it is still impossible to predict where (and when) these fragments may come down as that is dependant on many contributing factors, some of which are difficult to predict (e.g. the effect of fluctuating solar activity on the density gradient of the atmosphere). It will only be possible to predict this with some confidence in the final hours directly before re-entry.

Although the satellite is now without propulsion, its scientific sensors are still working. GOCE will continue to gather important scientific data on the Earth's gravity field until very shortly before its final demise.

The satellite controllers at ESOC have told me they have put the satellite in Fine Pointing Mode: a series of magnetic torques which react to the Earth's magnetic field keep the satellite stable in attitude (orientation), preventing it from tumbling, even though it has lost propulsion.

Since 2009, the GOCE satellite has gathered highly detailed data on the Earth's gravitational field and ocean surface heights.

Note: the apogee and perigee altitudes in the 2nd diagram were calculated with a fixed Earth radius of 6378 km, ignoring Earth oblateness

My last view of GOCE, an image taken on 29 September 2013 during a twilight pass over Leiden (click image to enlarge)

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