Monday 11 November 2013

GOCE re-entry photographed from the Falklands?

This has just appeared on Twitter:

Reported time and geographic location seem a match (21:20 Falkland time is 00:20 UT)!

Alas, poor GOCE, I knew him well...

click map to enlarge

Last night just after 0h UT, GOCE, ESA's Gravity Field and Steady-State Ocean Circulation Explorer, died an heroic death, plunging into the atmosphere while passing over the ice cold wastes of Antarctica, within minutes of passing over the Falkland islands.

ESA reported the decay time as "close to 01:00 CET on Monday 11 November" (= close to 00:00 UT, Nov 10-11).

USSTRATCOM gives a final TIP placing decay at 11 Nov 00:16 UTC +/- 1 m near 56 S 60 W.

My initial last pre-decay forecast, made in a haste late last evening after returning from a full day surveying in the field (later more on that...), was too early.

This was before the final few orbits for GOCE were published, and before I learned from Alan Pickup of a secret setting in SatAna and SatEvo that makes it possible to tweak details that are important in the last few orbits at very low altitude. My tweet at that time:

As this window was including a pass over Australia, I also tweeted:

ESA however next reported having received telemetry from a GOCE pass at 22:42 UT from Troll station on Antarctica, making clear GOCE was still alive and functioning while only just above 110 km altitude!

So my 22:10 UT forecast was wrong. We now know it was wrong by an hour two hours, actually.

Alan Pickup mailed me around that time about some 'hidden' experimental options in SatAna and SatEvo that take into account spacecraft dimensions and some dimension-related effects that are significant at very low altitudes only.

Together with the addition of two more orbital updates that have since appeared, I have therefore re-done the exercise, as an "aftercast".

With solar flux at 154, a 0.3 day tle arc (the last 5 available orbit updates) processed in SatAna and the result then fed into Satevo, and setting the length of GOCE at 5.0, I get re-entry at:

11 Nov 00:13 UT +/- 14 m
69 S, 52 W

This is only 3 minutes from the time given by USSTRATCOM.

In the map on the top of this post, the blue dot gives the USSTRATCOM position, the red dot and red line give the SatAna + SatEvo nominal  prediction and window.

Below is the SatEvo result in 3D, looking towards the south polar region:

I am rather surprised about how well (after tweaking some internal settings) the final SatEvo result compares to USSTRATCOM's final TIP. Kudo's to Alan who wrote the software! (of course, and Alan agrees, the near-perfect match can be a lucky coincidence).

The diagram above shows how quickly GOCE dropped in the end. The last available orbital elements from an epoch about an hour before reentry, are for a perigee altitude of only 110 km! A day earlier the perigee was still at 150 km altitude.

One of the most amazing things about the re-entry of GOCE is that the spacecraft retained its drag-reducing attitude right up to the end. The designers of the spacecraft deserve some serious kudo's for that.

Of all the ways a spacecraft can go, GOCE died gracefully and heroically!  GOCE, clutching on to life to the bitter end, victim of the same forces that it helped map in so much detail. Now let us mourn our brave little spacecraft...

(17 Mar 2009 - 11 Nov 2013)
(here imaged 1.5 months before it's re-entry)

Saturday 9 November 2013

Brief update on Goce (9 Nov): one day from reentry!

My current re-entry forecast for GOCE, made with Alan Pickup's SatAna and SatEvo software, is re-entry in a 11.3 hr window centered on Nov 10.806 UT.

Any deviation from the nominal value is more likely to be towards the later part of that window (i.e. up to the early hours of Nov 11 is possible) unless attitude control fails earlier, in which case it will come down earlier.

Due to circumstances I will not be able to update my forecasts tomorrow. I will do an "aftercast" on Monday.

Friday 8 November 2013

Another brief update on #GOCE, 8 Nov 2013

The average orbit of GOCE is now below 172 km altitude, with perigee below 170 km. It is hence approaching the critical value of 150 km.

Due to the effects of several recent solar outbursts on the atmosphere, drag levels went up and fluctuated over the past 1.5 days (see ESA's blog here and the diagram below). This makes forecasting difficult. Forecasts at different times today varied as a result, but they all are somewhat earlier in time than they were yesterday.

The forecast at the moment of writing (SatEvo prediction on a 1-day SatAna arc up to Nov 8.55 UT) is for a 1-day window centered on Nov 10.7 UT. With the same caveat as in my previous posts: this forecast assumes GOCE will keep its drag-reducing attitude up to re-entry, which is by no means certain.

USSTRATCOM has issued two first TIP-bulletins for GOCE, yesterday and today. The last of these forecasts is re-entry at Nov 11.12 UT, +/- 48 hours.

Thursday 7 November 2013

Brief GOCE update, 7 November

This morning the average orbital altitude of GOCE had dropped to 181 km. The orbital droprate is now near 6 km per day. There is no sign yet that attitude control is failing.

The nominal re-entry forecast is slightly shifting backwards. My current forecast using Alan Pickup's SatAna and SatEvo software is re-entry in a 1.5 day uncertainty window centered on Nov 11.17 UT. This is assuming that the attitude control will hold until decay (see discussions in previous posts).

Wednesday 6 November 2013

Brief (semi-) daily GOCE update, 6 November

GOCE's perigee is now below 183.5 km, the average orbit has dropped to 187 km (from 227.5 km originally). The orbital droprate is now close to 5 km/day.

The nominal forecast re-entry window is currently a two-day period centered on Nov 10.90 UT. However, see the caveat in my previous post. If GOCE loses its current drag-reducing attitude (flight orientation), the re-entry forecast above will turn completely obsolete.

A more elaborate explanation of factors involved can be found in my previous post, including an explanation on why re-entry forecasts for GOCE must be approached very cautiously.

Forecasts were made using Alan Pickup's SatAna and SatEvo software: for an explanation on the workings of this software and factors of influence, see an earlier post here.

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.

Sunday 3 November 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)