1. ESA UNCLASSIFIED – Releasable to the Public GOCE re-entry campaign B. Bastida Virgili, T. Flohrer, S. Lemmens, H. Krag ESA Space Debris Office, HSO-GR, ESA/ESOC

2. ESA UNCLASSIFIED – Releasable to the Public Outline ESA’s Space Debris Office activities for the re-entry Coordination within the Inter-Agency Space Debris Coordination Committee (IADC) re-entry test campaign Technical aspects (space debris perspective) – Pre-campaign expectations – Attitude evolution – Challenges for predicting the re-entry epoch Results Outlook Summary

3. ESA UNCLASSIFIED – Releasable to the Public ACTIVITIES FOR GOCE REENTRY ESA Space Debris Office

4. ESA UNCLASSIFIED – Releasable to the Public GOCE – recap of some relevant data for re-entry predictions First ESA re-entry object that is followed by re-entry campaign Drag-compensating ion propulsion – Fuel depletion on 21-Oct-2013 (215 km × 233 km x 96.55deg) Flight dynamics operational assumptions on areas – Equivalent drag area: 1.0352m 2 – Lift area: 6.0492m 2 – Lateral area: 11.210 m 2 Environment – Mean solar flux of F 10.7 = 117±3 SFU – average daily flux values of F 10.7 = 151±8.4 SFU – Modest geomagnetic activities during final phase (reaching a peak A p = 22 on 09-Nov-2013)

5. ESA UNCLASSIFIED – Releasable to the Public ESA’s Space Debris Office activities for the re-entry of GOCE Organise support tracking (and potentially, imaging data) – TIRA Support IADC and ESA re-entry campaigns, ensure 24/7 on-call availability phase – Perform ESA independent Re-entry Predictions with increasing frequency up to every new orbit determination – Organise and administer IADC campaign and contribute for ESA Interface with Flight Dynamics and with Flight Control Team – Re-entry prediction depend on the attitude evolution of GOCE (aerodynamic cross- section can vary by factors) Running a GOCE model in Scarab 3.1L to assess – Attitude evolution after loss of control – Cross-section characteristics during tumbling Support to Public Relations

6. ESA UNCLASSIFIED – Releasable to the Public COORDINATION WITHIN THE IADC RE-ENTRY TEST CAMPAIGN GOCE Re-entry

7. ESA UNCLASSIFIED – Releasable to the Public Coordination within the Inter-Agency Space Debris Coordination Committee (IADC) re-entry test campaign The IADC was formally established in 1993 to: – promote the exchange of information on space debris research activities between member space agencies, – to facilitate opportunities for cooperation in space debris research, – to review the progress of on-going cooperative activities, – and to identify debris mitigation options. The IADC Terms of Reference request that at least one annual re-entry prediction exercise shall be conducted to verify the functionality of prediction tools. – Full re-entry campaign or re-entry test campaign (as for GOCE in 2013) – Campaign 2013-1: GOCE spacecraft (2009-013A, #34602)  16th test campaign, 11 of (at that time) 12 IADC member agencies participated in the test campaign Campaign administration, web-based front-end hosting and maintenance by ESA/ESOC by Space Debris Office

8. ESA UNCLASSIFIED – Releasable to the Public IADC re-entry test campaign 2013/1 timeline Sep-2013: consensus of IADC members to use GOCE as test object for the 2013-1 IADC Re-entry Campaign 21-Oct-2013: Start of the 2013-1 GOCE was expected to enter into a phase of attitude-controlled fine-pointing mode (FPM) until the attitude controllers would be unable to cope with the atmospheric torques (20mN, later 50mN, …) and then the satellite would enter in a phase of fully uncontrolled flight. 11-Nov-2013 01:50 UTC: re-entry confirmation reentry confirmation received from US Strategic Command 12-Nov-2013 09:50 UTC: Formal closure (USSTRATCOM, through their ”Space-Track” web portal, confirmed that a final assessment of the 80 km atmospheric interface pass of GOCE led to a re-entry at 11-Nov-2013 00:16 UTC ~60W/56S  Centre of impact window: 11-Nov-2013 00:23 UTC, ( ∼ 7 minutes to reach a 10 km reference altitude ~65W/41S) for a fictitious intact satellite

9. ESA UNCLASSIFIED – Releasable to the Public TECHNICAL ASPECTS FROM THE SPACE DEBRIS PERSPECTIVE GOCE Re-entry

10. ESA UNCLASSIFIED – Releasable to the Public Pre-campaign expected events during decay phase Variation of the cross sectional projection After Xenon depletion: ~2 weeks FPM, A*CD around 3.5 (close to stable flying) When mean drag force level reaches 20mN (at ca. 200km mean altitude) – Attitude controllers unable to cope with the atmospheric torques – Phase of fully uncontrolled flight  passivation and transmitters turned off Assessment of c D as function of aspect angle via cross-section projection from SCARAB model of GOCE (NB: today also possible through ESA DRAMA tool) Ca. 1m² Ca. 11m² A*C d Time 0 3.5 5.5 11 Xenon depletion Ca. 2 weeks (attitude control) Drag-free mode Campaign startsTransmitters off A*C D to be estimated (expected around 5.5 but with range from 2.2 (aerodynamically stabilised) to 11 (randomly tumbling) Mean drag level of 20mN Did not happen

11. ESA UNCLASSIFIED – Releasable to the Public Attitude evolution Parametric simulation using SCARAB3.1L – Centre of pressure evaluation match design – Variation on release altitude (i.e. drag level) – Propagation with uncontrolled attitude (after release)

12. ESA UNCLASSIFIED – Releasable to the Public Attitude evolution If released below 160km, remains stable  observed! See S. Lemmens for radar measurements

13. ESA UNCLASSIFIED – Releasable to the Public Challenges for predicting the re-entry epoch Daily coordination with Flight Control Team and Flight Dynamics experts to improve predictions Different sources of orbital data Segment wise consideration of different (fixed) C D values Asymmetric re-entry window capability (cf. next slide) Last prediction based on OD epoch 23:12UTC  acquisition, tracking, full OD and re-entry prediction within ~30min (!) timeframe, and, eventually, ~1h before re- entry(!!)  great team effort

14. ESA UNCLASSIFIED – Releasable to the Public Uncertainty window vs. FPM loss example for Nov 4 Orbit epoch 2013/11/04-13:01:15 mNdays from curr reentry epoch (days from curr) reentry epoch (days from FPM stop)20% unc (h) 20% unc (h) from FPM stop10% unc (h) 10% unc (h) from FPM stop 2602.2 10.56 5.28 270.52.552.0512.249.846.124.92 2912.881.8813.8249.0246.9124.512 311.53.21.715.368.167.684.08 3423.551.5517.047.448.523.72 372.53.91.418.726.729.363.36 4034.31.320.646.2410.323.12 453.54.551.0521.845.0410.922.52 5144.920.9223.6164.41611.8082.208 594.55.250.7525.23.612.61.8 7055.60.626.882.8813.441.44 895.55.920.4228.4162.01614.2081.008 12866.250.25301.2150.6 2626.56.60.131.680.4815.840.24

15. ESA UNCLASSIFIED – Releasable to the Public RESULTS GOCE Re-entry

16. ESA UNCLASSIFIED – Releasable to the Public What really happened Stable fine-pointing mode until very final phase of the re-entry Attitude control maintained in a head-on, minimum drag configuration during the re-entry campaign System exceeded its specification by a factor of 5 or more (attitude controllers worked until the end) Re-entry test object with constant area-to-mass ratio (!!) GPS receivers kept operating  very accurate orbit determinations Known aerodynamic cross-section  unique insight into the accuracy of used density models Finally, no passivation required

17. ESA UNCLASSIFIED – Releasable to the Public Mean apogee and perigee altitude decay profile Moderate orbit eccentricity (about ±18 km per orbit) States from TLE, FDyn (GPS and Ranging), TIRA with ESA Orbit Determination

18. ESA UNCLASSIFIED – Releasable to the Public Predicted re-entry windows 20mN assumption 50mN Phasing to “up to” no fail assumption  and resulting asymmetric window no fail assumption  switch to “classical” decay with constant A/m

19. ESA UNCLASSIFIED – Releasable to the Public Prediction on 07:38UTC Nov 10th Europe was considered cleared after 07:38UTC Nov 10 prediction

20. ESA UNCLASSIFIED – Releasable to the Public Prediction on 20:18 UTC Nov 10th (3 rd last ESA Prediction) Last tracked TROLL passage Predicted Centre of Impact Window

21. ESA UNCLASSIFIED – Releasable to the Public Final orbit around COIW

22. ESA UNCLASSIFIED – Releasable to the Public Final Re-entry: 00:16UTC Nov 11th Area where debris could potentially reach ground (projected from a previous GOCE re-entry break-up analysis) 80km altitude interface point reported by USSTRATCOM Eye witness from Falkland 00:20 UTC

23. ESA UNCLASSIFIED – Releasable to the Public OUTLOOK GOCE R-entry

24. ESA UNCLASSIFIED – Releasable to the Public Outlook on research opportunities In order to determine the on-ground risk prior to mission implementation, ESA uses specialised software to simulate the re- entry break-up This simulation considers the aerodynamic and aerothermal effects during the spacecraft/atmosphere interaction The data gathered during the GOCE re-entry down to altitudes slightly above 100km can help to validate the simulator: 1.Predicted decay rate (from telemetry and orbits – GPS and ground-based SLR/radar observations) 2.Predicted attitude motion (from telemetry) 3.Predicted temperature at selected components (payload sensors)

25. ESA UNCLASSIFIED – Releasable to the Public Summary GOCE was the first ESA re-entry object that was followed by re-entry campaign The IADC selected GOCE as re-entry test object to validate the functionality of tracking networks, orbit determination systems, re-entry prediction tools, and the IADC Re-Entry Events Database Re-entry prediction depend on the attitude evolution of GOCE (aerodynamic cross- section can vary by factors!) Attitude evolution after loss of control and cross-section characteristics during tumbling previously assessed But: stable fine-pointing mode until very final phase of the re-entry, head-on, minimum drag configuration during the re-entry campaign  constant area-to-mass ratio Very accurate orbit determinations from GPS data  unique insight into the accuracy of used density models Daily re-entry predictions by Space Debris Office, last prediction based on OD epoch 23:12UTC  acquisition, tracking, full OD and re-entry prediction within ~30min (!) timeframe, ~1h before re-entry(!!) Further research opportunities in re-entry and break-up analysis

26. ESA UNCLASSIFIED – Releasable to the Public Courtesy of Bill Chater (Falklands at 21:20 local time on 10 November) Centre of impact window on 11-Nov-2013 00:23 UTC: crossing of 10 km reference altitude at ~65W/41S for a fictitious intact satellite