The Deorbiting of GOCE – A Spacecraft Operations Perspective C. Steiger, M. Romanazzo, P.P. Emanuelli, R. Floberghagen, M. Fehringer 5th GOCE User Workshop, 27th Nov 2014, UNESCO, Paris

GRAVITY FIELD AND STEADY-STATE OCEAN CIRCULATION EXPLORER 1700 days of flight (launch 17/03/2009, re-entry 11/11/2013) Measuring gravity: S/C designed for drag environment in very low Earth orbit “Drag-free Mode” for science operations to cancel out non-gravitational forces (air drag) 4.8m×2.4m, weight 1050kg

MISSION PROFILE 2009–2013 Low orbit operations campaign Commissioning “Routine” mission at 260 km mean altitude 21/10/2013 fuel depletion Mean altitude [km] 11/11/2013 re-entry

OPERATIONS IN DRAG-FREE MODE –GEOMAGNETIC STORM AT 229 KM

RE-ENTRY OPERATIONS OVERVIEW Rapid orbital decay once fuel for ion engine is exhausted S/C remains functional, but re-entry can’t be controlled About 40 pieces of debris (total mass 250 kg) to impact ground “[..] Approach is to operate GOCE as long as possible in the decay phase, i.e. up to when [..] the performance of S/C or G/S does not allow to continue.” [GOCE Deorbiting Operations Plan] Preparations and special measures: IADC re-entry campaign; close coordination with ESOC Space Debris Office S/C and G/S performance evaluations (e.g. orbit prediction accuracy) Flight control team double shift, 2 orbit determinations runs/day, ground station engineering support, extended station coverage.. Extreme drag below 180km was expected to lead to an end of operations, yet.. S/C and G/S functioned until the end in conditions far outside the specs Last contact only 1.5h before re-entry on 11th Nov 00:16 UTC

ALTITUDE PROFILE

Along track drag force [N] ATMOSPHERIC DRAG 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 Drag levels on 10th Nov, last day before re-entry Last contact with S/C at 10th Nov 22:43 UTC Along track drag force [N] 00:00 04:00 08:00 12:00 16:00 20:00 24:00

TIME OFFSET VALUE (TOV) EVOLUTION Time Offset Value TOV [sec]: difference between expected and actual time of signal acquisition = measure for along track orbit error TOV within ±2 sec: S/C within antenna beam, automatic signal acquisition

S/C ATTITUDE CONTROL PERFORMANCE View from sun Attitude control with magnetic torquers (weak actuator) S/C designed to be aerodynamically stable S/C attitude nominal until the end (drag >> specs) Passive aerodynamic stabilisation played a major role Attitude error around Z over 1 orbit (90 min) X Y Sun Z Earth

S/C THERMAL EVOLUTION Central computer (CDMU) Battery (BAT) Nitrogen storage tank (NST) Power distribution unit (PCDU) Star trackers (STR) Ion propulsion electronics (IPCU)

10 NOV 2013: FINAL OPERATIONS All S/C FDIR disabled in the morning to keep going until the very end Last Kiruna pass at 18:56Z, after that only Troll (Antarctica) passes S/C nominal in last Troll pass at 22:43Z

GOCE re-entry captured by Bill Chater on 11th Nov 00:20 UTC 11 NOV 2013: RE-ENTRY GOCE re-entry captured by Bill Chater on 11th Nov 00:20 UTC