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Gravity Field and Steady-State Ocean Circulation Explorer

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Gravity Field and Steady-State Ocean Circulation Explorer
Artist's view of GOCE. Its sleek, aerodynamic design led it to be dubbed the 'Ferrari of space'[1]
Mission typeGravitational research
OperatorESA
COSPAR ID2009-013AEdit this at Wikidata
SATCATno.34602
Websitehttp:// esa.int/GOCE
Mission durationPlanned: 20 months
Final: 4 years, 7 months, 3 days
Spacecraft properties
ManufacturerThales Alenia Space
EADS Astrium
Launch mass1,077 kg (2,374 lb)
Dry mass872 kg (1,922 lb)
Dimensions5.3 m × 2.3 m (17.4 ft × 7.5 ft)
Power1,600 watts
Start of mission
Launch date17 March 2009, 14:21(2009-03-17UTC14:21)UTC[2]
RocketRockot/Briz-KM
Launch sitePlesetsk Cosmodrome
ContractorEurockot Launch Services
End of mission
DisposalOrbital decay
Declared21 October 2013(2013-10-22)UTC[3]
Last contact10 November 2013, 22:42 UTC[4]
Decay date11 November 2013, 00:16 UTC[5]
Orbital parameters
Reference systemGeocentric
RegimeSun-synchronous[6]
Perigee altitude254.9 km (158.4 mi)[6]
Apogee altitude254.9 km (158.4 mi)[6]
Inclination96.7 degrees
Epoch29 June 2010[6]
Transponders
BandS band
Frequency2 GHz
Bandwidthup to1.2 Mbit/sdownload
up to4 kbit/supload
GOCE mission insignia
Insignia for theGOCEmission
SMOS→

TheGravity Field and Steady-State Ocean Circulation Explorer(GOCE) was the first ofESA'sLiving Planet Programmeheavy satellitesintended to map in unprecedented detail theEarth's gravity field.The spacecraft's primary instrumentation was a highly sensitive gravitygradiometerconsisting of three pairs ofaccelerometerswhich measured gravitational gradients along threeorthogonalaxes.

Launched on 17 March 2009, GOCE mapped the deep structure of the Earth'smantleand probed hazardous volcanic regions. It brought new insight into ocean behaviour; this in particular, was a major driver for the mission. By combining the gravity data with information aboutsea surface heightgathered by other satellite altimeters, scientists were able to track the direction and speed ofgeostrophic ocean currents.The low orbit and high accuracy of the system greatly improved the known accuracy and spatial resolution of thegeoid(the theoretical surface of equal gravitational potential on the Earth).

The satellite's unique arrow shape andfinshelped keep GOCE stable as it flew through thethermosphereat acomparatively low altitudeof 255 kilometres (158 mi). Additionally, anion propulsionsystem continuously compensated for the variable deceleration due to airdragwithout the vibration of a conventional chemically poweredrocket engine,thus limiting the errors in gravity gradient measurements caused by non-gravitational forces and restoring the path of the craft as closely as possible to a purelyinertialtrajectory.

After running out of propellant, the satellite began dropping out of orbit and made an uncontrolled re-entry on 11 November 2013.[1]

Discoveries and applications

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Mission objectives

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  • To determine gravity-field anomalies with an accuracy of10−5m/s2(1mGal).[7][8][9][10]To increase resolution, the satellite flew in an unusually low orbit.
  • To determine thegeoidwith an accuracy of 1–2 cm.
  • To achieve the above at a spatial resolution better than 100 km.

Gravity map and model

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The final gravity map and model of the geoid will provide users worldwide with well-defined data product that will lead to:

  • A better understanding of the physics of the Earth's interior to gain new insights into the geodynamics associated with thelithosphere,mantle composition and rheology, uplift and subduction processes.
  • A better understanding of the ocean currents and heat transport.
  • A global height-reference system, which can serve as a reference surface for the study of topographic processes and sea-level change.
  • Better estimates of the thickness of polar ice-sheets and their movement.[11]

Findings

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The first Earth global gravity model based on GOCE data was presented at ESA's Living Planet Symposium, in June 2010.[6]

Initial results of the GOCE satellite mission were presented at theAmerican Geophysical Union(AGU) 2010 Fall (Autumn) Meeting by Dr Rory Bingham from Newcastle University, UK. The maps produced from the GOCE data show ocean currents in much finer detail than had been available previously.[12] Even very small details like theMann Eddyin theNorth Atlanticwere visible in the data,[13]as was the effect ofHurricane Igorin 2010.[14]Detailed analysis of GOCE's thruster and accelerometer dataserendipitouslyrevealed that it had detected theinfrasoundwaves generated by the2011 Tōhoku earthquake(whereupon it inadvertently became the firstseismographin orbit).[15]

Later results from the GOCE data exposed details in the Earth's mantle including mantle plumes, ancient subduction zones, and remnants of theTethys Ocean.[16]

Subsequent analysis of GOCE data has also provided new information about the geological makeup of the Antarctic continent, including the detection of ancient continent remnants and at least threecratonsbeneath the Antarctic ice.[17][18][19]

Operations

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Launch

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The first launch attempt on 16 March 2009 was aborted due to a malfunction with the launch tower.[20]

GOCE was launched on 17 March 2009 at 14:21 UTC from thePlesetsk Cosmodromein northern Russia aboard aRokot/Briz-KMvehicle.[21]The Rokot is a modifiedUR-100Nintercontinental ballistic missile that was decommissioned after the Strategic Arms Reduction Treaty. The launcher used the two lower liquid fuel stages of the original missile and was equipped with a Briz-KM third stage developed for precise orbit injection. GOCE was launched into aSun-synchronousdusk-dawn orbit with an inclination of 96.7° and an ascending node at 18:00. Separation from the launcher was at 295 km. The satellite's orbit then decayed over a period of 45 days to an operational altitude, planned at 270 km. During this time, the spacecraft was commissioned and the electrical propulsion system checked for reliability inattitude control[needs update].[22]

Operation

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Model of GOCE

In February 2010 a fault was discovered in the satellite's computer, which meant controllers were forced to switch control to the backup computer.

In July 2010, GOCE suffered a serious communications malfunction, when the satellite suddenly failed to downlink scientific data to its receiving stations. Extensive investigations by experts from ESA and industry revealed that the issue was almost certainly related to a communication link between the processor module and the telemetry modules of the main computer.[23]The recovery was completed in September 2010: as part of the action plan, the temperature of the floor hosting the computers was raised by some 7 °C (13 °F), resulting in restoration of normal communications.[24]

In November 2010, the due completion date for the original 20-month mission before it was delayed by the glitches, it was decided to extend the mission lifetime until the end of 2012 in order to complete the original work and carry out a further 18-month mission to improve the collected data.[25]

In November 2012 the orbit was lowered from 255 to 235 km (158 to 146 mi) to get higher resolution data, at which time fuel remained for another 50 weeks.[26]

End of mission and re-entry

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In May 2013 a further lowering to 229 km (142 mi) took place.

The satellite ran out of its xenon propellant in October 2013, at which time it would take2–3 weeksto re-enter.[27]On 18 October 2013,ESAreported that the pressure in the fuel system of GOCE's ion engine had dropped below 2.5 bar, which is the nominal operating pressure required to fire the engine.[28]Subsequently, end of mission was formally declared on 21 October when the spacecraft ran out of fuel; deprived of xenon, the ion drive stopped working at 03:16 UTC.[3][29]

On 9 November 2013, a published report indicated that the satellite was expected to re-enter within a day or two.[30][31]By this date, theperigeealtitude had decayed to 155 km (96 mi).[32]

On 10 November, ESA expected re-entry to occur between 18:30 and 24:00 UTC that day, with the most probable impact ground swath largely running over ocean and polar regions.[33]

Its descending orbit on 11 November 2013 passed overSiberia,the westernPacific Ocean,the easternIndian OceanandAntarctica.[34][35]The satellite finally disintegrated around 00:16UTCon 11 November near theFalkland Islands.[5][36]

Design

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Payload

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The satellite's main payload was the Electrostatic Gravity Gradiometer (EGG) to measure thegravity field of Earth.This instrument consisted of three pairs of capacitive accelerometers arranged in three dimensions that responded to tiny variations in the 'gravitational tug' of theEarthas it traveled along its orbital path. Because of their different position in the gravitational field they all experienced the gravitational acceleration of the Earth slightly differently. The three axes of thegradiometerallowed the simultaneous measurement of the five independent components of thegravity gradient tensor.

Other payload was an onboardGPSreceiver used as a Satellite-to-Satellite Tracking Instrument (SSTI); a compensation system for all non-gravitational forces acting on the spacecraft. The satellite was also equipped with alaserretroreflectorto enable tracking by ground-basedSatellite laser rangingstations.[37]

Power and propulsion

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GOCE flares to magnitude +2 as the 67.5 degree solar panel briefly mirrors sunlight (3 January 2010, 17:24:23.15 UTC).

GOCE's 5 × 1.1 m (16 × 4 ft) frame had fixedsolar panelscovering its sun-facing side, which produced 1,300wattsof power.[38]The panels were shaped to act as fins, stabilising the spacecraft while it orbited through the residual air in thethermosphere.

Theion propulsionelectric engine, designed and built atQinetiQ's space centre in Farnborough, England, ejectedxenonions at velocities exceeding 40,000 m/s (140,000 km/h; 89,000 mph), which compensated for the orbital decay losses. GOCE's mission ended when the 40 kg (88 lb) xenon fuel tank emptied.[7]The dual Kaufman-type ion thrusters could produce up to 20millinewtons(0.0045 lbf) of thrust.[39]

Although its predicted lifetime was 20 months, an ESA report in June 2010 suggested that unusually low solar activity (meaning a calmer upper atmosphere, and hence less drag on the craft) meant the fuel would last longer than its predicted 20 months—possibly into 2014. In reality, the end of mission was formally declared on 21 October 2013 after 55 months, with the final 11 months in a lower orbit (with greater air density and therefore greater fuel use).[40]

See also

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  • GRACE(NASA; DLR; in orbit 2002–2017) and the follow-up mission GRACE-FO
  • SLATS(JAXA), also used ion thrusters to maintain altitude, 2017-2019
  • Satellite gravimetry

References

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  1. ^ab"GOCE completes its mission".European Space Agency.Retrieved11 November2013.
  2. ^"GOCE launched and in orbit".European Space Agency. 17 March 2009.Retrieved10 October2013.
  3. ^ab"ESA's GOCE mission comes to an end"(Press release). European Space Agency. 21 October 2013. No. 33-2013.Retrieved10 July2017.
  4. ^Scuka, Daniel (11 November 2013)."ESOC update – 23:50CET".European Space Agency.Retrieved28 December2016.
  5. ^abScuka, Daniel (11 November 2013)."GOCE re-entry region".European Space Agency.Retrieved28 December2016.
  6. ^abcde"GOCE giving new insights into Earth's gravity".European Space Agency. 29 June 2010.Retrieved29 June2010.
  7. ^ab"ESA's Gravity Mission GOCE".European Space Agency. 16 October 2008. Archived fromthe originalon 10 September 2014.Retrieved26 October2008.
  8. ^Drinkwater, Mark; Haagmans, Roger; Kern, Michael; Muzi, Danilo; Floberghagen, Rune (February 2008)."GOCE: Obtaining a Portrait of Earth's Most Intimate Features"(PDF).Bulletin(133). European Space Agency: 4–13.
  9. ^Drinkwater, M. R.; Floberghagen, R.; Haagmans, R.; Muzi, D.; Popescu, A. (July 2003)."GOCE: ESA's First Earth Explorer Core Mission"(PDF).Space Science Reviews.108(1–2). Kluwer Academic: 419–432.Bibcode:2003SSRv..108..419D.doi:10.1023/A:1026104216284.S2CID121029480.
  10. ^Johannessen, J. A.; Balmino, G.; Le Provost, C.; Rummel, R.; Sabadini, R.; et al. (July 2003)."The European Gravity Field and Steady-State Ocean Circulation Explorer Satellite Mission: Its Impact on Geophysics"(PDF).Surveys in Geophysics.24(4): 339–386.Bibcode:2003SGeo...24..339J.doi:10.1023/B:GEOP.0000004264.04667.5e.hdl:1956/3796.S2CID55391538.
  11. ^"GOCE scientific objectives".European Space Agency.Retrieved11 November2013.
  12. ^Bingham, R. J.; Knudsen, P.; Andersen, O. B.; Pail, R. (December 2010).Using GOCE to estimate the mean North Atlantic circulation (Invited).American Geophysical Union, Fall Meeting 2010. 13–17 December 2010. San Francisco, California.Bibcode:2010AGUFM.G33B..08B.
  13. ^Amos, Jonathan (21 December 2010)."Goce gravity mission traces ocean circulation".BBC News.Retrieved21 December2010.
  14. ^"ESA missions highlighted at world's largest scientific conference".European Space Agency. 17 December 2010.Retrieved22 December2010.
  15. ^"1-tonne GOCE satellite falls to Earth Sunday night".CBC News.21 March 2009.Retrieved11 November2013.
  16. ^Amos, Jonathan (27 January 2014)."Europe's Goce gravity satellite probes Earth's mantle".BBC News.Retrieved27 January2014.
  17. ^"ESA's gravity-mapper reveals relics of ancient continents under Antarctic ice".ESA.7 November 2018.Retrieved13 March2019.
  18. ^Andrews, Robin George (23 November 2018)."Beneath Antarctica's Ice Is a Graveyard of Dead Continents".New York Times.Retrieved13 March2019.
  19. ^Ebbing, Jörg; Haas, Peter; Ferraccioli, Fausto; Pappa, Folker; Szwillus, Wolfgang; Bouman, Johannes (5 November 2018)."Earth tectonics as seen by GOCE - Enhanced satellite gravity gradient imaging".Scientific Reports.8(1).Nature Research:16356.Bibcode:2018NatSR...816356E.doi:10.1038/s41598-018-34733-9.PMC6218487.PMID30397250.
  20. ^Bonacina, Franco (16 March 2009)."Launch of ESA's gravity mapping satellite delayed".European Space Agency.
  21. ^Bonacina, Franco (17 March 2009)."ESA launches first Earth Explorer mission GOCE".European Space Agency.
  22. ^Fehringer, Michael; Andre, Gerard; Lamarre, Daniel; Maeusli, Damien (February 2008)."A Jewel in ESA's Crown"(PDF).Bulletin(133). European Space Agency: 14–23.
  23. ^Amos, Jonathan (21 August 2010)."Computer blow to Europe's Goce gravity satellite".BBC News.Retrieved22 August2010.
  24. ^"GOCE gravity mission back in action".European Space Agency. 7 September 2010.Retrieved29 September2010.
  25. ^"ESA's gravity mission granted 18-month extension".European Space Agency. 25 November 2010.
  26. ^Winder, Jenny (19 November 2012)."GOCE – How Low Can It Go?".Universe Today.
  27. ^"ESA's GOCE mission to end this year".European Space Agency. 13 September 2013.Retrieved11 November2013.
  28. ^Scuka, Daniel; Steiger, Christoph (18 October 2013)."GOCE fuel steadily runs out".European Space Agency.Retrieved11 November2013.
  29. ^Scuka, Daniel; Steiger, Christoph (21 October 2017)."Update on GOCE".European Space Agency.Retrieved10 July2017.
  30. ^Allman, Tim (9 November 2013)."'Don't panic': One-tonne satellite falling to Earth ".BBC News.Retrieved11 November2013.
  31. ^Henderson, Greg (9 November 2013)."'Ferrari Of Space' Crashing Back To Earth — Maybe Tomorrow ".NPR.Retrieved10 November2013.
  32. ^"GOCE - Orbit".Heavens Above.Retrieved11 November2013.
  33. ^Scuka, Daniel; Klinkrad, Heiner (10 November 2013)."GOCE re-entry forecast from ESA's Space Debris Office".European Space Agency.Retrieved11 November2013.
  34. ^"GOCE gives into gravity".European Space Agency. 11 November 2013.Retrieved11 November2013.
  35. ^Smith, Matt (10 November 2013)."Falling European satellite burns up upon re-entry".CNN.Retrieved11 November2013.
  36. ^O'Neill, Ian (11 November 2013)."Twitter: GOCE Burned Up Over Falkland Islands".Discovery News.Archived fromthe originalon 12 November 2013.
  37. ^Strugarek, Dariusz; Sosnica, Krzysztof; Jaeggi, Adrian (January 2019). "Characteristics of GOCE orbits based on Satellite Laser Ranging".Advances in Space Research.63(1). Elsevier: 417–431.Bibcode:2019AdSpR..63..417S.doi:10.1016/j.asr.2018.08.033.S2CID125791718.
  38. ^Amos, Jonathan (24 October 2008)."Goce gravity flight slips to 2009".BBC News.Retrieved26 October2008.
  39. ^"GOCE Mission".European Space Agency.Retrieved16 April2017.
  40. ^Amos, Jonathan (28 June 2010)."Goce satellite views Earth's gravity in high definition".BBC News.Retrieved30 June2010.
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