 Artist's view of GOCE. Its sleek, aerodynamic design led it to be dubbed the 'Ferrari of space'[1] | |||||||||
| Mission type | Gravitational research | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| Operator | ESA | ||||||||
| COSPAR ID | 2009-013A | ||||||||
| SATCATno. | 34602 | ||||||||
| Website | http://www.esa.int/GOCE | ||||||||
| Mission duration | Planned: 20 months Final: 4 years, 7 months, 3 days | ||||||||
| Spacecraft properties | |||||||||
| Manufacturer | Thales Alenia Space EADS Astrium | ||||||||
| Launch mass | 1,077 kg (2,374 lb) | ||||||||
| Dry mass | 872 kg (1,922 lb) | ||||||||
| Dimensions | 5.3 m × 2.3 m (17.4 ft × 7.5 ft) | ||||||||
| Power | 1,600 watts | ||||||||
| Start of mission | |||||||||
| Launch date | 17 March 2009, 14:21 (2009-03-17UTC14:21) UTC[2] | ||||||||
| Rocket | Rockot/Briz-KM | ||||||||
| Launch site | Plesetsk Cosmodrome | ||||||||
| Contractor | Eurockot Launch Services | ||||||||
| End of mission | |||||||||
| Disposal | Orbital decay | ||||||||
| Declared | 21 October 2013 (2013-10-22) UTC[3] | ||||||||
| Last contact | 10 November 2013, 22:42 UTC[4] | ||||||||
| Decay date | 11 November 2013, 00:16 UTC[5] | ||||||||
| Orbital parameters | |||||||||
| Reference system | Geocentric | ||||||||
| Regime | Sun-synchronous[6] | ||||||||
| Perigee altitude | 254.9 km (158.4 mi)[6] | ||||||||
| Apogee altitude | 254.9 km (158.4 mi)[6] | ||||||||
| Inclination | 96.7 degrees | ||||||||
| Epoch | 28 June 2010[6] | ||||||||
| Transponders | |||||||||
| Band | S band | ||||||||
| Frequency | 2 GHz | ||||||||
| Bandwidth | up to1.2 Mbit/s download up to4 kbit/s upload | ||||||||
| |||||||||
 Insignia for theGOCE mission SMOS → | |||||||||
TheGravity Field and Steady-State Ocean Circulation Explorer (GOCE) was the first ofESA'sLiving Planet Programmeheavy satellites intended to map in unprecedented detail theEarth's gravity field. The spacecraft's primary instrumentation was a highly sensitive gravitygradiometer consisting of three pairs ofaccelerometers which measured gravitational gradients along threeorthogonal axes.
Launched on 17 March 2009, GOCE mapped the deep structure of the Earth'smantle and 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 height gathered 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 andfins helped keep GOCE stable as it flew through thethermosphere at acomparatively low altitude of 255 kilometres (158 mi). Additionally, anion propulsion system continuously compensated for the variable deceleration due to airdrag without 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 purelyinertial trajectory.
After running out of propellant, the satellite began dropping out of orbit and made an uncontrolled re-entry on 11 November 2013.[1]
The final gravity map and model of the geoid will provide users worldwide with well-defined data product that will lead to:
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 fromNewcastle 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 Eddy in theNorth Atlantic were visible in the data,[13] as was the effect ofHurricane Igor in 2010.[14] Detailed analysis of GOCE's thruster and accelerometer dataserendipitously revealed that it had detected theinfrasound waves generated by the2011 Tōhoku earthquake (whereupon it inadvertently became the firstseismograph in 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 threecratons beneath the Antarctic ice.[17][18][19]
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 Cosmodrome in northern Russia aboard aRokot/Briz-KM vehicle.[21] The Rokot is a modifiedUR-100N intercontinental 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-synchronous dusk-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]
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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]
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 weeks to re-enter.[27] On 18 October 2013,ESA reported 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, theperigee altitude 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 Ocean andAntarctica.[34][35] The satellite finally disintegrated around 00:16 UTC on 11 November near theFalkland Islands.[5][36]
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 theEarth as 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 thegradiometer allowed the simultaneous measurement of the five independent components of thegravity gradient tensor.
Other payload was an onboardGPS receiver 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 alaserretroreflector to enable tracking by ground-basedSatellite laser ranging stations.[37]
GOCE's 5 × 1.1 m (16 × 4 ft) frame had fixedsolar panels covering its sun-facing side, which produced 1,300 watts of power.[38] The panels were shaped to act as fins, stabilising the spacecraft while it orbited through the residual air in thethermosphere.
Theion propulsion electric engine, designed and built atQinetiQ's space centre in Farnborough, England, ejectedxenon ions 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]
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