Gravity Field and Steady-State Ocean Circulation Explorer

GOCE

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
SATCAT № 34602
Website www.esa.int/SPECIALS/GOCE/index.html
Mission duration 20 months (planned)
55 months (to announced end of mission
in October 2013; re-entry November 2013)
Spacecraft properties
Manufacturer Thales Alenia Space
EADS Astrium
Launch mass 1,077 kilograms (2,374 lb)
Dry mass 872 kilograms (1,922 lb)
Dimensions 5.3 by 2.3 metres (17.4 ft × 7.5 ft)
Power 1600 watts
Start of mission
Launch date 17 March 2009, 14:21:00 UTC (2009-03-17UTC14:21Z)[2]
Rocket Rokot/Briz-KM
Launch site Plesetsk 133/3
Contractor Eurockot
End of mission
Disposal Fuel depletion, decayed
Last contact 10 November 2013, 22:42 UTC (2013-11-10UTC22:43Z)
Decay date 11 November 2013, 00:16 UTC
Orbital parameters
Reference system Geocentric
Regime Sun-synchronous[3]
Perigee 254.9 kilometres (158.4 mi)[3]
Inclination 96.7 degrees
Epoch main mission, March 2009 - November 2012
Transponders
Band S Band
Frequency 2 GHz
Bandwidth up to 1.2 Mbit/s download
up to 4 kbit/s upload
Instruments
EGG: Electrostatic Gravity Gradiometer
SSTI: Satellite-to-Satellite Tracking Instrument
LRR: Laser Range Reflector

Living Planet Programme

The Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) was the first of ESA's Living Planet Programme satellites intended to map in unprecedented detail the Earth's gravity field. The spacecraft's primary instrumentation was a highly sensitive gravity gradiometer consisting of three pairs of accelerometers which measured gravitational gradients along three orthogonal axes.

GOCE mapped the deep structure of the Earth's mantle 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 about sea surface height gathered by other satellite altimeters, scientists were able to track the direction and speed of geostrophic ocean currents. The low orbit and high accuracy of the system greatly improved the known accuracy and spatial resolution of the geoid (the theoretical surface of equal gravitational potential on the Earth).

The satellite's unique arrow shape and fins helped keep GOCE stable as it flew through the upper thermosphere at an altitude of 255 kilometres (158 mi). Additionally, an ion propulsion system continuously compensated for the variable deceleration due to air drag without the vibration of a conventional chemically powered rocket 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 purely inertial trajectory.

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

Discoveries and applications

Mission objectives

Gravity map and model

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

Findings

The first Earth global gravity model based on GOCE data was presented at ESA’s Living Planet Symposium, in June 2010.[3]

Initial results of the GOCE satellite mission were presented at the American 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.[9] Even very small details like the Mann Eddy in the North Atlantic were visible in the data,[10] as was the effect of Hurricane Igor (2010).[11] Detailed analysis of GOCE's thruster and accelerometer data serendipitously revealed that it had detected the infrasound waves generated by the 2011 Tōhoku earthquake (whereupon it inadvertently became the first seismograph in orbit).[12]

Later results from the GOCE data exposed details in the Earth's mantle including mantle plumes, ancient subduction zones, and remnants of the Tethys Ocean.[13]

Operations

Launch

GOCE was launched from the Plesetsk Cosmodrome in northern Russia with a Rokot/Briz-KM vehicle on 17 March 2009 at 14:21 UTC.[14] The Rokot is a modified UR-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 a Sun-synchronous dusk-dawn orbit with an inclination of 96.70° 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 in attitude control.[15]

The first launch attempt on 16 March 2009 was aborted due to a malfunction with the launch tower.[16] Liftoff occurred successfully at 14:21 GMT on 17 March 2009. The Rokot launcher delivered the satellite northward over the Arctic. About 90 minutes later, after one orbital revolution and two Briz-KM upper-stage burns, the spacecraft was successfully released into a circumpolar orbit at 280 km altitude with 96.7° inclination to the Equator. Soon after the separation, contact was successfully established with the satellite.[17]

Operation

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.[18] 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 – resulting in restoration of normal communications.[19]

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 till the end of 2012, in order to complete the original work and carry out a further 18-month mission to improve the collected data.[20]

In November 2012 the orbit was lowered from 255 km to 235 km to get higher resolution data, at which time fuel remained for another 50 weeks.[21]

End of mission and re-entry

In May 2013 a further lowering to 229 km took place.

The satellite ran out of propellant (xenon) in mid-October 2013, at which time it would take 2–3 weeks to re-enter.[22]

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.[23] Subsequently end of mission was formally declared on 21 October when the ion drive stopped working at 03:16 UTC.[24]

On 9 November 2013, a published report indicated that the satellite was expected to re-enter within a day or two.[25][26] By this date, the perigee altitude had decayed to 155 km.[27]

On 10 November ESA expected re-entry to occur between 18:30 UTC - 24:00 UTC, on 10 November; the most probable impact ground swath largely running over ocean and polar regions.[28]

Its descending orbit on 11 November 2013 passed over Siberia, the western Pacific Ocean, the eastern Indian Ocean and Antarctica.[29][30] The satellite finally disintegrated at around 1 a.m CET, according to the European Space Agency, near the Falkland Islands.[31]

Design

Payload

The satellite's main payload was the Electrostatic Gravity Gradiometer (EGG) to measure the gravity field of Earth. They were arranged in three pairs of ultra-sensitive accelerometers arranged in three dimensions that responded to tiny variations in the 'gravitational tug' of the Earth as it travelled 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 the gradiometer allowed the simultaneous measurement of the five independent components of the gravity gradient tensor.

Other payload was an onboard GPS 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 a laser retroreflector to enable tracking by ground-based lasers.

Power

GOCE (09-013A) flaring briefly to magnitude +2 on this image as the 67.5 degree solar panel briefly mirrors sunlight to the observer at 17:24:23.15 UTC (3 January 2010).

GOCE had fixed solar panels, which produced 1,300 W of power and cover the Sun-facing side of GOCE.[32]

The ion propulsion electric engine, designed and built at QinetiQ's space centre in Farnborough, England, ejected xenon ions at velocities exceeding 40,000 m/s, which compensated for the orbital decay losses. GOCE's mission ended when the 40 kg xenon fuel tank emptied (with a predicted lifetime of about 20 months).[4]

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 mission would be able to extend past its predicted 20 months due to fuel savings - possibly into 2014. In reality, the end of mission was formally declared on 21 October 2013 after 55 months, after being deployed for its final 11 months in a lower orbit (with greater air density and therefore greater fuel use).[33]

The 5 m × 1 m frame incorporates the fixed solar panels as fins to stabilise the spacecraft while it orbits through the residual air in the thermosphere.

See also

References

  1. 1 2 "GOCE completes its mission / GOCE / Observing the Earth / Our Activities / ESA". Esa.int. Retrieved 2013-11-11.
  2. ↑ "GOCE launched and in orbit". ESA. 17 March 2009. Retrieved 10 October 2013.
  3. 1 2 3 GOCE giving new insights into Earth’s gravity, ESA, retrieved 29 June 2010
  4. 1 2 "Homepage - ESA's gravity mission GOCE". European Space Agency. 16 October 2008. Retrieved 26 October 2008.
  5. ↑ ESA Bulletin n. 133 (PDF), ESA, February 2008
  6. ↑ Drinkwater, M R; et al. (2003). "GOCE: ESA’s first Earth Explorer Core mission" (PDF). Space Science Reviews, vol. 108: pp. 419–432,. Kluwer Academic Publishers. Retrieved 24 August 2011.
  7. ↑ Johannessen J J; et al. (2003). "The European hiExplorer Satellite Mission: Impact in hi". Surveys in Geophysics, 24, 4, pp. 339-386. Springer.
  8. ↑ "GOCE scientific objectives". Esa.int. Retrieved 2013-11-11.
  9. ↑ Bingham, R J; et al. (2010). "Using GOCE to estimate the mean North Atlantic circulation (Invited)". Abstract presented at 2010 Fall Meeting, AGU, San Francisco, Calif., 13-17 Dec. American Geophysical Union. Retrieved 22 December 2010.
  10. ↑ Jonathan Amos, Science correspondent, BBC News (21 December 2010). "Goce gravity mission traces ocean circulation". BBC News website, Science & Environment. BBC News. Retrieved 21 December 2010.
  11. ↑ "ESA missions highlighted at world's largest scientific conference". ESA GOCE website, News. European Space Agency. 17 December 2010. Retrieved 22 December 2010.
  12. ↑ "1-tonne GOCE satellite falls to Earth Sunday night - Technology & Science - CBC News". Cbc.ca. 2009-03-21. Retrieved 2013-11-11.
  13. ↑ Jonathan Amos Science correspondent, BBC News (27 January 2014). "Europe's Goce gravity satellite probes Earth's mantle". BBC News. Retrieved 2014-01-27.
  14. ↑ GOCE site, ESA, retrieved 4 March 2009
  15. ↑ ESA Bulletin n. 133 (PDF), ESA, February 2008
  16. ↑ Launch of ESA’s gravity mapping satellite delayed, ESA, 16 March 2009
  17. ↑ ESA launches first Earth Explorer mission GOCE, ESA, 17 March 2009
  18. ↑ Computer blow to Europe's Goce gravity satellite, BBC News Online, 21 August 2010, retrieved 22 August 2010
  19. ↑ GOCE fully operational again, ESA, 29 September 2010
  20. ↑ ESA's gravity mission granted 18-month extension, ESA, 25 November 2010
  21. ↑ "GOCE – How Low Can It Go?". Universetoday.com. 19 November 2012.
  22. ↑ ESA's GOCE mission to end this year, Asdnews.com, retrieved 2013-11-11
  23. ↑ GOCE fuel steadily runs out, Nlogs.esa.int, retrieved 2013-11-11
  24. ↑ GOCE End Of Mission, Esa.int, retrieved 2013-11-11
  25. ↑ 'Don't panic': One-tonne satellite falling to Earth, Bbc.co.uk, retrieved 2013-11-11
  26. ↑ Henderson, Greg (9 November 2013). "'Ferrari Of Space' Crashing Back To Earth — Maybe Tomorrow". NPR. Retrieved 10 November 2013.
  27. ↑ "GOCE - Orbit". Heavens-above.com. Retrieved 2013-11-11.
  28. ↑ "GOCE re-entry forecast from ESA’s Space Debris Office | Rocket Science". Blogs.esa.int. Retrieved 2013-11-11.
  29. ↑ "GOCE gives into gravity". esa.int. Retrieved 11 November 2013.
  30. ↑ "Falling European satellite burns up upon re-entry". CNN. Retrieved 11 November 2013.
  31. ↑ http://news.discovery.com/space/twitter-goce-burned-up-over-falkland-islands-131111.htm#mkcpgn=rssnws1
  32. ↑ Amos, Jonathan (24 October 2008). "Goce gravity flight slips to 2009". BBC News. Retrieved 26 October 2008.
  33. ↑ "BBC News - Goce satellite views Earth's gravity in high definition". BBC. 28 June 2010. Retrieved 30 June 2010.

External links

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