The Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) was the first of ESA's

Living Planet Programme The Living Planet Programme (LPP) is a programme within the European Space Agency which is managed by the Earth Observation Programmes Directorate. LPP consists of two classes of Earth observation missions (listed below) including research missio ...

satellite A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioiso ...

s 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

accelerometer An accelerometer is a tool that measures proper acceleration. Proper acceleration is the acceleration (the rate of change of velocity) of a body in its own instantaneous rest frame; this is different from coordinate acceleration, which is acce ...

s which measured gravitational gradients along three

orthogonal In mathematics, orthogonality is the generalization of the geometric notion of '' perpendicularity''. By extension, orthogonality is also used to refer to the separation of specific features of a system. The term also has specialized meanings in ...

axes. Launched on 17 March 2009, GOCE mapped the deep structure of the Earth's

mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...

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

thermosphere The thermosphere is the layer in the Earth's atmosphere directly above the mesosphere and below the exosphere. Within this layer of the atmosphere, ultraviolet radiation causes photoionization/photodissociation of molecules, creating ions; the ...

at a comparatively low altitude of . 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 A rocket engine uses stored rocket propellants as the reaction mass for forming a high-speed propulsive jet of fluid, usually high-temperature gas. Rocket engines are reaction engines, producing thrust by ejecting mass rearward, in accorda ...

, 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 In classical physics and special relativity, an inertial frame of reference (also called inertial reference frame, inertial frame, inertial space, or Galilean reference frame) is a frame of reference that is not undergoing any acceleration. ...

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

Discoveries and applications

Mission objectives

* To determine gravity-field anomalies with an accuracy of (1 mGal). To increase resolution, the satellite flew in an unusually low orbit. * To determine the geoid with an accuracy of 1–2 cm. * To achieve the above at a spatial resolution better than 100 km.

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: * A better understanding of the physics of the Earth's interior to gain new insights into the geodynamics associated with the lithosphere, 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.

Findings

The first Earth global gravity model based on GOCE data was presented at ESA's Living Planet Symposium, in June 2010. Initial results of the GOCE satellite mission were presented at the

American Geophysical Union The American Geophysical Union (AGU) is a 501(c)(3) nonprofit organization of Earth, atmospheric, ocean, hydrologic, space, and planetary scientists and enthusiasts that according to their website includes 130,000 people (not members). AGU's ...

(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. Even very small details like the Mann Eddy in the

North Atlantic The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe a ...

were visible in the data, as was the effect of

Hurricane Igor Hurricane Igor was a very large Cape Verde hurricane and the most destructive tropical cyclone on record to strike the Canadian island of Newfoundland. It originated from a broad area of low pressure that moved off the western coast of Africa on ...

in 2010. Detailed analysis of GOCE's thruster and accelerometer data serendipitously revealed that it had detected the infrasound waves generated by the

2011 Tōhoku earthquake Eleven or 11 may refer to: *11 (number), the natural number following 10 and preceding 12 * one of the years 11 BC, AD 11, 1911, 2011, or any year ending in 11 Literature * ''Eleven'' (novel), a 2006 novel by British author David Llewellyn *''E ...

(whereupon it inadvertently became the first

seismograph A seismometer is an instrument that responds to ground noises and shaking such as caused by earthquakes, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to form a seismograph. The outpu ...

in orbit). 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. 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 three

craton A craton (, , or ; from grc-gre, κράτος "strength") is an old and stable part of the continental lithosphere, which consists of Earth's two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging and ...

s beneath the Antarctic ice.

Operations

Launch

The first launch attempt on 16 March 2009 was aborted due to a malfunction with the launch tower. GOCE was launched on 17 March 2009 at 14:21 UTC from the

Plesetsk Cosmodrome Plesetsk Cosmodrome ( rus, Космодром «Плесецк», r=Kosmodrom "Plesetsk", p=kəsmɐˈdrom plʲɪˈsʲet͡sk) is a Russian spaceport located in Mirny, Arkhangelsk Oblast, about 800 km north of Moscow and approximately 200 ...

in northern Russia aboard a

Rokot Rokot (russian: Рокот meaning ''Rumble'' or ''Boom''), also transliterated Rockot, was a Russian space launch vehicle that was capable of launching a payload of into a Earth orbit with 63° inclination. It was based on the UR-100N (SS- ...

/ Briz-KM vehicle. The Rokot is a modified

UR-100N The UR-100N, also known as RS-18A is an intercontinental ballistic missile in service with Soviet and Russian Strategic Missile Troops. The missile was given the NATO reporting name SS-19 Stiletto and carries the industry designation 15A30. Deve ...

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 96.7° 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 in attitude control.

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. 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 , resulting in restoration of normal communications. 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. In November 2012 the orbit was lowered from to get higher resolution data, at which time fuel remained for another 50 weeks.

End of mission and re-entry

In May 2013 a further lowering to took place. The satellite ran out of its xenon propellant in October 2013, at which time it would take to re-enter. 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. 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. On 9 November 2013, a published report indicated that the satellite was expected to re-enter within a day or two. By this date, the

perigee An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any el ...

altitude had decayed to . 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. Its descending orbit on 11 November 2013 passed over

Siberia Siberia ( ; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive geographical region, constituting all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has been a part ...

, the western

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the conti ...

, the eastern

Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by ...

and

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...

. The satellite finally disintegrated around 00:16 UTC on 11 November near the

Falkland Islands The Falkland Islands (; es, Islas Malvinas, link=no ) is an archipelago in the South Atlantic Ocean on the Patagonian Shelf. The principal islands are about east of South America's southern Patagonian coast and about from Cape Dubouze ...

Design

Payload

The satellite's main payload was the Electrostatic Gravity Gradiometer (EGG) to measure the gravity 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 the

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...

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 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 A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The ...

retroreflector A retroreflector (sometimes called a retroflector or cataphote) is a device or surface that reflects radiation (usually light) back to its source with minimum scattering. This works at a wide range of angle of incidence, unlike a planar mirro ...

to enable tracking by ground-based

Satellite laser ranging In satellite laser ranging (SLR) a global network of observation stations measures the round trip time of flight of ultrashort pulses of light to satellites equipped with retroreflectors. This provides instantaneous range measurements of milli ...

stations.

Power and propulsion

GOCE's frame had fixed

solar panels A solar cell panel, solar electric panel, photo-voltaic (PV) module, PV panel or solar panel is an assembly of photovoltaic solar cells mounted in a (usually rectangular) frame, and a neatly organised collection of PV panels is called a photo ...

covering its sun-facing side, which produced 1,300

watt The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James ...

s of power. The panels were shaped to act as fins, stabilising the spacecraft while it orbited through the residual air in the

. The ion propulsion electric engine, designed and built at QinetiQ's space centre in Farnborough, England, ejected

xenon Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...

ions at velocities exceeding , which compensated for the orbital decay losses. GOCE's mission ended when the xenon fuel tank emptied. The dual Kaufman-type ion thrusters could produce up to of thrust. 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).