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

ESA The European Space Agency (ESA) is a 23-member international organization devoted to space exploration. With its headquarters in Paris and a staff of around 2,547 people globally as of 2023, ESA was founded in 1975 in the context of European ...

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 miss ...

heavy satellites intended to map in unprecedented detail the

Earth's gravity field The gravity of Earth, denoted by , is the net acceleration that is imparted to objects due to the combined effect of gravitation (from mass distribution within Earth) and the centrifugal force (from the Earth's rotation). It is a vector quantity ...

. The spacecraft's primary instrumentation was a highly sensitive gravity

gradiometer A gradiometer measures the gradient (numerical rate of change) of a physical quantity, such as a magnetic field or gravity. Types of gradiometer There are at least two types of gradiometer to measure magnetic fields: # ''Axial gradiometer''. ...

consisting of three pairs of

accelerometer An accelerometer is a device that measures the proper acceleration of an object. Proper acceleration is the acceleration (the rate of change (mathematics), rate of change of velocity) of the object relative to an observer who is in free fall (tha ...

s which measured gravitational gradients along three

orthogonal In mathematics, orthogonality (mathematics), orthogonality is the generalization of the geometric notion of ''perpendicularity''. Although many authors use the two terms ''perpendicular'' and ''orthogonal'' interchangeably, the term ''perpendic ...

axes. Launched on 17 March 2009, 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 Ocean surface topography or sea surface topography, also called ocean dynamic topography, are highs and lows on the ocean surface, similar to the hills and valleys of Earth's land surface depicted on a topographic map. These variations are exp ...

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 geoid ( ) is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent. This surface is exte ...

(the theoretical surface of equal gravitational potential on the Earth). The satellite's unique arrow shape and

fins A fin is a thin component or appendage attached to a larger body or structure. Fins typically function as foil (fluid mechanics), foils that produce lift (force), lift or thrust, or provide the ability to steer or stabilize motion while travelin ...

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 An ion thruster, ion drive, or ion engine is a form of electric propulsion used for spacecraft propulsion. An ion thruster creates a cloud of positive ions from a neutral gas by ionizing it to extract some electrons from its atoms. The ions ...

system continuously compensated for the variable deceleration due to air drag without the vibration of a conventional chemically powered

rocket engine A rocket engine is a reaction engine, producing thrust in accordance with Newton's third law by ejecting reaction mass rearward, usually a high-speed Jet (fluid), jet of high-temperature gas produced by the combustion of rocket propellants stor ...

, 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 an inertial space or a Galilean reference frame) is a frame of reference in which objects exhibit inertia: they remain at rest or in uniform motion relative ...

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

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 A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the lithospheric mantle, the topmost portion of the upper mantle that behaves elastically on time ...

, 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 science, atmospheric, Oceanography, ocean, Hydrology, hydrologic, Astronomy, space, and Planetary science, planetary scientists and enthusiasts that ...

(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 The Mann Eddy is a very small feature of ocean currents in the Atlantic. It is a persistent clockwise circulation in the middle of the North Atlantic ocean, specifically "a mesoscale anticyclone, adjacent to the path of the North Atlantic Current ...

in the

North Atlantic The Atlantic Ocean is the second largest of the world's five oceanic divisions, with an area of about . It covers approximately 17% of Earth's surface and about 24% of its water surface area. During the Age of Discovery, it was known for ...

were visible in the data, as was the effect of

Hurricane Igor Hurricane Igor was a very large tropical cyclone which became the most destructive 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 September& ...

in 2010. Detailed analysis of GOCE's thruster and accelerometer data

serendipitously Serendipity is an unplanned fortunate discovery. The term was coined by Horace Walpole in 1754. The concept is often associated with scientific and technological breakthroughs, where accidental discoveries led to new insights or inventions. Man ...

revealed that it had detected the

infrasound Infrasound, sometimes referred to as low frequency sound or incorrectly subsonic (subsonic being a descriptor for "less than the speed of sound"), describes sound waves with a Audio frequency, frequency below the lower limit of human audibility ...

waves generated by the

2011 Tōhoku earthquake Eleven or 11 may refer to: *11 (number) * One of the years 11 BC, AD 11, 1911, 2011 Literature * ''Eleven'' (novel), a 2006 novel by British author David Llewellyn *''Eleven'', a 1970 collection of short stories by Patricia Highsmith *''Eleven'' ...

(whereupon it inadvertently became the first

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

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 The Tethys Ocean ( ; ), also called the Tethys Sea or the Neo-Tethys, was a prehistoric ocean during much of the Mesozoic Era and early-mid Cenozoic Era. It was the predecessor to the modern Indian Ocean, the Mediterranean Sea, and the Eurasia ...

. 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 "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 rifting of contine ...

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 () is a Russian spaceport located in Mirny, Arkhangelsk Oblast, near the town of Plesetsk, from which it takes its name. Until 2025 and the commissioning of the Andøya Space, Andøya base in Norway, it was the only operati ...

in northern Russia aboard a

Rokot Rokot ( meaning ''Rumble'' or ''Boom''), also transliterated Rockot, was a Soviet Union (later 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 ( ...

Briz-KM The Briz-K, Briz-KM and Briz-M ( meaning ''Breeze-K, KM and M'') are Russian liquid-propellant rocket orbit insertion upper stages manufactured by Khrunichev State Research and Production Space Center and used on the Proton-M and Angara A5. T ...

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. Dev ...

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 A Sun-synchronous orbit (SSO), also called a heliosynchronous orbit, is a nearly polar orbit around a planet, in which the satellite passes over any given point of the planet's surface at the same local mean solar time. More technically, it is ...

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 Spacecraft attitude control is the process of controlling the orientation of a spacecraft (vehicle or satellite) with respect to an inertial frame of reference or another entity such as the celestial sphere, certain fields, and nearby objects, ...

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,

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. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...

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 ( ; , ) is an extensive geographical region comprising all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has formed a part of the sovereign territory of Russia and its predecessor states ...

, the western

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

, the eastern

Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or approximately 20% of the water area of Earth#Surface, Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia (continent), ...

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 (also known as the Antarctic Ocean), it contains the geographic South Pole. ...

. The satellite finally disintegrated around 00:16

UTC Coordinated Universal Time (UTC) is the primary time standard globally used to regulate clocks and time. It establishes a reference for the current time, forming the basis for civil time and time zones. UTC facilitates international communica ...

on 11 November near the

Falkland Islands The Falkland Islands (; ), commonly referred to as The Falklands, 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 from Cape Dub ...

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 Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

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

allowed the simultaneous measurement of the five independent components of the gravity gradient tensor. Other payload was an onboard

GPS The Global Positioning System (GPS) is a satellite-based hyperbolic navigation system owned by the United States Space Force and operated by Mission Delta 31. It is one of the global navigation satellite systems (GNSS) that provide geol ...

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'' originated as an acronym for light amplification by stimulated emission of radi ...

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

to enable tracking by ground-based

Satellite laser ranging Satellite laser ranging (SLR) is a method to measure the distance to satellites in a geocentric orbit. It consists of an astronomical observatory equipped with a laser that sends ultrashort pulses of light. The pulses hit the satellite and boun ...

stations.

Power and propulsion

GOCE's frame had fixed

solar panels A solar panel is a device that converts sunlight into electricity by using photovoltaic (PV) cells. PV cells are made of materials that produce excited electrons when exposed to light. These electrons flow through a circuit and produce direct ...

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

watt The watt (symbol: W) is the unit of Power (physics), 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 quantification (science), quantify the rate of Work ...

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

. The

electric engine, designed and built at

QinetiQ QinetiQ ( as in '' kinetic'') is a British defence technology company headquartered in Farnborough, Hampshire. It operates primarily in the defence, security and critical national infrastructure markets and run testing and evaluation capabili ...

's space centre in Farnborough, England, ejected

xenon Xenon is a chemical element; it has 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).