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GOCE gravity-mapping satellite falling back to Earth
BY STEPHEN CLARK
SPACEFLIGHT NOW

Posted: November 2, 2013


A sleek European satellite is coming back to Earth after depleting its supply of xenon propellant last month, and European Space Agency controllers say an uptick in solar activity could hasten the spacecraft's fiery fall from orbit.


Artist's concept of the GOCE satellite in orbit. Credit: ESA/AOES Medialab
 
Designed with an aerodynamic shape for its unusually low operating orbit, the Gravity Field and Steady-State Ocean Explorer, or GOCE, satellite measured the planet's global gravity field over its four-and-a-half years in space.

"This innovative mission has been a challenge for the entire team involved: from building the first gradiometer for space to maintaining such a low orbit in constant free-fall, to lowering the orbit even further," said Volker Liebig, ESA's director of Earth observation programs.

"The outcome is fantastic," Liebig said. "We have obtained the most accurate gravity data ever available to scientists. This alone proves that GOCE was worth the effort - and new scientific results are emerging constantly."

GOCE ran out of its fuel supply Oct. 21, leaving the spacecraft without a way to maintain its orbit and beginning a gradual slip from space and back into Earth's atmosphere.

As of Oct. 31, GOCE's altitude had dropped to 205 kilometers, or 127 miles, a loss of nearly 12 miles since the satellite ran out of fuel. Officials expect GOCE's decay rate to increase in the coming days as it dips into thicker layers of the atmosphere.

Recent eruptions from the surface of the sun could also be a factor. Solar activity causes Earth's atmosphere to balloon, meaning more air molecules are present at GOCE's altitude to impart more drag on the satellite, lower its velocity and reduce its altitude.

Without fuel, GOCE's mass is 1,002 kilograms, or 2,209 pounds. The spacecraft measures 17.4 feet long and more than 3 feet in diameter. Most of that will be annihilated by heat generated from friction as GOCE streaks into the atmosphere at more than 17,000 mph.

It's impossible to predict where GOCE will come down, but any small components which do survive re-entry will almost certainly fall in the ocean or in unpopulated areas. GOCE's ground track takes it over all of the planet's large land masses.

"Taking into account that two-thirds of Earth are covered by oceans and vast areas are thinly populated, the danger to life or property is very low," ESA said in a statement.

As of Nov. 1, GOCE's re-entry was predicted to occur between Nov. 5 and Nov. 10, according to Christoph Steiger, the mission's spacecraft operations manager at the European Space Operations Center in Darmstadt, Germany.


Several large, active regions on the sun burst out with about 20 eruptions between Oct. 25 and Oct. 28. Some were flares, some were coronal mass ejections, and at least one was a prominence eruption. This image from Oct. 26 combines imagery from the Solar Dynamics Observatory and the SOHO mission. An increase in solar activity translates to more atmospheric drag in low Earth orbit. Credit: NASA/ESA
 
The $474 million mission's science mission is over, but GOCE's GPS navigation receivers and gradiometer are still collecting data, yielding unparalleled information on atmospheric drag at such low altitudes.

GOCE's gradiometer instrument is actually three pairs of accelerometers mounted inside the satellite. They detect the tug of gravity as the craft flies around Earth, mapping global changes in gravity's pull.

Although the gradiometer is no longer pursuing information on Earth's gravity field, the sensors continue to measure the satellite's rate of deceleration caused by drag.

"The accurate accelerometers that are part of GOCE's gravity gradiometer instrument, in combination with information from its drag-free control system, can be used to detect the aerodynamic force on the satellite," said Eelco Doornbos from Delft University of Technology in the Netherlands.

"This force is caused by molecules and atoms in the upper layer of the atmosphere bouncing off the satellite's outer surfaces," Doornbos said in an ESA statement. "Through careful processing, we have been able to gain four years' worth of data on upper-atmospheric density and winds from these accelerations."

The GOCE mission - the first to launch in ESA's Earth Explorer program - was designed for a fiery farewell from the beginning.

GOCE does not carry a reservoir of conventional chemical rocket fuel. After the satellite's launch in March 2009, controllers activated GOCE's electrically-powered ion engine to maintain the craft's orbit at at an altitude of 255 kilometers, or 158 miles, for three years.

Last year, ESA commanded GOCE to reduce its altitude even further to obtain unrivaled data on the planet's lumpy gravity field. The lower the altitude, the better the science, according to researchers.

ESA touts GOCE's results as the best satellite measurements ever made of Earth's gravity field.

It mapped the Moho, the boundary between Earth's crust and mantle, with more precision than possible before GOCE. In another first, GOCE's gravity sensor also picked up sound waves generated by the March 2011 Tohuku earthquake off the Japanese coast, which spawned a deadly tsunami that killed more than 15,000 people.


ESA's GOCE mission has delivered the most accurate model of the 'geoid' ever produced, which will be used to further our understanding of how Earth works. Credit: ESA/HPF/DLR
 
Once processed and analyzed, data collected from GOCE's low-altitude orbit could shed light on ocean dynamics, such as eddy currents, according to ESA. A three-dimensional global map produced from GOCE data, called a geoid, shows Earth's surface as influenced only by gravity - without the tug of ocean tides and currents.

Geologists use the geoid as a reference for measurements of ocean currents, rising sea levels, and ice movements.

"The data GOCE acquired during these last months when the satellite was put in a lower orbit will give us more exciting details about ocean circulation, which we plan to release in mid-2014," said Marie-Helene Rio, a scientist from the French company CLS, which studies variability in ocean height.

GOCE's orbit skimming through the outermost layers of Earth's atmosphere meant it would rapidly drop from orbit once its 88 pounds of xenon gas xenon gas ran out, rendering the spacecraft's ion engine powerless to counteract drag.

With a distinctive arrowhead shape, GOCE has small winglets and a tail fin help stabilize the spacecraft as it circles Earth. Steiger said GOCE remains pointed along its ground track to minimize its cross-section and reduce the effect of drag, buying the mission a few extra weeks before re-entry.

GOCE's appearance has earned it the nickname "Ferrari of space."

Follow Stephen Clark on Twitter: @StephenClark1.

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