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Experts say fall of European satellite poses little risk
BY STEPHEN CLARK
SPACEFLIGHT NOW

Posted: November 8, 2013


A European research satellite is expected to succumb to atmospheric drag and make an uncontrolled re-entry Sunday or Monday, disintegrating from ferocious aerodynamic forces more than 50 miles up and scattering a trail of debris along a 500-mile ground track.


Artist's concept of the GOCE satellite in orbit. Credit: ESA/AOES Medialab
 
The European Space Agency says its $466 million Gravity Field and Steady-State Ocean Explorer, or GOCE, satellite was orbiting at an altitude of less than 110 miles Friday. With its propellant tank drained, GOCE's fall back to Earth could occur anywhere.

But the odds of hitting someone are staggeringly low. You are more likely to be hit by a meteorite, and your chances of winning the lottery are much better.

"The risk to the population on ground will be minute," said Heiner Klinkrad, head of the European Space Agency's space debris office in Darmstadt, Germany. "Statistically speaking, it is 250,000 times more probable to win the jackpot in the German Lotto than to get hit by a GOCE fragment. In 56 years of space flight, no man-made space objects that have re-entered into Earth's atmosphere have ever caused injury to humans."

Fragments from the arrowhead-shaped satellite will most likely fall in the ocean or on unpopulated land, but it is impossible to predict exactly when atmospheric drag will be sufficient to force GOCE to re-enter. Little is known about the dynamics of the upper atmosphere, which can fluctuate with solar activity.

GOCE ran out of xenon fuel Oct. 21. Ground controllers are still in contact with the satellite, which continues to gather data with its gravity-detecting instrument, but there is no longer a way to maintain its orbit. The fuel exhaustion precipitates a gradual slip from space and back into Earth's atmosphere.

Before it emptied its fuel tank, the satellite was already operating in an unusually low orbit about 140 miles above Earth. Flying in such an orbit allowed GOCE to collect unrivaled data on Earth's global gravity field, revealing its lumpy variability and mapping the Moho, the boundary between the crust and mantle, with more precision than ever before.

The effect of atmospheric drag in GOCE's low orbit required constant thrusting from the satellite's electrically-powered, xenon-fueled ion engine, depleting its 88-pound supply of xenon gas in four-and-a-half years since the mission launched in March 2009.

Engineers fitted GOCE with wings and tail fins, giving the satellite the appearance of an airplane and helping the slim spacecraft glide through the outer reaches of the atmosphere.

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.

Rune Floberghagen, GOCE's mission manager at ESA, said engineers have analyzed the satellite's re-entry using independent and in-house computer models.

"We expect most of the spacecraft to burn up in the Earth's atmosphere, but there are some pieces that, according to their analysis, could be expected to reach the surface," Floberghagen said in an interview with Spaceflight Now.

Between 20 percent and 25 percent of GOCE's mass could survive the fiery plunge through the atmosphere, according to Floberghagen and Klinkrad. The rest of the satellite should completely burn up.

"When the spacecraft reaches altitudes below 100 kilometers (62 miles), then atmospheric density will drastically increase. Starting with a velocity of about 25,000 kilometers per hour (15,534 mph), air drag with the concomitant aerodynamic pressure and heating will cause a break-up of the spacecraft at approximately 80 kilometers (49 miles) altitude, causing a large number of fragments," Klinkrad said in a post on ESA's website.


Photo of GOCE inside a space environment simulator at ESA's test center in the Netherlands. Credit: ESA
 
Built by an industrial consortium led by Thales Alenia Space of Italy and Astrium GmbH of Germany, GOCE does not carry large spherical propellant tanks, which commonly reach the surface when derelict rockets and satellite re-enter the atmosphere. That removes one source of potential debris.

"The biggest worry from GOCE's perspective is the core of the instrument," Floberghagen said. "The instrument is made of carbon-carbon. Carbon is abrasive, and it will erode and burn, but we have massive, strong and solid structures, in particular the core structure that holds this gradiometer instrument. That is expected to survive, along with part of the instrument itself, the sensors and so on."

The gradiometer, mounted near the center of the satellite, includes three pairs of identical accelerometers that can detect minuscule gravity changes as the spacecraft flies around Earth.

"We expect, to be absolutely clear, that about one-quarter of the mass, in a train of fragments, may eventually hit the ground," Floberghagen said. "This is amounting to something in the order of a maximum of 250 kilograms (551 pounds) in a train of fragments that will be spread along a trajectory which could be up to 900 kilometers (559 miles) long. There is no such thing as an impact point; there will be an impact trajectory."

According to Floberghagen, the analysis shows the largest chunk of GOCE that could fall to Earth's surface is less than 90 kilograms, or 198 pounds.

About 100 tons of space debris falls to Earth every year, Floberghagen said, so the re-entry of 1.1-ton GOCE satellite represents 1 percent of the total mass of man-made objects coming back to Earth annually.

"We think everybody should put this re-entry in perspective," Floberghagen said.

"It's the first re-entry of this kind of an ESA satellite in more than 25 years," Floberghagen said. "The [political] climate now is very different. There's a lot of focus on space debris, not only with cleaning space but also on re-entry and the risks associated with re-entry. We need to be aware of this because, sooner or later, someone is going to be unlucky and the be first one to hit something."

The re-entry of GOCE comes two years after NASA's decommissioned Upper Atmospheric Research Satellite, or UARS, made an uncontrolled re-entry over the Pacific Ocean after it garnered global attention in the weeks leading up to its demise.

The risk from UARS was higher than from GOCE, which is six times less massive than the NASA atmospheric research satellite.

Follow Stephen Clark on Twitter: @StephenClark1.

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