Curtain falls on GOCE gravity probe mission
By David Szondy
October 21, 2013
ESA announced on Monday that its Gravity field and steady-state Ocean Circulation Explorer (GOCE) has ended its extended mission to map the Earth’s gravitational field. Orbiting the Earth at an altitude of 224 km (139 mi), the unmanned probe, known as the “Ferrari of space” because of its streamlined shape, has run out of fuel for the ion engine that kept it in orbit and is expected to reenter the Earth’s atmosphere within two weeks.
Launched in March 2009 from Russian’s Plesetsk Cosmodrome, GOCE was developed by an industrial consortium of 45 companies in over 13 European countries. The octagonal 1,100-kg (2,425-lb) satellite was designed to always show the same side to the Sun to power its four body-mounted and two wing-mounted solar panels that are backed up by a lithium battery in the bow.
It was designed to measure the Earth’s gravitational field to within one millionth of a gravity, which required it to orbit at low altitude. Because of the absolute precision required, it is essentially a single instrument with no moving parts rather than a satellite carrying instrument packages.
GOCE was nicknamed the “Ferrari of space” because of its symmetrical, somewhat streamlined shape with winglets to reduce drag due to it orbiting so low that it is passing through wisps of the upper atmosphere. In the stern is an ion engine to keep it in orbit because a conventional rocket would cause too much vibration and would have had a much more limited life.
The heart of GOCE is the Electrostatic Gravity Gradiometer (EGG) for the measurement of gravitational differences between a set of test masses inside the satellite by means of six 3-axis accelerometers mounted in a diamond configuration in an ultra-stable structure. This is the first time the measurement of gravity gradients in all directions in space has been attempted.
In addition to the EGG, GOCE also has a GPS tracking system to provide additional measurements of the satellite’s orbit by triangulating with 12 GPS satellites. If this wasn't precise enough, there’s also a Laser Retroreflector to allow the satellite’s position to be precisely measured by ground stations.
GOCE has provided the most accurate and detailed 3D map yet of the Earth’s gravity field, with the data supplied by GOCE allowing scientists to learn a great deal about the structure of the Earth and its oceans. One major achievement was the creation of the “geoid,” which is the shape of an ideal global ocean as it would appear under only the influence of rotation and gravity and not tides and wind. It provides a zero point for Earth scientists to measure from and shows that, far from uniform, sea level is altered by the Earth’s uneven gravitational field and affects ocean circulation and local sea levels. GOCE was able to map the geoid to an accuracy of less than 2 cm (0.8 in).
Other scientific milestones for GOCE’s gravity measurements included mapping the dynamic topography and circulation patterns of the oceans, creating the first global high-resolution map of the Moho (the boundary between Earth’s crust and mantle), measuring the thickness and movements of the polar ice caps, and acting as the first seismometer in orbit when it detected changes in air density in the form of infrasound caused by the earthquake that hit Japan on March 11, 2011.
GOCE completed its primary mission in April 2011, which is when mission planners expected the ion engine would be exhausted by. However, solar activity is mostly what causes the atmosphere to expand and cause orbital drag and this activity was lower than expected over the past decade. As a result, there was less drag and, consequently, enough fuel to extend the mission.
But by this year, it was clear that GOCE would soon run out of fuel, so in anticipation of the mission’s end, the satellite was brought down from an altitude of 255 km (158 mi) to 224 km (139 mi) to improve its resolution and accuracy.
“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,” says Volker Liebig, ESA’s Director of Earth Observation Programmes. “The outcome is fantastic. 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.”
On October 21, ESA declared the GOCE mission at an end when the satellite ran out of xenon fuel for its ion engine. About 350 kg (771 lb) of xenon is left in the tank, but on that day, the fuel pressure dropped below 2.5 bar, which is the minimum pressure needed to feed the engine.
GOCE is expected to hit the atmosphere in about two weeks. During this time, the satellite will continue to send back data until the atmosphere causes the systems to stop working, after which, it will be shut down by mission control. It’s expected to break up in the atmosphere, though the exact time and location won’t be known until later. ESA’s Space Debris Office will monitor the re-entry and provide updated predictions.
The video below shows GOCE’s technology.
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