GOCE satellite achieves drag-free perfection (w/Videos)

May 26, 2009
GOCE orbit is so low that it experiences drag from the outer edges of Earth's atmosphere. The satellite's streamline structure and use of electric propulsion system counteract atmospheric drag to ensure that the data are of true gravity. Credits: ESA - AOES Medialab

(PhysOrg.com) -- ESA's gravity mission GOCE has achieved a first in the history of satellite technology. The sophisticated electric propulsion system has shown that it is able to keep the satellite completely free from drag as it cuts through the remnants of Earth's atmosphere - paving the way for the best gravity data ever.

Volker Liebig, ESA's Director of Programmes commented, "I am very pleased to see another world premier by GOCE, after having already demonstrated the most sensitive gradiometer technology ever flown in space."

Launched on 17 March and currently progressing through the commissioning phase, GOCE (Gravity field and steady-state Ocean Circulation Explorer) is set to measure Earth's gravity field with unprecedented accuracy. However, to do this means that the has to orbit Earth as low as possible where the gravitational signal is stronger, but also where the fringes of the atmosphere remain.

To ensure that the gravity measurements taken by GOCE are of true gravity, the satellite has to be kept stable and in 'free-fall'. Any buffeting from residual air the satellite encounters along its orbital altitude of just 250 km could potentially drown the gravity data. This posed a technological challenge - the satellite structure had to be as aerodynamic as possible and a system had to be developed that would continually and instantaneously compensate for variations in air drag.

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The need for GOCE to fly in a low orbit means it has to be equipped with a system to compensate for the atmospheric drag it will experience at this exceptionally low altitude. The animation shows a close-up of the ion-propulsion assembly, which keeps the satellite in a smooth trajectory - free from all effects except the gravity field itself. Two winglets provide additional aerodynamic stability. Credits: ESA - AOES Medialab

Along with its sleek design, GOCE is able to achieve drag-free flight by employing an electric ion propulsion system mounted at the back of the satellite, relative to its direction of flight. Unlike conventional fuel-driven engines, the system uses electrically-charged xenon to create a gentle thrust. The system continually generates tiny forces between 1 and 20 millinewtons (mN), depending on how much drag the satellite experiences as it orbits Earth.

GOCE was recently switched to drag-free mode as part of the commissioning and instrument calibration activities. The system was found to be working perfectly, demonstrating that the electric ion thruster-based control system automatically produces the right amount of thrust to achieve drag-free flight. This could not be demonstrated at ground level before GOCE launched since it is impossible to create exact in-orbit flight conditions in a laboratory.

Michael Fehringer, ESA's System Manager for GOCE said, "We were very pleased with the results when the ion propulsion system and the gradiometer, which is the main instrument, were commissioned separately a couple of weeks ago. Now, both systems work together flawlessly - the gradiometer continually senses the air drag and feeds the ion-propulsion system with commands to produce thrusts exactly opposite to the drag being experienced. Initial data indicate that this drag compensation system works 10 times better than what we had expected."

Not only does this mean that GOCE will deliver clean gravity data but it also marks a significant step in satellite technology. GOCE is the first-ever mission to fly drag free in low-Earth orbit using an electric . Other missions using advanced electric propulsion technology only have the system turned on for short periods of time. Once operational, GOCE's system will be constantly compensating for the variations in air drag as it orbits Earth.

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GOCE will map the global variations in the gravity field with extreme detail and provide a unique model of Earth's gravity field and its geoid. The geoid is the surface of equal gravitational potential defined by the gravity field and is crucial for improving our understanding of ocean circulation, sea-level change and terrestrial ice dynamics, all of which are affected by climate change. GOCE-derived data will also provide new insight into processes occurring in the lithosphere and upper mantle. In addition, data will be used for practical applications such as surveying and levelling. Credits: ESA - AOES Medialab

Rune Floberghagen, ESA's GOCE Mission Manager stated that, "Knowing that the drag-free control system works perfectly means we now have everything in place to carry out the complex process of calibrating the gradiometer instrument. Once calibration has been completed we will be able to see the true excellence of GOCE's gravity-field measurements."

Over its life of about 24 months, will map global variations in the gravity field with extreme detail and accuracy. This will result in a unique model of the 'geoid', which is the surface of equal gravitational potential defined by the - crucial for deriving accurate measurements of ocean circulation and sea-level change, both of which are affected by climate change. GOCE-derived data are also much needed to understand more about processes occurring inside the Earth and for use in practical applications such as surveying and levelling.

Provided by European Space Agency (news : web)

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User comments : 11

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Ronan
5 / 5 (2) May 27, 2009
Fantastic! I've read a bit about this satellite before now, and I'm glad to hear that all is going as it should be--or better, as it happens. I'm really looking forward to reading about the science that it will make possible. Congratulations and kudos to the ESA and everyone involved in this project!
denijane
5 / 5 (1) May 27, 2009
Cool! I wish GOCE all best! And I can't wait for the data :)
superhuman
5 / 5 (3) May 27, 2009
GOCE is the first-ever mission to fly drag free in low-Earth orbit using an electric propulsion system.

This is a blatant lie, it is NOT "drag free," not anymore then a helicopter hovering in the air is "gravity free." Compensating for drag is something completely different then removing drag.

This tendency to distort reality for the sake of publicity is appalling especially in the case of ESA a scientific organization whose goal should be to educate people about physics instead of misleading them.
seanpu
5 / 5 (2) May 27, 2009
ah true superhuman, but we can say that drag is being monitored and attempts are being made to adjust for the observed drag, therefore we are nearly drag-free.

you are rights, it impossible to be completely drag-free, there has to be a feedback loop and its during that feedback that drag is adjusted for.

but, in fairness, knowing the drag being imparted helps to adjust other measurements against that observed drag.

well pointed out, they shouldn't try to blind the general public with impossibilities, but that is the way of science these days so we can only make each other aware of these things via such forums as this.
Egnite
5 / 5 (1) May 27, 2009
Aye pretty poor scientific report but we're getting quite used to Physorg exagerating headlines and stories. Still very exciting although the ion drive is the exciting part imo, not a drag calculating/compensating device.
bmcghie
5 / 5 (2) May 27, 2009
I must say, it's nice to hear of something being fired up for the first time and having it work BETTER than expected. That doesn't happen everyday. Hats off to the ESA.
lengould100
not rated yet May 27, 2009
denijane is anxious for the data. I wonder if the true data from this experiment will be made public? Seems to me it would make finding boomer subs childs play.
djp
not rated yet May 28, 2009
Lets get to the big elephant in the article shall we. Ion Propulsion drive? Really! By all means, please provide more details about how this technology was achieved and who made it!
Birger
not rated yet May 30, 2009
Answer to djp: Small ion drives have been in use for some time to keep geostationary satellites in place (they drift sideways because of irregularities in Earths geoid). Since previous ion drives burned themselves out after some time, the START-1 technology demonstrator tested a more durable engine a year ago, spiralling all the way up to the Moon. All these engines use the reaction force of ions accelerated with energy taken from photoelectric cells. This force is very small and cannot be used by a vehicle starting from the surface of a planet, but adds up to a high total velocity for interplanetary travel. The Soviets planned to use ion drives for a manned mission to Mars, but the country collapsed before the technology had been developed.
Birger
not rated yet May 30, 2009
Mapping the Earth's geoid with high precision might help with a very prosaic but economically important task: Searching for metal ores using airborne gravimetry. To find the tiny anomalies that signal ore bodies requires the aircraft with the gravimetry detector to use a satellite positioning system to compensate for the bumps and deviations in the flight path, but also a map of the general backround of the gravitational field so systematic errors can be eliminated.
Also, a gravimetric map will provide the locations of the ca. 100 000 seamounts in the ocean, a cornucopia for marine biologists studying the deep-water corals who often live on the peaks of these underwater mountains.
ShotmanMaslo
not rated yet May 31, 2009
Isnt it beautiful?

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