October 24, 2008

GOCE Gravity Flight Pushed Back Another Year

Ongoing technical problems with its launcher has forced Europe's gravity mission to be bumped to next year.

The arrow-shaped GOCE satellite will map tiny variations in the pull of gravity experienced across the world, giving scientists a clearer insight into how the oceans move, and provide a universal reference to measure height anywhere on Earth.

But reliability concerns over its Russian rocket means a lift-off is now unlikely before February.

The delay is frustrating for the Gravity Field and Steady-State Ocean Circulation Explorer team(GOCE).

Immense technical difficulties during the satellite's construction had already put it two years behind schedule.

The GOCE was due to go into orbit in Spring on a modified intercontinental ballistic missile, known as the Rockot, from the Plesetsk Cosmodrome in north-west Russia.

But an investigation was undertaken into the performance of a different, failed rocket system that shared key components.

And soon after, GOCE's own launcher then became the focus of separate investigations that have now resulted in a further, extended delay.

Scientists pinpointed the problem on the guidance and navigation units of the Breeze upper stage that has been added to the ICBM to inject satellite payloads into their correct orbits.

Throughout GOCE's mission, it will sense the very subtle gravity anomalies that exist across the planet.

The idea that Earth's pull is the same everywhere is a popular misconception. In reality, it differs slightly from place to place.

This is because the Earth is not a perfect sphere - it is flatter at the poles, fatter at the equator. Its interior layers are also not composed of uniform shells of homogenous rock - some regions are thicker or denser.

That makes for an irregular distribution of mass; and as everything that has mass is pulled by gravity, that tug becomes irregular, too.

GOCE will carry a special device known as a gradiometer to measure these anomalies, where the variations are almost imperceptible.

The gradiometer incorporates precision-built accelerometers that sense accelerations that are as small as 1 part in 10,000,000,000,000 of the gravity experienced on Earth.

But GOCE needs to fly low, below 300km for optimal performance. This is difficult because the residual atmosphere at that altitude can jostle the spacecraft and introduce "noise" into the data.

Activity on the Sun is linked indirectly to how much air is present at the flight altitude. The Sun heats the upper reaches of the atmosphere, altering its density on an 11-year-cycle.

Should the Sun gets more active in the coming months and years, as expected, GOCE may have to fly higher than was anticipated, limiting the resolution of its data.


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