Satellite to begin gravity quest

By Jonathan Amos
Science reporter, BBC News

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A European spacecraft will begin its quest this week to make the most detailed global map of the Earth's gravity field.

The arrow-shaped Goce satellite can sense tiny variations in the planet's tug as it sweeps around the world at the very low altitude of just 255km.

The map will help scientists understand better how the oceans move.

It should also give them a universal reference to compare heights anywhere across the globe.

Goce was launched from the Plesetsk Cosmodrome in north-west Russia in March.

Engineers have since commissioned the spacecraft, satisfying themselves that all its systems are working properly.

But the satellite has had to wait until now for the right conditions to start its science campaign.

"We've been in the so-called eclipse mode where the Sun doesn't shine fully on the solar panels, but now we are entering the measurement mode," said Dr Volker Liebig, the director of Earth observation at the European Space Agency (Esa).

"Being able to fly low is very good for the measurements, and the Sun is so low in its activity at the moment that we can fly lower than we expected; and that will give us a better signal," he told BBC News.

The data return will be the first to come from Esa's Earth Explorer programme, which is sending up a fleet of small satellites tasked with acquiring key information on issues of environmental concern.

GRAVITY FIELD AND STEADY-STATE OCEAN CIRCULATION EXPLORER

1. The 1,100kg Goce is built from rigid materials and carries fixed solar wings. The gravity data must be clear of spacecraft 'noise'

2. Solar cells produce 1,300W and cover the Sun-facing side of Goce; the near side (as shown) radiates heat to keep it cool

3. The 5m-by-1m frame incorporates fins to stabilise the spacecraft as it flies through the residual air in the thermosphere

4. Goce's accelerometers measure accelerations that are as small as 1 part in 10,000,000,000,000 of the gravity experienced on Earth

5. The UK-built engine ejects xenon ions at velocities exceeding 40,000m/s; the engine throttles up and down to keep Goce at a steady altitude

6. S Band antenna: Data downloads to the Kiruna (Sweden) ground station. Processing, archiving is done at Esa's centre in Frascati, Italy

7. GPS antennas: Precise positioning of Goce is required, but GPS data in itself can also provide some gravity field information

The precision with which Goce expects to make its gravity measurements means every disturbance the satellite could experience has to be minimised.

This includes avoiding the rapid and deep swings in temperature that come from different parts of the spacecraft going into and out of sunlight.

Goce data will be used to construct an idealised surface called a geoid More details

Goce now flies with the Sun's rays constantly streaming on to one side of the satellite. Its permanently shadowed side radiates heat out into space.

To make its map, the satellite carries a set of six state-of-the-art accelerometers housed in a device called a gradiometer.

As the spacecraft "bumps" through Earth's gravity field, the accelerometers sense fantastically small deviations - as small as one part in 10,000,000,000,000 of the gravity experienced at the Earth's surface.

But to make the most of this sensitivity, Goce flies extremely low - much lower than is normal for Earth observation satellites.

Mission controllers have been fortunate in that solar activity which acts to excite the Earth's atmosphere and raise its upper reaches has been exceptionally calm. The density of residual air molecules that might otherwise buffet the spacecraft and introduce further noise into the data has been greatly reduced as a consequence.

In the zone

Goce can now fly lower than its originally planned science altitude of 268km. Controllers currently have the spacecraft orbiting at 254.75km. They are throttling its ion engine up and down to keep it within a 50m band.

Given that the errors in the gravity map double with every 20km of additional altitude, the agency now expects a very fruitful science campaign.

"We've been trying to discover just how good our measurements are," Dr Rune Floberghagen, Esa's Goce mission manager, told BBC News.

"We've taken our measurements and we've compared them with the state-of-the-art models of the Earth's gravity field that are available today. And what we see is what we hoped to see: our measurements basically follow the trend but certainly do contain a lot more high-frequency, finer-scale information."

There is a misconception that the Earth's pull is the same everywhere. In reality, it differs ever so slightly from place to place.

One of the reasons is that our planet 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.

Although the variations in the gravity field are subtle, they have an important influence on many Earth systems.

Without a sense of how gravity helps pull water from one location to another, scientists cannot have a full understanding of ocean currents.

And without knowing where gravity shifts mean sea-level from location to location, scientists cannot properly compare the heights of objects, such as the mountain ranges on different continents.

GRAVITY FIELD AND STEADY-STATE OCEAN CIRCULATION EXPLORER

1. Goce senses tiny variations in the pull of gravity over Earth

2. The data is used to construct an idealised surface, or geoid

3. It traces gravity of equal 'potential'; balls won't roll on its 'slopes'

4. It is the shape the oceans would take without winds and currents

5. So, comparing sea level and geoid data reveals ocean behaviour

6. Gravity changes can betray magma movements under volcanoes

7. A precise geoid underpins a universal height system for the world

8. Gravity data can also reveal how much mass is lost by ice sheets

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