It's different

Space cartography of gravity

 
Published: Sunday 31 August 2003

a new gravity map of the Earth suggests if you want to feel one per cent lighter you should be in India, where the pull of gravity is a bit less than elsewhere due to the type of mantle material beneath the country's crust.

The map, a colourful tapestry of space-based cartography, shows the little variations in the Earth's gravitational field depending on location. The amount of gravity in any point of the Earth is proportional to the amount of mass of that area. The more the mass, the stronger is its gravitational force.

The map is the first of many gravity maps to be generated by the twin satellites of grace -- the Gravity Recovery and Climate Experiment -- underway by nasa and the German Aerospace Centre. The differences in gravity can reveal much about the structure of the Earth's interior and the oceans. They can reveal Earth's past too as well as help researchers study current climate and sea level changes. Earth's polar ice caps used to be larger than they are today, and the weight of that ice flattened the Earth a bit at the poles. As ice melted off, the land beneath it --such as Northern Canada and Scandinavia -- rebounded and the planet grew more spherical, resulting in changes in the gravitational field. Such uprisings continue today. grace will allow researchers to weigh the Earth's ice sheets and study sea level changes by tracking the mass of ice.

The grace satellites measure gravitational fluctuations by flying in a caravan formation, one after the other, and constantly tracking the distance between each other with the help of microwave range finders. As the leading satellite passes over an area of greater gravity, it feels it and reacts by pulling the trailing craft. The satellites fly about 220 kilometres apart, with the ranging finding system between them accurate down to a micron. The researchers plan to generate a new gravity map about every 30 days -- the length of time it takes for the satellites to pass over the Earth once. About 60 maps will be made during the mission's five-year timeframe.

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