Looking at the Earth through different eyes: the GRACE mission


Adapted from a talk given at #CSIROTweetup by Dr Daniel Shaddock (laser systems) Dr Paul Tregoning (geo data) ANU, supplemented with data from the GRACE mission home page. Image snarfed from the NASA/JPL GRACE page

Most people associate going into space with exploring objects not on Earth, but many satellites do in fact the opposite: they look back at Earth from space. A twin set of satellites are doing this in an innovative way that is proving very useful for many areas of science.

The GRACE (Gravity Recovery and Climate Experiment) satellites were launched in 2002. They are two identical units that orbit the Earth at a height of 500km, in exactly the same orbit, but one following the other by a distance of 220km. Through a microwave ranging system that very accurately determines the distance between the two satellites, they measure gravity.

How does this work?

Well, Earth is not homogenous, and therefore neither is its gravity homogenous. Because gravity is mass, variations are due to mass differences. This could be through features on the outside of the planet (mountains, ice caps, large bodies of water), or because of the flow of material inside the Earth. Water is lighter than rock, and ice is lighter than water.

As discussed elsewhere, gravity affects the speed of orbiting objects. When passing over a patch with higher gravity, an orbiting object will speed up. Apply this to our two satellites, and when they fly over a mountain range, the first one will speed up, increasing the distance between the two, and then return to normal speed, then the second one speeds up, decreasing the distance again. By very accurately measuring the distance between the two, you can calculate the gravity of the material underneath. These are very small changes. We’re talking about a variation in the distance between the two satellites of 50nm.

When collated into models, this data gives a very detailed picture of the flow of mass on Earth. The data only shows that a mass difference has occurred, and needs careful consideration as to the cause of this difference. Things that can be measured are the decrease of mass in polar ice caps, changes in water content of soil (for example during the Queensland floods) or seasonal water patterns (the annual filling of the Amazon basin), the depth, salinity and temperature of the ocean, and shifts of mass due to large earthquakes. It can be used to look at water reserves, melting of ice sheets, ocean tides and the effect of earthquakes. If you hear that an earthquake has shifted the ocean floor or that the ice caps are melting faster than previously thought, and you wonder how people get the data to make these statements, this is how.

These data has been invaluable to hydrologists, climatologists, geologists and atmospheric scientists. GRACE resolution is too small to be of any use in prospecting, but already mining companies use gravity gradiometry in aircraft, using the same basic principle, but with different equipment.

Didn’t know this sort of research was being conducted in Australia? Here is speaker Dr Daniel Shaddock with a piece of equipment developed at ANU for the successor of the GRACE mission.

He also spoke about the closely related field of Gravitational Wave Astronomy and some possible grand projects that look at gravitational influences of objects on the opposite end of the scale: galaxies and black holes.

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