Sunday, 26 May 2019

Mission launches to weigh Earth’s water

Grace mission has gone into orbit to weigh the water on Earth. SpaceX rocket climbed above California before heading south towards Antarctica.

In a joint US-German mission, the Grace satellites were launched on Tuesday aboard a SpaceX rocket from the Vandenberg Air Force base in California. It will take a number of weeks before start gathering data.

The satellites were assembled in Europe by Airbus. The first Gravity Recovery and Climate Experiment (Grace) ran from 2002 to 2017.

The first Gravity Recovery and Climate Experiment (Grace) ran from 2002 to 2017.

The follow-on mission again draws heavily on expertise from Europe, in particular from the German Research Centre for Geosciences (GFZ).

The Grace duo will obtain data by executing a carefully calibrated pursuit in orbit.

The lead spacecraft will lurche and drag through the Earth’s uneven gravity field and the second satellite will follow 220km behind, measuring changes in their separation to the nearest micron (a thousandth of a millimetre).

The Grace concept is brilliant at sensing the big changes that occur in the hydrological cycle.

“There was a period in 2011 when sea-level rise slowed down and went in the other direction very briefly,” explained Nasa project project scientist Dr Frank Webb.

“From the Grace data we could see there were heavy rain seasons in Australia and South America, and that equivalent of mass was going into storage on land. Eventually, it was released back to the oceans and sea-level rise continued.”

The ice sheets are losing about 400 gigatonnes to the oceans every year. One of the great contributions from the first Grace mission was to confirm the scale of change at the poles.

Satellites carrying altimeters can weigh the ice sheets every year by measuring the change in shape of Antarctica and Greenland – but Grace provided independent insight through its gravity assessments. Antarctica was seen to be losing 120 billion tonnes of ice a year and Greenland 280 billion tonnes.

“With the launch of Grace-FO, we can now continue to detect changes in the ice mass, to determine the extent to which ice is being lost, and find out if there has been any acceleration,” said Prof Helen Fricker from the Scripps Institution of Oceanography to BBC News.

The new satellites carry a microwave-ranging instrument the same technology used in the previous Grace, but now they have a laser system incorporated to give a 10 times improvement in precision. Scientists are still hopeful they can get significant gains in performance.

The new Grace pair will use both microwave and laser-ranging to measure their separation.

There is the possibility of a future Grace-like gravity mission pulled into the European Commission’s Sentinel Earth-observation programme.

The same has already happened with the US-French Jason series, which has been measuring sea-surface height since 1992.

Future Jasons will be known as the Sentinel-6 mission – a status that has helped secure long-term funding.

“The ‘e’ in Grace stands for ‘experiment’, but the data is now being used for services, such as flood monitoring. My strong opinion is that it could be a Sentinel,” said Prof Frank Flechtner, the Grace-FO project manager at GFZ, told BBC News.

The total cost of Grace-FO was $520m and the mission should last for at least five years.


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