A European spacecraft will begin its quest this week to make the most detailed¹ global map of the Earth's gravity field.

一艘欧洲宇宙飞船本周将要绘制详细的全球重力场图片。

Goce can fly much lower than was expected

The arrow-shaped Goce satellite can sense tiny variations in the planet's tug² as it sweeps around the world at the very low altitude（高度，海拔） 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（日蚀，月蚀） mode where the Sun doesn't shine fully³ 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.

The precision with which Goce expects to make its gravity measurements means every disturbance⁴ 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 now flies with the Sun's rays constantly streaming on to one side of the satellite. Its permanently⁵ 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（倾斜计）.

As the spacecraft "bumps" through Earth's gravity field, the accelerometers sense fantastically small deviations⁶（脱离，偏离） - 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 density⁷ of residual⁸ air（残余空气） molecules⁹ that might otherwise buffet¹⁰（打击） the spacecraft and introduce further noise into the data has been greatly reduced as a consequence.