Way beyond apples

  • In orbit, the GOCE satellite remains facing the Sun. The spacecraft is equipped with four body-mounted and two wing-mounted solar panels that tolerate temperatures as high as 160 degrees C and as low as -170 degrees C.
  • GOCE in preparation at Thales Alenia Space facility in Turin, Italy.
  • GOCE will significantly advance our understanding of the physics and dynamics of the Earth's interior. Its detailed mapping, along with seismic data, is expected to shed new light on the processes causing earthquakes and volcanic activity.
  • (A) A circle with Earth's equatorial diameter (~40 000 km). A gyroscope following this circular path in empty space will always point in the same direction, as indicated by the arrows above. (B) Earth's mass warps spacetime inside the circle into a cone, formed by removing a pie-shaped wedge (dotted lines). This reduces the circle's circumference by 28 mm. A gyroscope will now change its orientation while tracing the conical path, as shown in the drawing above.
  • The GOCE gradiometer contains three pairs of proof masses positioned at the outer ends of three 50 cm-long orthogonal arms. Because of their different position in the gravitational field, they all experience the gravitational acceleration of the Earth slightly differently.
  • Artist's concept of the Gravity Probe B spacecraft orbiting Earth to measure spacetime, a four-dimensional description of the Universe that includes height, width, length and time. Image credit: Nasa
Date:30 April 2009 Tags:, , , , ,

New European satellite will map Earth's gravity as never before

Three centuries after Newton supposedly watched an apple fall from a tree and was inspired to publish his Universal Law of Gravitation, we still don't fully understand this fundamental force of Nature. Although scientists can predict and measure its effects to an astonishing degree of accuracy, they have yet to verify the existence of gravitational waves, and cannot tell us why there appears to be more gravity in our Universe than can be explained by general relativity.

But they're getting there.

Last month, the European Space Agency (ESA) launched a sophisticated satellite to investigate the Earth's gravitational field with unprecedented resolution and accuracy. The 1-ton Gravity field and steady-state Ocean Circulation Explorer (GOCE) was placed in a low-altitude orbit by a Russian Rockot vehicle, launched from the Plesetsk Cosmodrome in northern Russia.

GOCE data will be crucial for obtaining accurate measurements of ocean circulation and sea-level change, both of which are affected by climate change. The data will help us to better understand processes occurring inside the Earth that are linked to volcanoes and earthquakes.

ESA's spacecraft carries a highly sensitive gradiometer to measure the variations of the gravity field in three dimensions. Its data will provide a high-resolution map of the "geoid" (the reference surface of the planet) and of gravitational anomalies. Such a map will not only greatly improve our knowledge and understanding of the Earth's internal structure, but will also be used to provide much better reference data for ocean and climate studies, ocean circulation and sea levels.

To make the mission possible, ESA collaborated with a consortium of 45 European companies and the science community to overcome some daunting technical challenges. The spacecraft had to be designed to orbit the Earth at close enough quarters to gather high-accuracy gravitational data while being able to filter out disturbances caused by the remaining traces of the atmosphere in low-Earth orbit (at an altitude of only 260 km). Their efforts resulted in a slender, 5 m-long arrowhead shape for optimum aerodynamic performance, with low-power ion thrusters to compensate for atmospheric drag. The octagonal, 1 100 kg satellite is equipped with two winglets to provide additional aerodynamic stability.

To measure gravity, there can be no disturbances from moving parts, so the entire spacecraft is actually one extremely sensitive measuring device. It is the first space mission to employ "gradiometry" – the measurement of gravitational differences between an ensemble of test masses inside the satellite, using a set of six 3-axis accelerometers mounted in a diamond configuration.

The satellite consists of a central octagonal tube with seven internal floors that support the equipment and electronic units. Two of the floors support the gradiometer, which is mounted at the heart of the satellite, close to its centre of mass. The structure is built largely of carbon fibre-reinforced plastic sandwich panels to guarantee stable conditions under varying temperatures, and to limit mass.

In orbit, the same side of the satellite always faces the Sun. This side carries four body-mounted and two wing-mounted solar panels capable of tolerating temperatures as high as 160 degrees Celsius and as low as minus 170 degrees. The internal equipment is protected against the high temperatures by multi-layer insulation blankets positioned between the solar panels and the main body of the satellite. The side that faces away from the Sun is used as a radiator to dissipate heat into space.

The gradiometer will for the first time measure gravity gradients in all directions. It is specifically designed for the stationary gravity field, measuring the geoid and gravity anomalies with high accuracy and high spatial resolution. The satellite also carries a GPS receiver to be used as a satellite-to-satellite tracking instrument, and a laser retroreflector that allows its precise orbit to be tracked by a global network of ground stations through the Satellite Laser Ranging Service. This will provide accurate positioning for orbit determination and data products.

To achieve the mission's crucial scientific objectives, the satellite must orbit as low as possible in order to sense minute variations in the Earth's gravitational field. This presents unique challenges for the entire flight operations team at ESOC, the European Space Operations Centre, but particularly so for the flight dynamics specialists responsible for determining GOCE's orbit, calculating its position and orientation and – together with the Flight Control Team – reacting to any onboard contingency.

Explains Marco Antonio Garcia Matatoros, lead flight dynamics co-ordinator: "We orbit on the edge of reentry; if anything goes wrong, we have very little time before we start dropping down – about eight days before we get dangerously low."

The spacecraft's cutting-edge electric engine generates up to 20 milliNewtons of ion thrust, which is roughly equal to the weight (on Earth) of a few drops of water. But this is sufficient to enable GOCE to maintain its nominal altitude at about 270 km and slowly lift it to safe altitudes during eclipse seasons (when less sunlight means less power for the electric propulsion). The fuel tank holds 40 kg of Xenon propellant, sufficient for a 30-month mission.

GOCE is the first ESA satellite employing "drag-free control" (in which the satellite is in pure free-fall around the Earth) and the first to use electric propulsion to continually compensate for atmospheric drag.

Now it's just a question of time. The more we know about the workings of our planet, the better equipped we'll be to look after it.

What is gravity, anyway?
We know that gravity is a purely attractive force: it can only pull, never push, and it's generated by any object with mass. But frankly, when it comes to understanding its effects at really large scales, we're still in the dark.

Italian Galileo Galilei was one of the first scientists to investigate the way objects are caused to move. However, it was not until Isaac Newton studied “gravity” that we began to really understand this feature of the Universe. In 1687, Newton published his Universal Law of Gravitation, providing mathematical expressions that would precisely describe the way objects moved in a gravitational field. (For the record, it’s thought unlikely that his discovery came about when an apple fell on his head.)

Not only did the expressions work for objects falling to the ground on Earth, but they also explained how the planets moved through the night sky. Unsurprisingly, this is heralded as one of the greatest works of science. There was only one observation his work could not explain: Mercury’s elliptical orbit was gradually moving around the Sun, a phenomenon known as “precession”.

In the early 20th century, Albert Einstein developed his theory of general relativity, in which he described gravity as a deformation in space caused by the presence of massive objects, similar to the way a heavy ball would warp a sheet of rubber. (He called it the “space-time continuum”). This deformation “told” smaller things how to move through space, so they either went into orbit or fell on to the larger celestial object.

This was a very different way to visualise space. In the past, it was thought that space was filled with a fluid known as ether. When no one could prove the existence of the ether, people began to think of space as simply empty a huge expanse of nothing. General relativity not only explained the precession of Mercury, it made a number of surprising predictions that, over the subsequent decades, have been observed to be true.

Among them was that light passing by a massive object would be deflected from its original path, and that light escaping from a gravitational field would lose energy. (In fact, satellite-based navigation systems such as GPS take this second effect into account in order to pinpoint precisely the location of their users.)

The only prediction of general relativity yet to be tested is that certain celestial objects will emit gravitational waves that will ripple through the fabric of space. The joint ESA/Nasa mission LISA will attempt to detect these. For now, however, the clues are mounting that general relativity itself is incomplete. Certain observations suggest that there is more gravity in our Universe than can be explained by general relativity – for example, as evidenced by the so-called Pioneer anomaly. The Pioneer 10 and 11, Galileo and Ulysses spacecraft are being slowed down for a reason that has yet to be understood.

On a considerably larger scale, certain galaxies spin as if they were generating more gravity than appears possible. One explanation may be that vast quantities of exotic “dark matter” exist in the Universe – or it may be that general relativity is not the final theory of gravity.

Gravity Probe B was a Nasa physics mission to experimentally investigate Einstein's 1916 general theory of relativity – his theory of gravity. The spacecraft used four spherical gyroscopes and a telescope, housed in a satellite orbiting 642 km above the Earth, to measure two extraordinary effects predicted by the general theory of relativity (the second effect having never before been directly measured):

Geodetic effect – the amount by which the Earth warps the local spacetime in which it resides.

Frame-dragging effect – the amount by which the rotating Earth drags its local spacetime around with it.

The experiment set out to test these two effects by precisely measuring the displacement angles of the spin axes of the four gyros over the course of a year and comparing the results with predictions from Einstein’s theory. The project did not go entirely according to plan, and the jury is still out on whether the mission’s R7 billion price tag was justified by the results.

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