A sleek satellite that is set to make the most detailed map of the Earth’s gravity took to the skies on Tuesday. The probe is expected to make important contributions to ocean current measurements and climate models.
After a day’s delay to work out problems with the probe’s launch tower, the Gravity field and steady-state Ocean Circulation Explorer (GOCE), launched on Tuesday from the Plesetsk Cosmodrome in Russia, which lies some 800 km north of Moscow.
If all goes well, the satellite will assume an orbit some 285 km above the Earth, gradually falling to an altitude of 268 km, where the probe will take much of its science data. It will remain in orbit for at least two years, beginning science operations in late August or early September when the probe will have sufficient solar power to do its observations.
The Earth’s mass is not uniformly distributed. By flying low, GOCE will be able to map these variations in unprecedented detail.
But this orbit will also subject GOCE to the drag of tenuous gas at the edge of Earth’s atmosphere. If its orbit were uncorrected, the probe might only last a few months before falling back to Earth, says mission manager Rune Floberghagen.
To counteract this drag, the satellite has an aerodynamic shape and an ion-propulsion engine that continuously jettisons charged xenon atoms to keep its orbit stable.
Stability is important for the probe, which was built to have no moving parts that could interfere with the measurements of the satellite’s three pairs of accelerometers.
The team hopes GOCE will measure deviations in the Earth’s gravity field at a spatial resolution higher than 100 km. That is a factor of three better than NASA’s GRACE satellites, a pair of probes that launched in 2002 to measure how Earth’s gravity field changes in time, largely as water is transported around the globe.
GOCE will map the Earth’s average gravity field to build up a picture of the geoid, an idealised model of the Earth’s surface where the gravitational potential is identical at every point (see this illustration). If the Earth were covered with a perfectly still ocean, not subject to winds or ocean currents, the water would naturally take this shape.
The lack of a high-resolution geoid has hampered measurements of ocean currents, says Lee-Lueng Fu, chief scientist for the US-European Jason-1 and Jason-2 satellites, which are used to map ocean levels.
Over the last 15 years, satellites have been using sea level changes to measure ocean currents. Currents may cause swells or troughs on the order of metres high, but spatial variations in Earth’s gravity mean some points on the ocean are naturally 100 or more metres higher than others, Fu says.
“We cannot really compute the current [ocean current] speed because this sea surface shape is primarily determined by gravity,” Fu told New Scientist.
By measuring this background shape, GOCE could refine measurements of ocean current, which could be used to get a better estimate of how heat is transported by the oceans.
Since oceans absorb most of the Earth’s excess heat, the gravity map will ultimately help refine climate models. “This mission is an important step forward to allow us to make more reliable projections,” Fu says. “We’ve been waiting for such a mission for a long time.”