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Gravity mapper ascends to space atop Rockot booster

Posted: March 17, 2009

Europe launched a slender, winged ion-driven satellite Tuesday to glide through the upper atmosphere for nearly two years measuring Earth's gravity field with unprecedented precision.

Credit: ESA
The European Space Agency commissioned the GOCE mission to create a highly detailed, exceptionally accurate map of the planet's geoid, a global model illustrating subtle variations in the gravity field if oceans were motionless.

But oceans aren't stationary. Ocean currents and changing sea levels can have drastic consequences on Earth's climate.

"GOCE has scientists all over the world anxiously waiting to get their hands on the data because we are providing new information to the scientists that they never have been able to use before," said Rune Floberghagen, GOCE mission manager.

Scientists use the geoid's reference surface to weigh against measurements of ocean activity. The comparisons allow scientists to more accurately study ocean circulation and sea level changes.

"This surface is used as a reference for measurements of height, including measurements of sea level heights," said Danilo Muzi, GOCE program manager. "Therefore, if we have a very precise geoid, then we will know whether the sea level is rising every year, and by how much."

Scientists will combine GOCE's measurements with altimeter data from the Envisat observatory to gauge ocean currents far more accurately than currently possible, ESA officials said.

Sea level and ocean circulation are intimately linked to climate change, according to scientists.

"The goal is to understand the way in which the Earth system works, and the way in which aspects such as ocean circulation help to moderate the climate and help to influence climate variability," said Mark Drinkwater, GOCE mission scientist.

GOCE data will also provide a new understanding of tectonic activity that could lead to better forecasts of earthquakes and volcanoes. Scientists expect changes in Earth's interior to show up in the satellite's gravity measurements.

Scientists say other applications for GOCE's measurements include surveying and construction, which rely on exact height measurements aided by accurate gravity models.

The mission is valued at 350 million euros, or about $450 million, according to ESA.

The arrow-shaped 2,319-pound satellite blasted off at 1421 GMT (10:21 a.m. EDT) from the Plesetsk Cosmodrome in northern Russia. The 95-foot-tall Rockot launcher, a modified SS-19 ballistic missile, roared away from the snowy, forested spaceport on 420,000 pounds of thrust.

The Rockot turned north from Plesetsk, dropping its first stage into the Barents Sea two minutes after launch. About 50 seconds later, the booster jettisoned the payload fairing shielding GOCE during the early portion of the flight.

The launcher's second stage shut down about five minutes after liftoff and separated from the Rockot's Breeze KM upper stage 21 seconds later.

The Breeze KM, fueled by hydrazine and nitrogen tetroxide, ignited for a nine-minute burn to propel GOCE into an elliptical parking orbit.

The upper stage fired again for 13 seconds at 1539 GMT (11:39 a.m. EDT) to reach a circular orbit with a planned altitude of 173 miles and an inclination of 96.7 degrees.

Spacecraft separation occurred as scheduled at 1551 GMT (11:51 a.m. EDT). Controllers acquired the first signals from GOCE a few moments later through a ground station in Kiruna, Sweden.

Engineers confirmed the spacecraft survived the trip to space and was responding to commands from the ground.

A launch attempt Monday was scrubbed moments before liftoff, when the Rockot pad's mobile gantry did not retract from the rocket as scheduled during the final minutes of the countdown.

GOCE's launch was previously postponed from September after rocket officials discovered a failure in the guidance and navigation system of the Rockot's Breeze KM upper stage. Technicians had to replace the rocket's gyroscope and refurbish its power system, delaying launch to this year.

Engineers will begin testing the satellite's systems and science instruments during the next few weeks. Controllers will also monitor GOCE as its orbit naturally decays to an altitude of about 162 miles during the next 45 days.

GOCE's ion engine will be started to maintain the craft's orbit during its operational mission, which is expected to last about two years.

The highly efficient propulsion system is powered by 88 pounds of xenon gas.

Officials expect the ion engine will keep GOCE in orbit for up to three years, and scientists hope the mission can be extended for at least a year beyond its current baseline. GOCE's usage of xenon and orbital lifetime will largely depend on solar activity, which expands the atmosphere and increases drag on orbiting satellites.

Because of GOCE's unusually low altitude, the satellite will likely by destroyed during re-entry within weeks after exhausting its supply of xenon.

GOCE has to fly in a low orbit to make high-resolution measurements because the pull of gravity is stronger closer to Earth.

The sleek-looking satellite, built by Thales Alenia Space and Astrium, is based on a unique aerodynamic platform designed to reduce drag that could cause the spacecraft to prematurely re-enter the atmosphere.

"No European satellite before has flown at his altitude," Muzi said. "At this altitude, we still have some effects due to the atmosphere, and it is for this reason that the satellite is this shape."

Small winglets and a tail fin will help stabilize the spacecraft as it circles Earth. Engineers want to limit use of chemical thrusters because they could disrupt GOCE's gravity studies.

GOCE stretches more than 17 feet long and 3 feet wide. The satellite has a low cross-section to diminish the force of drag.

"It looks more like a space plane actually than a satellite," Muzi said.

The primary instrument, called a gradiometer, is mounted near the satellite's center of gravity to take advantage of a pristine environment free of disturbances that could spoil the sensor's measurements.

The gradiometer includes three pairs of identical accelerometers that can detect minuscule gravity changes as small as one-ten-trillionth of the gravity experienced on Earth, according to ESA.

The accelerometers are mounted on three arms facing in the satellite's direction of travel, perpendicular to the flight path, and toward the center of the Earth.

The instrument will compute the difference in the tug of gravity measured by each accelerometer as GOCE flies around the globe at more than five miles per second. The six different readings will be consolidated to measure the geoid with a resolution of less than 1 inch.

"This instrument has three flying test masses inside of it, which feel the pull of Earth's gravity in a slightly different way," Floberghagen said. "By comparing the pull on pairs of these objects every second for about 20 months, we will to derive a complete map of the Earth's gravity field."

GOCE's measurements should have a spatial resolution better than 100 kilometers, or about 62 miles. The resolution is a significant improvement over data gathered by the U.S.-German GRACE mission and the German-led CHAMP satellite, Drinkwater said.

The accelerometers are 100 times more sensitive than similar sensors flown on the GRACE mission. GOCE data will be 1,000 times more precise than information collected by CHAMP, according to Drinkwater.

GRACE was designed to measure changes in the planet's gravity field over time. GOCE's focus is to study the force in much finer detail.

Scientists are eager to explore collaboration between GOCE, GRACE and CHAMP, which continue to operate after nearly a decade in space. Tuesday's launch of GOCE also came on the seventh anniversary of GRACE's launch.

"The fact that three such gravity missions are flying at the same time offers a rare and unique opportunity," Drinkwater said.

Cooperative research between the missions could begin soon after GOCE is calibrated and scientists confirm the satellite is producing good data.

"Once the GOCE products have received the quality stamp, we will distribute them to the scientific community before turning our attention to value-added products that exploit the synergies amongst each gravity mission," Drinkwater said.

GOCE, which stands for the Gravity field and steady-state Ocean Circulation Explorer, is the first of six ESA Earth Explorer satellites built to focus on aspects of processes affecting the planet's climate.

"This new chapter that we see opening with the launch of GOCE demonstrates the potential ESA has to contribute on the world scene in Earth observation, and I think what we have is a pallet of extremely exciting new scientific missions," Drinkwater said.

GOCE is the first ESA satellite dedicated to Earth science since the agency launched Envisat in 2002. Envisat is a flagship-class observatory designed for a broad range of science investigations.

"The size has changed, but the rationale remains the same: to provide the best science our technology can deliver for the maximum benefit of the science community and ultimately the citizens of Europe and the world," said Jean-Jacques Dordain, ESA's director general.

The next Earth Explorer, named SMOS, could launch as soon as July to detect soil moisture and salt levels in oceans. SMOS will also launch on a Rockot vehicle.