GOCE's electric ion propulsion engine switched on

Apr 06, 2009
GOCE is a sleek five metre-long spacecraft about one metre in diameter. It consists of a central octagonal tube with seven internal 'floors' that support the equipment and electronic units. The spacecraft is built largely of carbon-fibre reinforced plastic sandwich panels to guarantee stable conditions and minimise mass. Forming the heart of the satellite, the gradiometer is mounted close to the satellite’s centre of mass. The gradiometer structure is based on ultra-stable carbon-carbon technology. GOCE is the first mission to employ the concept of gradiometry in space. Credits: ESA - AOES Medialab

(PhysOrg.com) -- GOCE's sophisticated electric ion propulsion system has been switched on and confirmed to be operating normally, marking another crucial milestone in the satellite's post-launch commissioning phase.

The success of GOCE's ultra-sensitive gravity measurements depends on finely controlling the satellite's orbit and speed. The push from the thruster must be just enough to compensate for the tiny amount of drag generated by the few wisps of atmosphere at GOCE's orbital height.
No normal jet engine could do this, but GOCE's sophisticated electric ion propulsion system can.

This cutting-edge system does not burn fuel like a regular . Instead, it is supplied with xenon from a 40-kg tank, which is converted to fast-moving ions - naked xenon atoms that have had some of their electrons stripped away by an electric discharge generated from solar energy. The ions are ejected toward the rear, giving a very gentle, steady and smooth thrust.

The electric ion propulsion system comprises two redundant thruster units mounted externally on the last panel of the satellite. The thrusters can be throttled between 1 and 20 millinewtons (mN), which will be set automatically, depending on the actual real time drag that the satellite experiences once is in routine operations.

This video is not supported by your browser at this time.
The need for GOCE (Gravity Field and Steady-State Ocean Circulation Explorer) to fly in a low orbit means it has to be equipped with a system to compensate for the atmospheric drag it will experience at this exceptionally low altitude. The animation shows a close-up of the ion-propulsion assembly, which keeps the satellite in a smooth trajectory - free from all effects except the gravity field itself. Two winglets provide additional aerodynamic stability. Credits: ESA - AOES Medialab

This is a fantastically small force - a thrust of a few mN is similar to the weight of a few drops of water on Earth. Yet, by thrusting continuously during GOCE's routine operations, it is sufficient to maintain a 'freefall' orbit.

"On Tuesday last week, we fired the B-unit of the electric ion propulsion, stepping it through 1, 3 and 8.3 mNs of thrust. On Thursday, we fired the A-unit. Both are performing nominally," said Operations Manager Juan Piñeiro, speaking at, ESA's European Space Operations Centre (ESOC) in Darmstadt, Germany.

"Next comes the 'wake up' for the instruments, which puts us in the position to start operating the first-ever gradiometer to be flown in space. After confirming they work, we will be ready for intensive testing of the satellite and of the scientific data-processing system on the ground, which will be managed at ESA's Earth Observation Centre in Frascati, Italy," said Rune Floberghagen, GOCE Mission Manager.

The Flight Control Team is now working from the GOCE Dedicated Control Room at ESOC, and has continued with a series of operational checks since the end of the critical Launch and Early Orbit Phase on 20 March.

Prior to commissioning the electric ion propulsion, GOCE was in free flight (not powered) and is now at an altitude of approximately 280 km, gently decaying at a rate of about 190 m/day.

GOCE is the first ESA satellite employing drag-free control, in which the satellite is in pure freefall around Earth, and is one of the first-ever satellites to use electric ion propulsion to continually compensate for atmospheric drag. The Drag-Free and Attitude Control Subsystem (DFACS) performs autonomous determination and control of the satellite's attitude pointing, angular movements and linear and angular accelerations.

The DFACS uses, among other inputs, signals from the Electrostatic Gravity Gradiometer (EGG) - the primary on board scientific instrument - to provide information on the drag experienced by GOCE. The propulsion is then varied automatically to maintain the required velocity and altitude.

GOCE also orbits at an extremely low altitude, so atmospheric drag and solar activity have important effects on the trajectory. Solar heating causes the upper atmosphere to expand, increasing the amount of drag that GOCE experiences at a given altitude.

"Solar activity is now in an extended minimum period, so the atmosphere is actually less thick and GOCE is experiencing less drag than forecast," says Marco Antonio Garcia Matatoros, GOCE Lead Flight Dynamics Coordinator.

Source: European Space Agency (news : web)

Explore further: Europe sat-nav launch glitch linked to frozen pipe

add to favorites email to friend print save as pdf

Related Stories

GOCE successfully completes early orbit phase

Mar 20, 2009

ESA's GOCE satellite was formally declared ready for work at 01:00 CET on 20 March. During the critical Launch and Early Orbit Phase beginning with separation from its booster on 17 March, GOCE was checked ...

GOCE satellite: Critical operations ongoing

Mar 19, 2009

(PhysOrg.com) -- After liftoff 17 March, ESA's GOCE spacecraft is performing very well, having achieved an extremely accurate injection altitude of 283.5 km, just 1.5 km lower than planned. The Mission Control ...

September launch for ESA's gravity mission GOCE

May 27, 2008

A new launch date has been set for GOCE. The change of date is due to precautionary measures taken after the malfunction of an upper-stage section of a Russian Proton launcher. Now confirmed not to affect ...

SMART-1 set for more lunar science

Sep 26, 2005

ESA’s SMART-1 mission in orbit around the Moon has had its scientific lifetime extended by ingenious use of its solar-electric propulsion system (or ‘ion engine’).

Launch of European gravity probe delayed

Mar 16, 2009

The launch of a pioneering European satellite designed to map Earth's gravity field was delayed due to technical problems and will take place Tuesday, Russia's Khrunichev Space Centre said.

March launch planned for GOCE gravity mission (Video)

Feb 04, 2009

(PhysOrg.com) -- ESA is now gearing up to return to Russia to oversee preparations for the launch of its GOCE satellite - now envisaged for launch on 16 March 2009. This follows implementation of the corrective ...

Recommended for you

Europe sat-nav launch glitch linked to frozen pipe

8 hours ago

A frozen fuel pipe in the upper stage of a Soyuz launcher likely caused the failure last month to place two European navigation satellites in orbit, a source close to the inquiry said Wednesday.

Cyanide ice in Titan's atmosphere

10 hours ago

Gigantic polar clouds of hydrogen cyanide roughly four times the area of the UK are part of the impressive atmospheric diversity of Titan, the largest moon of Saturn, a new study led by Leiden Observatory, ...

Video: Alleged meteor caught on Russian dash cam (again)

13 hours ago

Thanks to the ubiquity of dashboard-mounted video cameras in Russia yet another bright object has been spotted lighting up the sky over Siberia, this time a "meteor-like object" seen on the evening of Saturday, Sept. 27.

User comments : 2

Adjust slider to filter visible comments by rank

Display comments: newest first

joefarah
4.5 / 5 (2) Apr 06, 2009
Why doesn't the ISS use ion propulsion to help minimize drag? A few thrusters in the right places could make a fair bit of difference in fuel requirements.
Bob_Kob
not rated yet Apr 07, 2009
Probably too late now.