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Fig. 1. Artist's impression of Double Star in orbit around the Earth

For more recent information please go to:

Since their dual launches in July and August 2000, ESA's four Cluster spacecraft have been flying in formation around the Earth, sending back the first detailed, three-dimensional information about the magnetosphere and its interaction with the solar wind.

This unique examination of the magnetic bubble that surrounds our planet is about to be enhanced still further as the result of Double Star, a groundbreaking collaboration involving ESA and the Chinese National Space Administration (CNSA).

The Double Star programme involves the launches of two satellites - the first already placed in orbit on 29 December 2003 and a second scheduled for June 2004 - each carrying experiments provided by European and Chinese institutes. This will enable scientists to analyse data sent back simultaneously from no fewer than six spacecraft, each located in a different region of near-Earth space. The simultaneous, six-point study should provide new insights into the mysterious mechanisms that trigger magnetic storms and brilliant auroral displays in polar skies.

A History of Collaboration

The history of scientific collaboration between China and ESA began in 1980 through the signing of a document facilitating the exchange of information between ESA and the Commission for Science and Technology of China. Twelve years later, the Chinese approached ESA with a proposal to establish a Data and Research Centre in Beijing for Europe's cornerstone Cluster mission.

China's request to participate in an international space programme for the first time culminated in an official cooperation agreement that was signed on 25 November 1993. As a result of this agreement, a number of Chinese scientists and engineers were hosted by ESA and the institutes of Cluster principal investigators (PIs), while five of their compatriots became co-investigators on Cluster.

The next step came in 1997, when Professor Zhen-Xing Liu of the Chinese Centre for Space Science and Applied Research (CSSAR) gave a presentation on the proposed Double Star programme at the Cluster Science Working Team meeting. Six Cluster PIs responded to the invitation to participate by offering to provide flight spare models of their Cluster experiments.

In September 1999, the ESA Director General was invited by the Administrator of the Chinese National Space Administration to discuss ESA / China collaboration in space activities and more particularly collaboration on the Double Star Programme. Shortly after the Double Star Phase A report was presented to ESA and to the Cluster Science Working Team. Once again, the European response was very positive.

Fig. 2: ESA Director General, Antonio Rodotà, signs the historic Double
Star collaboration agreement with the CNSA Administrator, Luan Enjie,
9 July 2001.

Further progress was made in September 2000, when Double Star was given the green light by the Chinese government. In April 2001, a European delegation composed of ESA representatives and European PIs visited CNSA in order to finalise the preparation of the agreement between the two agencies and to review the project's status. Finally, on 9 July 2001, an historic agreement to develop the joint mission was signed at ESA Headquarters in Paris by the ESA Director General, Antonio Rodotà, and the CNSA Administrator, Luan Enjie.

"This agreement marks a significant advance for international co-operation in the exploration and peaceful use of outer space," said Mr. Rodotà. "It is one of the most important landmarks in scientific collaboration since ESA and the People's Republic of China first agreed to exchange scientific information more than 20 years ago."

"The Double Star programme will be just the first step in substantial co-operation between the Chinese National Space Administration and ESA," said Mr. Luan Enjie. "The signing of today's agreement paves the way not only for reciprocal co-operation between scientists, but for the establishment of comprehensive co-operation between the two agencies."

Under the agreement, ESA agreed to contribute 8 million Euros to the Double Star programme. This funding would be used for refurbishment and pre-integration of the European instruments, acquisition of data for 4 hours per day and co-ordination of scientific operations.

China's contribution would include the two spacecraft buses, eight scientific experiments, launch and operations.


Since the signing of the landmark agreement, rapid advances have been made - despite cultural and language differences and geographical separation.

A much less predictable obstacle was the outbreak of SARS (Severe Acute Respiratory Syndrome) in April, which meant that planned meetings had to be cancelled and replaced by videoconferences, remote data transfer, web cams and interactive messaging dialogues. Bodo Gramkow, ESA's payload project manager for Double Star, had to obtain special permission to travel to China while the global alert issued by the World Health Organisation was still in force, but through the generous assistance of the CNSA and CSSAR, those involved could continue the work and save the mission. Gramkow and three engineers from Astrium subsequently stayed in an empty hotel.

Despite such unforeseen problems, the programme remained on course for completion by the end of 2003, as a result of the hard work and dedication of all involved. After 18 months of intense interface definition work, the hardware test phase began in autumn 2002 with a successful compatibility test of European and Chinese equipment at Imperial College, London.

Fig. 3: Double Star assembly/instrument testing etc. with European staff? [Image to come]

In parallel, the assembly of the spacecraft structural-thermal model in China culminated with the successful completion of the environmental test programme in February 2003. Despite the inevitable delays caused by the SARS outbreak, the payload and the CSSAR subsystems were delivered to Beijing by 10 July for integration into the TC-1 flight spacecraft (TC are the initials of Tan-Ce, which means Explorer). The installation of Chinese equipment at ESA's Villafranca ground station was also successfully completed.

The launch campaign for the first Double Star satellite began in mid-November, and lasted five weeks. As planned, the launch took place before the end of the year, at 19:06 UT on 29 December 2003 (Fig. 4). The 1st and 2nd stages of the Long March 2C/SM rocket fired for 7 minutes, then the solid fuel upper stage injected the spacecraft into its operational orbit.

The TC-1 spacecraft was placed in a 570 x 78,970 km orbit with a 28.5 degree inclination. Although the apogee is about 12,000 km higher than expected, due to over-performance of the upper stage engine, this should not affect the scientific objectives. Indeed, the higher orbit means that the spacecraft will now be able to observe the Earth's bow shock, which was not in the original science plan. Preliminary estimates indicate that the number of conjunctions with Cluster have decreased slightly, but this will be compensated by the longer time interval of the individual conjunctions.

Fig. 4: Launch of TC-1 on Long March 2C/SM rocket
The second satellite (TC-2) is now being assembled prior to launch in July 2004, when it will join the Cluster flotilla in a polar orbit. Meanwhile, the operational lifetime of Cluster has been extended for 3 years until 2005, in order to enable the unique, six-spacecraft investigation to take place.

"We have made remarkable progress, thanks to all scientists and engineers who have worked with such dedication on the project," said Philippe Escoubet, ESA's project scientist for both Double Star and Cluster. "In less than three years, China has developed the two spacecraft and their instruments, while the refurbishment of the European instruments has been completed."

The check-out of the TC-1 spacecraft started early January 2004 with the successful deployment of the solid boom carrying the FGM magnetometer. However, a second boom that carries the search coil for the STAFF/DWP experiment failed to deploy. After an investigation by Chinese spacecraft engineers, a second attempt was made on 16 February, but this, too, was unsuccessful. Subsequent analysis showed that the stability of the spacecraft was unaffected and its spin axis is, as expected, close to the north ecliptic pole.

FGM started commissioning on 8 January and the instrument electronics were checked out successfully after 4 days. At the time of writing, data acquisition is taking place to calibrate the instrument. STAFF/DWP has also finished its initial commissioning and the instrument is functioning nominally. However, as a result of the undeployed boom, the STAFF/DWPsensor detects more interference and noise coming from the spacecraft. The principle investigator team responded to this challenge immediately and is adapting the data acquisition in order to reduce this interference.

The PEACE (electron detector) experiment started its first electronic tests on 20 January and is working nominally. During this phase, scientists enjoyed their first opportunity to conduct combined observations when increased activity of the Sun resulted in a M6.1 flare on 21 January, followed by the arrival of an interplanetary shock at the Earth around 01:35 UT the next day. The increased pressure of the solar wind (more than 5 times higher than normal) resulted in a large compression of the magnetosphere. The bow shock was pushed toward Earth and detected in the PEACE data as it passed the TC-1 satellite, located at an orbital altitude of 12.6 Earth radii (80,360 km). The Cluster spacecraft, which were in the solar wind at the time, also observed the interplanetary shock. The good conjunction of spacecraft orbits will provide excellent study opportunities for similar events in the future.

Commissioning of the ASPOC (ion emitter) experiment began on 24 January and concluded successfully after 5 days with all four emitters working perfectly. This instrument will keep the spacecraft "grounded" by compensating for its surface charging, thus enabling very good low energy particle detection.

HIA, the fourth European instrument on TC-1, started its first check-out on 4 February. Electronics checks lasted two days and verified that all components were in perfect shape. High voltages were raised slowly to around 1500 V and the first ions were detected on 9 February. In-flight calibrations continued up to 13 February, when the instrument was declared fully functional by the PI team.

In the coming weeks, the instruments will be further calibrated and the data distribution system will be commissioned. The commissioning review will take place on 10 March, after which the nominal mission operation phase is expected to start.

The Cluster Legacy

Each Double Star spacecraft carries eight scientific instruments, with half of the overall number provided by European institutions. The key aspect of Europe's participation in Double Star was the inclusion of seven instruments identical to those flying on the Cluster spacecraft, plus one extra instrument provided by the National University of Ireland. The first European instruments to be flown on Chinese satellites are accompanied by a further eight experiments provided by Chinese institutes.

The fact that most of the European science payload was derived from Cluster 'spares' made it possible to prepare and launch them on Double Star during the limited operational lifetime of ESA's quartet.

"By flying experiments identical to those on Cluster, we have been able to reduce costs and development time," explained Bodo Gramkow. "This minimised the risks and helped us to ensure that we would be able to meet the spacecraft development schedule."

Table 1: Double Star Scientific Payload

Equatorial Double Star (TC-1)

Polar Double Star (TC-2)





Active Spacecraft Potential Control (ASPOC)

K. Torkar, IWF, Graz, Austria

Neutral Atom Imager (NUADU)

S. McKenna-Lawlor, Ireland U., Ireland

Fluxgate Magnetometer (FGM)

C. Carr IC, UK

Fluxgate Magnetometer (FGM)

T. Zhang, IWF, Austria

Plasma Electron and Current Exp. (PEACE) #

A. Fazakerley, MSSL, Dorking, UK

Plasma Electron and Current Exp. (PEACE) #

A. Fazakerley, MSSL, Dorking, UK

Hot Ion Analyzer (HIA), sensor 2 of CIS

H. Reme, CESR, Toulouse, France

Low Energy Ion Detector (LEID) *

Q. Ren and J.B. Cao, CSSAR,China

Part of Spatio-Temporal Analysis of Field Fluct.(STAFF) + Digital Wave processor (DWP)

N. Cornilleau/H. Alleyne, CETP, Velizy, France and Sheffield U. UK

Low Frequency Electromagnetic Wave detector (LFEW) *


Z. Wang and J.B. Cao, CSSAR, China

High Energy Electron Detector (HEED) *

W. Zhang and J.B. Cao, CSSAR, China

High Energy Electron Detector (HEED) *

W. Zhang and J.B. Cao, CSSAR, China

High Energy Proton Detector (HEPD) *

J. Liang and J.B. Cao, CSSAR, China

High Energy Proton Detector (HEPD) *

J. Liang and J.B. Cao, CSSAR, China

Heavy ion detector (HID) *

Y. Zhai and J.B. Cao, CSSAR, China

Heavy ion detector (HID) *

Y. Zhai and J.B. Cao, CSSAR, China


* Instrument built by China
# PEACE includes only one sensor on each spacecraft

Fig. 5: Artist's Impression: Double Star Payload (or flight config.)

The European-provided instruments are:

  • Active Spacecraft Control (ASPOC) (PI: K. Torkar, IWF, Austria)- This instrument prevents a build up of positive electrical charge on the spacecraft by emitting ions of indium into space. (TC-1 only)
    · Hot Ion Analyser (HIA) (PI: H. Reme, CESR, France) - One of the spare sensors from the CIS instrument on Cluster, this instrument analyses the distribution of ions in the surrounding space plasma during each four-second spin of the spacecraft. (TC-1 only)
  • Fluxgate Magnetometer (FGM) (PIs: C. Carr, IC, UK and T. Zhang, IWF, Austria) - Two magnetometers attached to a 3.5 metre boom measure the local magnetic field and magnetic waves. They are able to make high resolution measurements with up to 22 samples per second. (TC-1 and 2)
  • NeUtral Atom Detection Unit (NUADU) (PI: S. McKenna-Lawlor, Nat. Univ. of Ireland) - Based on an instrument flying on ESA's Mars Express mission, this is an advanced particle detector that is designed to monitor energetic neutral atoms in the Earth's magnetosphere and perform imaging of the Earth's Ring Current. (TC-2 only)
  • Plasma Electron and Current Experiment (PEACE) (PI: A. Fazakerley, MSSL, UK) - This instrument is designed to measure the density, temperature and velocity of low-to-medium energy electrons. The spare instrument for Cluster has been split into two instruments: one sensor will fly on each Double Star spacecraft. (TC-1 and 2)
  • Spatio-Temporal Analysis of Field Fluctuations (STAFF) (PI: N. Cornilleau-Wehrlin, CETP, France) and Digital Wave Processing Experiment (DWP) (PI: H. Alleyne, Sheffield Univ., UK) - A magnetometer on the end of a 3.5 metre long boom looks at waves (rapid variations in the magnetic fields), particularly in regions where the solar wind interacts with the magnetosphere. Low frequency data are analysed on the ground, while the magnetic components of the higher frequency waves are processed on board. It also has a particle correlator that enables variations in the electron population around the spacecraft to be compared with the wave measurements. (TC-1 only)

The European Payload Operations Service (EPOS) at Rutherford Appleton Laboratory in the UK is co-ordinating the scientific operations of the European payload. EPOS is also developing the Double Star data management system (DDMS) that distributes data to the users, and the Double Star quicklook web page (DSDS web) that will display the latest data from both European and Chinese instruments. One of the EPOS tasks is to disseminate planning data about orbit and geomagnetic events in order to facilitate co-ordination with other magnetospheric missions.

Data from the Double Star experiments will be relayed to the Chinese data centre in Beijing via the ESA ground station at Villafranca (Spain) and the Chinese ground stations in Beijing and Shanghai. A dedicated network line has been set up between Villafranca and Beijing to facilitate the transfer of data. The European Space Operations Centre (ESOC) is in charge of collecting the data from Villafranca, while CSSAR is collecting the data at Beijing and Shanghai. Once they have been decompressed and prepared for use, the data will be sent to the Austrian data centre in Graz, where they will be made available for the European PIs (Fig. 6). The PIs will then process the data and send them to the Austrian, French and UK data centres for distribution to the scientists.

Fig. 6: The Double Star Science Data System

It is envisaged that a series of joint workshops and publications will result from this close collaboration. Meanwhile, the first announcement of opportunity to select Guest Investigators for the Double Star mission was issued in June 2003, with a "very positive reaction" from the European scientific community, according to Escoubet. Eight proposals were selected (see Table 2), covering various aspects of magnetospheric physics and co-ordinating Double Star and Cluster measurements.

Table 2: Selected Guest Investigators




D. Boscher, Onera, Toulouse, F

Radiation Environment Research from Multiple Monitors using Double Star

S. Bucher, IRF-U, S

Magnetosphere-ionosphere coupling, field-aligned currents and ion flow near the dayside cusp and auroral zone using ground-based, double star and Cluster

M. Dunlop, RAL, UK

Co-ordinated Cluster- DSP measurements in the cusp and at the magnetopause

J. Jahn, SWRI, USA

Multi-Spacecraft Energetic Neutral Atom Observations of Magnetospheric Processes

F. Pitout, ESTEC, NL

Double Cusps and Reconnection Hypotheses

T. Pulkinen, FMI, Finland

Global Understanding of Storms in the Inner Magnetosphere

S. Shwartz, QMW, UK

Kinetic Processes and Conditioning in the Dayside Magnetosheath and Equatorial Geomagnetic Tail

J. Wild, Leicester U., UK

A coordinated in-situ and remote-sensing investigation of magnetosphere-ionosphere coupling exploiting Double Star, Cluster and ground-based experiments”

The Joint Mission

As its name suggests, Double Star will involve two satellites - each designed, developed, launched and operated by the CNSA - flying in complementary orbits around the Earth.

Each cylindrical satellite has a mass of 280 kg and generates electrical power from solar cells which are exposed to sunlight as the spacecraft spins on its axis. Both will be launched by upgraded, three-stage Long March 2C rockets, but different launch sites will be used.

Fig. 7: Double Star
Artist's Impression

Double Star Spacecraft (launch configuration) Characteristics

            • Mass: 280 kg
            • Design: Cylindrical, diameter 2.1 m, height 1.2 m
            • Alignment: Spin axis perpendicular to ecliptic
            • Power: 6.33 m2 solar array generating 280W (BOL)
            • nickel-cadmium batteries
            • Lifetime: Equatorial (TC-1) minimum 18 months
            • polar (TC-2) minimum 12 months.

First to go was the "equatorial" spacecraft (TC-1), which was launched from the southern launch centre at Xichang on 29 December. This spacecraft will investigate the Earth's huge magnetic tail (magnetotail), the region where electrically charged particles (mainly ions and electrons) are accelerated towards the planet's magnetic poles by a process known as reconnection. Its nominal lifetime is 18 months.

The "polar" satellite (TC-2), which will be launched from Taiyuan, south west of Beijing, will concentrate on physical processes taking place over the magnetic poles and the development of aurorae (Northern and Southern Lights). It will follow a 700 x 39,000 km polar orbit with a period of 12 hours. TC-2 is expected to operate for at least one year.

The positions and orbits of the Double Star and Cluster spacecraft have been carefully orchestrated so that they perform a synchronised dance around the planet. While the Cluster quartet follow elongated orbits that carry them about one third of the distance to the Moon, Double Star will fly much closer to Earth, following very different paths.

By studying the Earth's magnetosphere from different perspectives, the six spacecraft will enable scientists to obtain simultaneous data on the changing magnetic field and populations of electrified particles in different regions of near-Earth space.

Fig. 8: Cluster and Double Star TC-1 orbits in February
2004. On this day, Cluster was in the solar wind and
TC-1 at the Earth's bow shock.

Unique Insights

Planet Earth is continually subjected to a bombardment of energetic particles originating 150 million kilometres away in the Sun. Most of the particles arrive in the solar wind, a continuous stream of protons and electrons that flows past our planet. From time to time, enormous explosions known as coronal mass ejections may blast billions of tonnes of material into space at such high speeds that they reach the Earth in only a few days.

Fortunately, the Earth's magnetosphere usually protects the planet from the cosmic onslaught, forcing the solar wind to flow around it like an island in a stream. However, the magnetic bubble can be severely compressed by coronal mass ejections and particles can break through the defences at two weak points in the defences, known as the polar cusps.

Particles that leak into the magnetosphere may eventually spiral down the magnetic field lines towards the Earth, generating the spectacularly beautiful, but harmless, polar auroras. In contrast, other phenomena, such as magnetic storms, can have serious consequences for human activities - including power cuts, damaged satellites and communication blackouts.

Fig. 9: Double Star and Cluster orbits in August 2004

Although some important insights into the physical processes taking place in near-Earth space have already been provided by the Cluster quartet, the additional perspectives offered by the two Chinese satellites will significantly enhance our understanding of the interaction between the magnetosphere and the solar wind.

Whereas the Cluster spacecraft orbit between 19,000 and 119,000 km from Earth, periodically sweeping in and out of the magnetosphere, the Double Star duo will study regions closer to home. For example, in August 2004, when Cluster is flying far down the magnetotail, Double Star will be able to examine magnetospheric processes and activity taking place nearer to the Earth (Fig. 9). Six months later, when Cluster is flying across the bow shock, the turbulent boundary between the magnetosphere and the solar wind, Double Star will be able to study activity at the cusps and close to the magnetopause.

A typical example of how both missions will interact is the study of the substorms that are produced when particles pick up energy and are accelerated towards the planet's poles, creating very bright auroras. Cluster was designed to study the mechanisms that produce these substorms far away in the magnetotail. However, a few years ago, some scientists suggested that the substorms might be generated closer to the Earth, in regions that can best be studied by Double Star. The joint mission will enable both hypotheses to be tested.

Similarly, while Cluster is flying at high altitude through the polar cusps - funnel-like openings in the magnetosphere above the magnetic poles - Double Star will be able to conduct simultaneous studies at lower altitudes. This will enable scientists to study in much greater detail the 'doors' used by electrically charged particles from the solar wind to descend into Earth's upper atmosphere."Space is very big, but the four Cluster satellites are very close to each other, only a few hundred kilometres apart," explained Escoubet. "Although this enables us to observe small regions in great detail, we need more satellites to study the magnetosphere on a larger scale. By flying similar instruments simultaneously in complementary orbits on board Double Star, we expect our understanding of the Sun-Earth connection to improve substantially."

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This page was first created on 25 March, 2004 and was last updated on 8 May, 2009.
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