RSS feed
SOHO, a remarkable 10 years in orbit
November 30, 2005
EADS Astrium was the prime contractor for the ESA satellite SOHO, launched into orbit on 2 December 1995 on a mission to observe the sun. Originally designed for a service life of only two tears, the Solar and Heliospheric Observatory has achieved an amazing 10 years of operation. The remarkable events of summer 1998, when contact was lost with the satellite and then restored, will surely go down in the annals of space history. Thanks to the skill and dedication of the engineers, and the robustness of the satellite and its instruments, the SOHO mission was saved and then prolonged. The satellite is now expected to remain in operation until at least 2007, enabling it to cover an entire cycle of sunspot activity (11 years).
More than 3,200 scientific researchers use data from SOHO for their work. SOHO is a unique solar observatory for scientists thanks to its position at the first Lagrangian point and its twelve instruments, capable of analysing all the regions of the sun, from its interior to the energetic particles carried by the solar wind.
Helioseismology instruments measure the oscillations on the surface of the sun. Using these measurements, scientists have mapped the convection currents of hot gas beneath the surface and identified how twists in the magnetic fields cause them to break through the surface, forming sunspots. SOHO's instruments can even look right through the sun to the active surface zones on the other side – zones that are invisible from the Earth. This makes it possible to predict solar flares several days in advance. These massive eruptions of gas have a noticeable effect on the Earth’s outer reaches, causing magnetic storms, the aurora borealis and sometimes even triggering power blackouts in the electricity grid.
Another of the satellite’s instruments is capable of detecting an area of activity on the hidden face of the sun by detecting the emission of hydrogen atoms in space beyond the sun’s surface – in the same way as the pencil of light from a lighthouse becomes visible when it lights up the clouds.
Instruments designed to observe the sun’s 'atmosphere' or corona have enabled scientists to analyse the mighty flares known as coronal mass ejections (CME). These occur almost daily with the sun spitting out millions or occasionally billions of tonnes of gas at a velocity of several hundred kilometres per second.
SOHO is also the greatest comet hunter of all times. Its data have led to the discovery of more than 1,000 comets, the majority by amateur astronomers in places all over the world (even Australia and China) who look for bright spots in the images taken by SOHO that are moving relative to the sun. Enormous patience and dedication are needed to analyse the images down to the very last detail, illustrating the passionate enthusiasm that news and images from the SOHO mission invoke among Internet users.
By the end of its life, SOHO will have supplied data throughout an entire sunspot cycle, in which solar activity was it its lowest point in 1996 and reached a peak in 2000 (the photo below shows the solar storm of 14 July 2000).
ORBITING AT 1.5 MILLI0N KILOMETRES FROM THE EARTH
SOHO’s orbit places the satellite in the perfect position to observe the sun and its influence on the Earth. It is situated around the first Lagrangian point L1, a point in space where the gravitational pull of the sun and of the Earth-moon system are in stable equilibrium. L1 is located approximately 1.5 million kilometres from the Earth along the Earth-sun axis (the total distance between Earth and the sun is 150 million kilometres). From this vantage point, the satellite has a permanent view of the sun, towards which it is pointed with very great accuracy while at the same time being sheltered against dynamic perturbations.
The specific advantages of this rather unusual orbit are that it allows the sun to be observed from outside the Earth’s magnetosphere (and thus without disturbing the solar wind), and at the same time avoid interruptions during periods when the sun is eclipsed by the Earth or the moon. As well as permitting more or less continuous observation of the sun, this places the satellite in a very stable thermal environment.
TECHNICAL DATA
The satellite is 3.8 metres high and has a wingspan of 9.5 metres with its solar panels deployed. Its launch weight was 1850 kilograms.
EADS Astrium is Europe’s leading satellite system specialist. Its activities cover complete civil and military telecommunications and Earth observation systems, science and navigation programmes, and all spacecraft avionics and equipment.
EADS Astrium is a wholly owned subsidiary of EADS SPACE, which is dedicated to providing civil and defence space systems. In 2004 EADS SPACE had a turnover of €2.6 billion and 11,000 employees in France, Germany, the United Kingdom and Spain.
EADS is a global leader in aerospace, defence and related services. In 2004, EADS generated revenues of €31.8 billion and employed a workforce of more than 110,000.
Press contacts:
EADS SPACE (FR), Rémi ROLAND +33 (0) 1 42 24 27 34
EADS SPACE (GER), Mathias PIKELJ +49 (0) 7545 8 91 23
EADS SPACE (UK), Jeremy CLOSE +44 (0)14 3877 3872
SOHO: lost and found Rescue mission at 1.5 million kilometres from the Earth
25 June 1998: SOHO has disappeared from the screens of the mission control team at the Goddard Space Center. It is impossible to re-establish contact with the satellite, which was launched two and a half years earlier and had been functioning perfectly up till now. Over the next 4 months, a team of a dozen Astrium engineers who had helped to build the satellite will work side by side with ESA and NASA experts in an attempt to recover the spacecraft.
During the first few weeks, the team elaborates a variety of hypotheses and scenarios that might be able to shed light on the satellite’s possible state and whereabouts, on the basis of the last known remote sensing data (at what speed and in which direction was it rotating around its axis, on what path is it drifting?). At the instant it disappeared from view, its solar panels were no doubt perpendicular to the sun, but as the months go by the position of the satellite with respect to the sun will change, hopefully enabling the solar panels to capture light again, and thus supply the energy that is needed for a subsequent rescue mission.
On 23 July, the large radiotelescope at Arecibo in Porto Rico reports detecting a radar echo from SOHO.
Its position is located, and the trajectory on which it is drifting through space. It is possible to establish that the satellite is spinning around its own axis at a velocity of one revolution per minute.
The next step is to make contact with the satellite’s receivers, and to do this the rescue team needs to define the best parameters to use in this case (frequencies, command sequence and rate of transmission).
3 August, a glimmer of hope. For the first time in 6 weeks, SOHO responds for just a few seconds to the commands being transmitted without cease from the ground. From 8 August onward, it becomes possible to glean more information from the satellite: the temperature is extremely low – between 50°and 60° below zero Celsuis – and solar energy is only available for 30 seconds of each revolution. Despite these extreme conditions, there is a chance that it might be possible for the satellite to execute the commands needed to recharge its batteries.
The next stage is to de-ice the hydrazine tank which will allow the satellite to be reoriented to the correct attitude. This job will take about 3 weeks.
On 16 September, the satellite is restored to its correct orientation; it stops spinning about its axis and its solar panels are once again pointing towards the sun.
The next job in hand is to verify and recalibrate the instruments. By 3 November 1998, the 12 instruments on board are again functioning normally.
But on 21 December, its last gyroscope fails. SOHO is therefore placed in a “rescue” mode to ensure its safety.
Once again, the Astrium and ESA teams sprang quickly into action, perfecting and uploading a software program, on 2 February 1999, that would enable the satellite to resume its functions, but without the aid of a gyroscope.
The SOHO rescue mission and the loss of the gyroscope had the unexpected side effect of enabling the engineers to develop an attitude control system that works without a gyroscope and has proven to be more resistant to the effect of solar flares than conventional gyroscope-based systems. This was an added advantage for the research community, given that the peak of sunspot activity occurred in 2000-2001, and thus at a time when the software was meanwhile operating minus gyroscope, and consequently more robust.
Photos available on ESA-int website
Its position is located, and the trajectory on which it is drifting through space. It is possible to establish that the satellite is spinning around its own axis at a velocity of one revolution per minute.
The next step is to make contact with the satellite’s receivers, and to do this the rescue team needs to define the best parameters to use in this case (frequencies, command sequence and rate of transmission).
3 August, a glimmer of hope. For the first time in 6 weeks, SOHO responds for just a few seconds to the commands being transmitted without cease from the ground. From 8 August onward, it becomes possible to glean more information from the satellite: the temperature is extremely low – between 50°and 60° below zero Celsuis – and solar energy is only available for 30 seconds of each revolution. Despite these extreme conditions, there is a chance that it might be possible for the satellite to execute the commands needed to recharge its batteries.
The next stage is to de-ice the hydrazine tank which will allow the satellite to be reoriented to the correct attitude. This job will take about 3 weeks.
On 16 September, the satellite is restored to its correct orientation; it stops spinning about its axis and its solar panels are once again pointing towards the sun.
The next job in hand is to verify and recalibrate the instruments. By 3 November 1998, the 12 instruments on board are again functioning normally.
But on 21 December, its last gyroscope fails. SOHO is therefore placed in a “rescue” mode to ensure its safety.
Once again, the Astrium and ESA teams sprang quickly into action, perfecting and uploading a software program, on 2 February 1999, that would enable the satellite to resume its functions, but without the aid of a gyroscope.
The SOHO rescue mission and the loss of the gyroscope had the unexpected side effect of enabling the engineers to develop an attitude control system that works without a gyroscope and has proven to be more resistant to the effect of solar flares than conventional gyroscope-based systems. This was an added advantage for the research community, given that the peak of sunspot activity occurred in 2000-2001, and thus at a time when the software was meanwhile operating minus gyroscope, and consequently more robust.
Photos available on ESA-int website