– Eigil Friis-Christensen – “The Father of Swarm”
– Ensuring Data for the Scientific Community
Nearly all scientific missions provide some answers; however, mostly new questions arise. This goes for the Danish Ørsted satellite. Initiated by Denmark and almost completely Danish, the satellite has through the years in space provided the scientists with a flow of new information about the Earth’ magnetic fields, but not even this mission has acquired all the answers to all the questions. New and unanswered questions arose, giving way for a Danish proposal for an even more advanced mission. Not only one satellite, but a “Swarm”.
The Ørsted Satellite was, in spite of the size, only 60 kilograms, epoch-making for detecting and understanding the magnetic field. The satellite sends measurements of the Earth’s magnetic field to scientists, who use these data to devise advanced models of the Earth’s magnetic field. Developed by scientists at DTU Space (Denmark Technical University) in close cooperation with NASA, the models are now used, among other applications, all over the world searching for oil and minerals.
Launched in 1999 and still partly operative the satellite continuously provides a wealth of new information as well as new questions.
Danish Idea for Better Resolution
DTU Space was heavily involved in the Ørsted satellite. As a result, Danish scientists in 1996, led by Professor Eigil Friis Christensen, proposed a new project based on six small identical satellites in different orbits for even more detailed mapping. To select the influence of the solar wind from the Earth’s geomagnetism it is necessary to have at least two satellites, but a larger number provide even better resolution. One of the reasons is that influence of the solar wind is largest on the dayside of the Earth, not so intense on the night side.
In the first competition the Goce mission was selected by ESA as the forth Earth Explorer mission.
Then, in 2002, an even more detailed mission proposal was submitted to ESA by a European consortium of members from the Ørsted consortium, led by DTU Space, Denmark. The reason for selecting their proposal as the fifth Earth Explorer mission in 2004, may be due to their advantage of a highly united team at Ørsted.
However, the Phase A completed in 2005 resulted in a final mission based on only three satellites. By placing two satellites near each other, one gets a stereoscopic view. A third satellite will by launched to a higher orbit, but for the rest nearly identical, but the orbit plane changes continually, and after three years the orbit plane will be perpendicular to the other two, providing a three-dimensional view of the magnetic fields. A constellation of several satellites for earth observation is a new and innovative method for the scientists.
Constellation of Satellites
To use several satellites and higher resolution the Swarm team can better separate the different sources from each other, providing better information about the activity in the core, however, it also increases information about other influences like the solar wind, ocean currents etc.
Through data from the mission, the scientists can study the activity in the core, the magnetic content in the crust, the stream systems in the ionosphere under the satellites orbit, and the magnetosphere outside the orbit. This system of activities creates the well-known Aurora Borealis.
To separate the different sources the team can deliverer separate models for example from the core, the crust, the oceans etc., and all these models, sometimes combined with data from other sources, can be used of the scientific community for further research.
Swarm is a scientific mission, but the data will also be useful for commercial applications. One field is within oil exploration and oil drilling. To steer the drill bit to more accurate positions deep in the mantle it is necessary to have knowledge about the geomagnetism in the underground. Ørsted already delivers such data and Swarm can continue this service.
The nominal lifetime for Swarm is three years, but experience from previous satellites indicates that the satellites can deliver data for more than that, if the project is going well. It is expected that when useful data is accessible, more commercial applications will be developed.
The main mission is to deliver data about the geomagnetism; however, it will also provide much more knowledge about the geomagnetic fields in the magnetosphere. To this day, several satellites have sampled data about the Earth’s magnetic fields, but none with the accuracy Swarm will provide.
The objective of the Swarm mission is to provide a survey of the geomagnetic field and its temporal evolution, in order to gain new insights into the Earth system by improving our understanding of the Earth’s interior and its environment.
Instruments Developed in Denmark and Sweden
DTU Space not only proposes and heads the project; the institute also develops and builds some of the instruments on-board. The core instrument in the mission is the Vector Field Magnetometer placed on the deployable boom at each satellite. The magnetometers are further refinements of those used aboard the Ørsted, SAC-C and CHAMP satellites, and the positive experience from previous missions has played an important role in developing the magnetometers designed for Swarm.
The Vector Field Magnetometer makes high precision measurements of the magnitude and direction of the magnetic field, i.e. the field’s vector. The orientation of the vector is determined by the star tracker assembly, which provides attitude data.
The institute also delivers the star tracker assembly, or the Stellar Compass. The instrument main task is always to detect the satellites’ position in space. Like the main instrument, the Stellar Compass is well proven and used in several missions before. Just now, a duplicate installed at the American Juno probe is its way towards Jupiter.
In addition to the Danish involvement, the Swedish Institute of Space Physic delivers the Novel Langmuir Probe as a part of the Electric Field Instrument. In addition, Ruag Space Sweden has developed and delivered most of the antennas onboard.
The satellites were launched at the same launcher and two satellites were placed side by side at an altitude of 460 km, but lowered of atmospheric drag to about 300 km at three years. The third satellite gets an orbit at 530 km and with a slightly different inclination. The satellite’s orbits drift resulting in the upper satellite crossing the path of the lower two at an angel of 90 degree in the third year of operations.
The Next Step?
Plans for investigation of the magnetic field with even better resolution than Swarm, is already in the idea phase. This time the team will propose a much larger swarm of identical satellites, all in different orbits around the Earth. Not as large as Swarm, but more like Ørsted and launched as piggyback together with other satellites, also in this case, mostly like Ørsted. More knowledge and better understanding of processes require more data and better data. Ten new years ahead, we may possibly see a new swarm of Danish satellites in space.