The European Space Agency is in the countdown phase to one of the highlights of its scientific year: the launch of its three-spacecraft Swarm mission to study the Earth’s magnetic field in unprecedented detail.
Having arrived at Plesetsk Cosmodrome in Archangel, about 800km north of Moscow, in separate Ilyushin Il-76 transport flights from Munich, the three 500kg Astrium-built spacecraft are now being fuelled and integrated with a single Rockot launcher for their 14 November trip to orbit.
One aloft, the trio – flying in rough formation – will independently measure the Earth’s magnetic field strength. Scientists on the ground will pore over the differences between their measurements, rather than merely their absolute values, in a bid to disentangle the contribution to the magnetic field of the Earth’s rotating, liquid core, its mantle, crust and oceans, the upper atmosphere region known as the ionosphere, and the interaction between the Solar wind and the ionosphere’s outer shell, the magnetosphere.
The scientific objective is to better understand how the Earth’s magnetic field is generated, how it varies around the globe and why this field that protects us from space weather appears to be weakening. To achieve that aim, ESA and prime contractor Astrium have had to employ some unusual design tricks.
As with all missions to space, Swarm must survive launch. But, says mission manager Rune Floberghagen, with all three identical 468kg craft closely packed to fit on one launcher, there is an additional risk of collision in the first few seconds after release.
Although Swarm is ESA’s first constellation mission, once it is in space and clear of the Rockot launch vehicle, navigation becomes a “relatively flexible” concern and orbital manoeuvres to the three spacecraft can be carried out independently, says Floberghagen. Two of the satellites will initially orbit essentially side by side around the poles at an altitude of 460km, while the third flies about 80km higher. The lower pair need only be kept within about 1.4˚ latitude of each other. Being at the same altitude and close in horizontal position, says Floberghagen, it can be assumed that they will be affected equally by the magnetosphere and ionosphere, and hence those critical differences between the measurements they take can be assumed to stem from very local effects of the Earth’s crust, mantle and core.
However, he says, what is important is that their along-track separation is watched very closely. As these lower two will be on approximately the same orbital plane, they could collide over the poles if one is not kept about 10s ahead of the other – which translates into a gap of 70-80km. Beyond that, the relative positions of the three satellites are only roughly important; what is critical that ESA knows exactly where each one of them is, using GPS and their onboard star-trackers, in order to know which part of the Earth is being measured at any moment.
The third satellite, at 530km up, will initially fly on an orbital plane 0.6˚ off the lower pair’s, with the result that its orbit will, over the first three years of the mission, drift to a 90˚ separation. At that point, Floberghagen explains, when the lower pair are in the Earth’s shadow the higher one will be in daylight, and vice versa; thus at any given moment the difference between measurements taken by the lower pair and their higher partner should help reveal the Sun’s influence on the Earth’s magnetic field.
Over four years, the lower pair of Swarm satellites will see their altitude decay naturally – owing to the drag effect of residual atmosphere – to just 300km. Therefore, and obviously unusually for spacecraft, they are streamlined to minimise drag and the amount of propellant needed to hold altitude. The front end, with a surface area of just 1m², houses the first 3D ion imagers to go into orbit, to measure ionosphere characteristics.
OUT ON A LIMB
The other unusual design characteristic critical to the mission is that while the total length of each satellite is 9.1m, 4m of that is a trailing boom, on which are mounted the magnetometers and star-tracker. By putting the magnetic field instruments far behind the body of the spacecraft, they will sit in a magnetically clean region, clear of any magnetic disturbance from the spacecraft’s electrical systems. Once the boom is deployed – and it has been designed for extreme stability – the spacecraft has no moving parts, whose vibrations would disturb its instruments.
As the name suggests, Swarm relies on multiple spacecraft. Indeed, says Floberghagen, there had at one point been talk of up to seven units, but they were reduced to three to cut costs. The mission design, however, allows other spacecraft to join the constellation, in the probably unlikely event that another space agency were able to launch a companion.
Should one or even two of the spacecraft fail, the mission will still be scientifically useful, he adds. The first backup plan is to operate the lower pair – again, to focus on the differences between the measurements taken. If the mission were reduced to just one satellite, it could still measure the circular variation of the Earth’s magnetic field, hopefully shedding some light on one of its mysteries: the poles wander.
Indeed, magnetic North has moved about 2,000km since it was first measured in 1831, and outright North-for-South field inversions are a regular feature of Earth’s geological history. Indications are that a field inversion is imminent, an event that would certainly wreak havoc on navigation. So any new insight into the deep workings of the planet that may come from a Swarm of even one spacecraft will be of more than academic interest.