Mission to the Sun
How we are going to get a better look at our star than ever before
We live inside a bubble blown by the Sun.
The bubble is made of the solar wind – a stream of charged particles and magnetic field flung out from the Sun’s hot atmosphere .
When this tangle of particles and magnetic field reaches us, it interacts with the Earth’s magnetic field.
This field protects us, but when the Sun is in a particularly violent mood, extreme solar wind events like flares and coronal mass ejections can interrupt satellites and power grids, affecting our way of life.
Understanding the Sun’s moods, and how these influence the solar wind, is the aim of Solar Orbiter.
Solar Orbiter is a European Space Agency mission that has Imperial-built kit on board. The spacecraft will launch in February 2020 with the aim of getting an up-close look at the Sun. It will scope out our star from only 50 million kilometres distance – a fraction of the 150 million kilometres away Earth sits at.
The spacecraft will carry ten instruments on board, which will make two kinds of measurements:
• Remote measurements will be made by looking down at the Sun’s surface, through sophisticated cameras and telescopes.
• In-situ measurements will be made by sensing what’s happening in the immediate area of the spacecraft, such as with Imperial’s magnetometer, which measures the Sun’s magnetic field.
We’re used to thinking of the Sun as a uniform ball of light, but under the surface it is broiling with complex magnetic fields that can release huge amounts of energy, flinging fast-moving charged particles in our direction
The Earth’s magnetic field can be pictured like a bar magnet. Magnetic field lines come out from one pole, encompass the Earth, and connect back into the other pole.
The Sun, however, is writhing with much more tangled magnetic field lines looping in and out across its surface, getting stretched and squeezed and sometimes coming undone, releasing energy and particles. The particles, and with them the magnetic field, sweep out into space, changing and transforming as they go.
We have plenty of data about what these look like when they reach the Earth. But they change a lot during the journey, and measuring them closer to the Sun will allow researchers to better match what happens on the Sun’s surface – where the solar wind is created – to what blasts off into space. This could allow for better forecasts of ‘space weather’ – the effects of solar wind events at the Earth.
“We've got the combination of these wonderful instruments with a really interesting orbit,” Professor Horbury says, “which means that we get to go in close to the Sun and see what's going on its surface, and then see how that expands out into interplanetary space.”
Professor Tim Horbury
Professor Tim Horbury
Extreme challenges
With such an important mission, why hasn’t it been done before?
The challenges begin, as you might expect, with temperature. But it’s not simply the heat of the Sun the spacecraft and its instruments have to contend with, it’s the huge variations of temperature. Out in the vacuum of space, there’s no atmosphere to evenly distribute heat. So, the side of the spacecraft facing the Sun, which is covered by a heat shield, will reach up to 500⁰C. Just a few metres behind this, out on a boom, the Imperial team’s magnetometer instrument will operate at -140⁰C. On the way to the Sun, however, there will be other swings – flying past Venus, the instrument will get much hotter.
Out in space, the instruments will also have to contend with radiation that we’re shielded from on Earth. The Sun throws out a lot of charged particles, which can interfere with the electronics on board, so the kit must be tested to withstand the radiation environment expected in operation around the Sun.
Before all this, there are the tumultuous conditions of launch. Solar Orbiter is a relatively small spacecraft (less than 3m across) but needs to launch atop one of NASA’s largest rockets in order to gain the speed needed to reach the inner Solar System. This means a lot of vibration as it blasts off, and a large shock when the rocket fairing separates – the nose cone protecting Solar Orbiter from the atmosphere during ascent.
Helen O'Brien holding the magnetometer
Helen O'Brien holding the magnetometer
The Imperial team’s magnetometer has been extensively tested under all these conditions, sometimes using sophisticated equipment, and sometimes, in the case of fairing separation shock, using lower-budget tests (a ‘really huge hammer’ applied to a table the instrument was bolted to).
The instrument failed its first vibration tests
The instrument failed its first vibration tests
Every stage of testing involves compromise. Helen O’Brien, Instrument Manager, was responsible for the design and build of the magnetometer. She says: “The shock test was a large concern for us, because we use a ceramic material in the sensor that is really good for keeping stable under varying temperature, but is quite brittle. We were pleased when it survived the shock test, but initially it failed the vibration test, and we had to make the material thicker to compensate.”
Measuring the magnetic field
All those tests make sure the magnetometer is safe. But the team also has to make sure it works, and measures the right things once in space.
The instrument is known as a fluxgate magnetometer. It's made up of a circular magnetic core (grey, below) in two halves. The core is made up of a material sensitive to magnetism, where many of its atoms act like tiny magnets, called dipoles.
A wire coil (blue) wraps around the core, capable of inducing a magnetic field. Finally, there is a rectangular wire wrap (orange) that senses the action of the dipoles.
It works like this:
Testing sensitivity
At its surface, the Sun’s magnetic field is around twice that of the Earth’s, but in deep space it is tens of thousands of times smaller, so the magnetometer must be incredibly sensitive.
Our instrument is so sensitive, it could measure the magnetic field of an MRI machine from the other side of London
“This means, however, that we have to work hard to isolate it from the other instruments on the spacecraft. Metal objects and electrical circuits create small magnetic fields, so we have really strict requirements on the rest of the project – right down to the screws and the paint.”
Before launch, as one of the final stages of testing, the spacecraft was placed in a unique facility run by German company IABG in a forest south of Munich to avoid interference with human-generated magnetic fields.
The facility consists entirely of non-magnetic materials like wood and contains twelve 15-metre coils – nearly as large as the building – which create a consistent magnetic environment that cancels out the Earth’s own magnetic field, simulating outer space conditions.
The magnetometer was then tested while the spacecraft performed some operations that could potentially cause interference, such as extra currents generated by the cameras on board to take images of the Sun.
And this is just for one instrument. With all that incredible amount of expertise and care, carried out by groups across Europe and the USA, it’s no wonder it’s taken so long to come together.
The last missions to approach the Sun were the Helios 1 and 2 spacecraft, launched in the 1970s. Despite the long wait for a follow-up, Solar Orbiter will actually be the second Sun-focused spacecraft launched within two years. NASA’s Parker Solar Probe launched in 2018, and will go closer to the Sun than Solar Orbiter. However, this means it cannot carry the same suite of instruments Solar Orbiter can, as they would simply not work under such extreme conditions.
Instead, the two spacecraft will work in tandem, able to get a detailed picture of how events on the surface of the Sun play out into space.
The instruments on Solar Orbiter will switch on shortly after launch, many just for testing, but the magnetometer will start taking measurements and sending back data within 24 hours.
The spacecraft will then take around two years to get into its orbit around the Sun, flying past Earth and Venus several times in order to gain a boost from their gravity and propel itself closer to our star. Then, it will spend seven years collecting data, before its solar panels are predicted to degrade and it can no longer operate.
In that time, the team expects to gain unprecedented new insights into the Sun, and particularly how the solar wind is made and accelerated into space. Parker Solar Probe has already made new discoveries about the origins of the solar wind, and the two spacecraft working together should reveal insights scientists haven’t even predicted.
It’s an exciting time to be in solar physics.
As Professor Horbury says: “Solar Orbiter is going to do so much great science in helping us understand how the Sun works and how it affects all our lives, and there is a community of scientists, at Imperial, elsewhere in the UK, and around the world, who are itching to get their hands on our measurements.”
Illustrations and animations of Solar Orbiter all credit to ESA/ATG medialab. Animation of Earth's magnetic field and solar flares credit NASA GSFC/CIL/Bailee DesRocher. Photos of Tim Horbury and Helen O'Brien credit to Imperial College London/Thomas Angus. Illustrations of magnetometer credit to Imperial College London/Richard Palmer. Photos of spacecraft in testing facility credit ESA–S. Corvaja.
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