Scientists aim to gather data on star's mysterious atmosphere
Nasa probe set for seven-year mission to explore sun's corona
Early next month scientists are set to attempt a world-first - launching a probe right into the heart of the sun.
The daring mission will see Nasa's Parker Solar Probe get closer to our nearest star than any other spacecraft in history.
Decades in the planning, the mission's capsule will be exposed to the full blast of the sun’s heat and radiation, with temperatures exceeding a million degrees Celsius.
But the probe is designed to do far more than just survive. It will make repeated swoops down towards the sun's surface, gathering data about the so-called corona, the outermost – and most mysterious - part of the sun’s atmosphere.
For reasons as yet unknown, the corona is far hotter than even the physical surface of the sun. Scientists hope that the PSP will finally reveal the source of its energy.
This is not just a matter of tidying up loose ends. The corona regularly unleashes huge bursts of fast-moving particles into space. Known as coronal mass ejections (CMEs), they usually tear harmlessly across the solar system.
But every so often they strike our planet, causing geomagnetic storms and massive electrical disruption.
In March 1989, a CME took out part of Canada’s electrical grid, leaving six million people without power.
And in September 1859, an even more powerful CME was responsible for the so-called Carrington Event which blacked out telegraph systems right across Europe and America.
Today, if a similarly large solar storm struck again, experts predict it would cause mayhem across the world’s power grids and the Internet.
Understanding what causes CMEs and how they can be predicted is one of the prime goals of the PSP – named after Eugene Parker, the 91-year-old American solar physicist, and the first living scientist to be so honoured.
But simply reaching its target will be a major achievement.
Nasa has now explored every corner of the Solar System, right out to the distant dwarf planet Pluto. Yet despite being much closer to the Earth, travelling to the sun actually requires far more energy.
That’s because when it leaves the Earth, the PSP will be travelling at over 100,000 km/hr relative to the sun, because of the Earth’s orbital motion.
To complicate matters further, the PSP must then plunge deep into the sun’s titanic gravitational field, yet remain under control. At its closest approach, the probe will be travelling at almost 700,000 km/hr, as the pull of the sun’s gravity reaches its peak.
To get the PSP on its way, Nasa engineers have selected their most powerful booster, the Delta IV Heavy - often used for launching giant spy satellites into orbit.
But even that leviathan cannot get the PSP to its destination unaided.
In early October, the probe will swing around Venus, a planet similar in mass to the Earth, and use its gravitational field as a slingshot, boosting its energy even further.
Over the coming years, PSP will perform this encounter seven times, picking up ever more energy, and getting ever closer to the sun.
Then, sometime in June 2025, it will swoop down to within just 6 million km or so of its surface – 36 million km closer than any probe has gone before.
And that will take PSP into the incredible heat of the corona.
Visible to the naked eye only during solar eclipses, the corona surrounds the physical disk of the sun like a vast cloud of intensely hot particles. What baffles scientists is why it is so hot - its million-degree Celsius temperature vastly exceeding the 5,500°C of the sun’s surface.
The incredible temperature reflects the high speed of the particles within it. So the real question is: what force is accelerating these particles?
Scientists believe the best answer is the magnetic field of the sun, but exactly how this happens remains a mystery.
Once in the corona, the PSP will use its instruments and imaging devices to watch how particles leaving the sun become caught up in waves of magnetic field before being accelerated to incredible speeds of around 500,000 kph.
But to do this, the PSP must be able to withstand the fearsome conditions of the corona.
And strangely the temperature is not the biggest obstacle – it’s the heat, which isn’t the same thing.
It’s a distinction we’re actually all familiar with. Walking outside on a day where the air temperature is 50 degrees Celsius is unpleasant – but it isn’t agonising in the same way plunging your hand in 50°C water would be.
The difference is that air is around a thousand times less dense than water, and so carries far less heat at the same temperature.
It’s the same with the corona. Although its particles have a very high temperature - aka speed - they’re not very concentrated, and are therefore much less damaging than one might think.
Even so, the PSP will have to withstand furnace-like heat equivalent to around 1,400°C – more than enough to fry the probe’s electronics.
As a result engineers have equipped the PSP with a carbon heat-shield measuring 2.4 metres across and 115mm thick. With the sun-facing side coated with white ceramic paint to reflect back radiant heat, the shield keeps the instruments at just 30°C – warm, but well within their operating range.
Various other design features – including simple liquid water cooling – protect the rest of the probe.
Following its launch next month, the PSP’s engineers face a seven-year wait to discover if their ingenuity was up to the task. But if all goes well, the probe will finally reveal the cause of the sun’s temper tantrums, which can wreak such havoc on our planet.
Robert Matthews is Visiting Professor of Science at Aston University, Birmingham, UK