MIS­SION INTO THE SUN

Are we fi­nally ready to ex­plore the star that has con­tin­ued to re­main stub­bornly out of reach?

BBC Earth (Asia) - - Front Page - WORDS BY DR STU­ART CLARK

This sum­mer, NASA will launch one of its most am­bi­tious space mis­sions to date: the Parker So­lar Probe. Trav­el­ling at a blis­ter­ing 720,000km/h (450,000mph), the space­craft will re­peat­edly dive closer to the Sun than any pre­vi­ous space­craft in his­tory. It will ven­ture so close that the Parker So­lar Probe team refers to it as touch­ing the Sun. In fact, it will dive in and out of the Sun’s at­mos­phere, known as its corona. And it’s not go­ing to be alone up there.

In Fe­bru­ary 2019, the Euro­pean Space Agency (ESA) will launch a so­lar mis­sion of its own, called So­lar Or­biter. This craft will not go as close to the Sun as its NASA coun­ter­part but it will still be bathed in in­tense sun­light, al­most 500 times that ex­pe­ri­enced by a space­craft in Earth’s or­bit. Un­like Parker So­lar Probe, which spends only a short amount of time in the fierce heat as it dives in and out, So­lar Or­biter will stay put for years, watch­ing and mea­sur­ing the Sun.

“TRAV­EL­LING AT A BLIS­TER­ING 720,000KM/H, THE SPACE­CRAFT WILL RE­PEAT­EDLY DIVE CLOSER TO THE SUN”

Both of these mis­sions have a sin­gle goal: to find out more about the way elec­tri­fied gas known as plasma is launched from the Sun’s at­mos­phere out into space. This con­tin­u­ous stream is known as the so­lar wind. It car­ries en­ergy and the Sun’s mag­netic field through space, and un­der­stand­ing it could solve a prob­lem that’s been mys­ti­fy­ing sci­en­tists for decades and could be the key to safe­guard­ing our tech­no­log­i­cal so­ci­ety.

WHAT A WIND

When the so­lar wind col­lides with Earth, it can dis­rupt or even de­stroy elec­tri­cal tech­nol­ogy in or­bit and on the ground. One re­cent study by the US Na­tional Acad­emy of Sciences found that with­out ad­vance warn­ing, a huge so­lar flare, car­ried by the so­lar wind, could cause $2tr worth of dam­age in the US alone, and it would not be quick to fix. The re­port found that such an enor­mous so­lar flare could cause so much dam­age to power sta­tions that the US eastern seaboard could be left with­out power for a year. Europe is sim­i­larly vul­ner­a­ble. Yet while some­thing of this mag­ni­tude would only hap­pen once ev­ery cou­ple of hun­dred years, smaller storms hap­pen more fre­quently. Most of these cause lit­tle dis­rup­tion, but all have an ef­fect. In March 1989, for ex­am­ple, a small so­lar storm se­verely dam­aged a power trans­former on the Hy­dro-Québec power sys­tem. It took down their power grid for more than nine hours as emer­gency re­pairs were car­ried out. And more re­cently, in 2003, a se­ries of so­lar storms that took place around the Hal­loween pe­riod caused more than half of NASA’s satellites to mal­func­tion in some way, while aero­planes had to be re-routed away from po­lar lat­i­tudes be­cause of the large amounts of ra­di­a­tion as­so­ci­ated with the in­tense au­rora.

So while study­ing the Sun has never been more timely, the de­sire to do so stretches back be­fore the space age to the 19th Cen­tury, when a so­lar mys­tery was un­cov­ered. On 7 Au­gust 1869, as­tronomers gath­ered across Rus­sia and North Amer­ica to ob­serve a to­tal so­lar eclipse. In those fleet­ing min­utes of dark­ness, the sci­en­tists got to see some­thing not vis­i­ble at any other time: the ghostly veils of the so­lar corona, the Sun’s outer at­mos­phere. It was an ob­ject of fas­ci­na­tion for the as­tronomers of the day. Two of the as­tronomers, Charles Au­gus­tus Young and Wil­liam Hark­ness, were us­ing spec­tro­scopes to split the coro­nal light into its con­stituent wave­lengths. They knew that the var­i­ous chem­i­cal el­e­ments gave out light at spe­cific wave­lengths, and by mea­sur­ing these ‘spec­tral lines’ they would be able to es­tab­lish the chem­i­cal com­po­nents of the corona. Work­ing in­de­pen­dently, they both dis­cov­ered a green spec­tral line with a wave­length of 530.3nm. It caused great ex­cite­ment at the time be­cause there was no known chem­i­cal re­lated to this wave­length, so the as­tronomers thought they had dis­cov­ered a new ele­ment. They named it coro­nium.

It turned out that Young and Hark­ness were wrong, yet it wasn’t un­til the 1930s that sci­en­tists un­der­stood why. As­tro­physi­cists Wal­ter Gro­trian and Bengt Edlén con­ducted lab­o­ra­tory ex­per­i­ments and found that iron could give out that green light, but only if it were heated to an ex­traor­di­nar­ily hot 3,000,000°C, turn­ing it into an elec­tri­cally changed gas known as a plasma. With this re­al­i­sa­tion the real mys­tery was born. What ex­actly is heat­ing the Sun’s corona to 3,000,000°C?

“WITH­OUT AD­VANCE WARN­ING, A HUGE SO­LAR FLARE, CAR­RIED BY THE SO­LAR WIND, COULD CAUSE $2TR WORTH OF DAM­AGE IN THE US ALONE”

The mag­ni­tude of the prob­lem is enor­mous be­cause the sur­face of the Sun is a mere (as­tro­nom­i­cally speak­ing) 6,000°C. “It de­fies the laws of physics and na­ture. It’s like wa­ter flow­ing up hill. You move away from a heat source and it should get cooler not hot­ter,” says Ni­cola Fox, mis­sion project sci­en­tist at the Johns Hop­kins Univer­sity Ap­plied Physics Lab­o­ra­tory. “What hap­pens in this re­gion that sud­denly ac­cel­er­ates all of this coro­nal ma­te­rial to tem­per­a­tures ex­ceed­ing 3,000,000°C? It is mys­tery num­ber one,” says Fox.

And if that wasn’t a big enough co­nun­drum, there is a sec­ond, re­lated mys­tery. The gas breaks away from the Sun just where the tem­per­a­ture peaks. “If you think of the Sun as a gi­ant grav­i­tat­ing star, it is go­ing to hang onto its ma­te­rial. And yet the plasma is able to break away and move out and bathe all of the plan­ets,” says Fox.

This is the so­lar wind. It is made mostly of hy­dro­gen and he­lium. The iron that be­trayed the corona’s great tem­per­a­ture ac­tu­ally makes up just a tiny frac­tion of its com­po­si­tion. The so­lar wind car­ries with it the Sun’s mag­netic field and streams out into space at about 1,600,000km/h (1,000,000mph). It bathes the plan­ets, and when it col­lides with the Earth, it sparks the stun­ning au­ro­ras that shine in the po­lar skies.

STAY COOL

As­tronomers say that the ac­cel­er­a­tion of the so­lar wind oc­curs at about 10 so­lar radii (one so­lar ra­dius is equal to the ra­dius of the Sun). “That’s where Parker So­lar Probe is go­ing, it’s a sci­en­tif­i­cally im­por­tant re­gion of space,” says Im­pe­rial Col­lege London’s Prof Tim Hor­bury, who is a co-in­ves­ti­ga­tor on Parker So­lar Probe’s FIELDS in­stru­ment.

Through its se­ries of ex­traor­di­nar­ily close en­coun­ters with the Sun, Parker So­lar Probe will re­peat­edly ex­plore this key re­gion. It will sur­vive its plunge thanks to an in­no­va­tive ther­mal pro­tec­tion sys­tem (TPS). This heat shield is made of two plates sep­a­rated by a layer of car­bon foam. The layer that faces the Sun is white and re­flec­tive. The foam it­self is dif­fuse and light, and is com­posed of 97 per cent air. It was de­vel­oped and man­u­fac­tured es­pe­cially for the space­craft and is one of the key tech­nolo­gies that has en­abled the mis­sion to take place. It is just over 11cm thick, and will be heated to around 1,377°C dur­ing its close so­lar passes. On the other side of the TPS, where the space­craft is lo­cated, the de­sign will al­most com­pletely dis­si­pate the heat, re­duc­ing it to a com­fort­able room tem­per­a­ture of around 21°C.

So­lar Or­biter’s heat shield takes a dif­fer­ent ap­proach be­cause it has to with­stand lower but con­stant heat­ing. Its max­i­mum tem­per­a­ture is likely to be around 520°C, but it is not go­ing to head out to the or­bit of Venus to cool down, like the Parker So­lar Probe. So­lar Or­biter’s heat­shield is pitch black rather than white and re­flec­tive, as this means it will ab­sorb heat and ra­di­ate it back out into space. It is made from ti­ta­nium cov­ered with a pro­tec­tive skin called So­larBlack, which is de­rived from a char­coal-based pig­ment made of burnt an­i­mal bones. This pig­ment

“THE SO­LAR WIND BATHES THE PLAN­ETS, AND WHEN IT COL­LIDES WITH THE EARTH, IT SPARKS STUN­NING AU­RO­RAS”

is a type of black cal­cium phos­phate and is widely used for fer­tiliser and metal al­loy pro­duc­tion, and for fil­ter­ing heavy met­als out of wa­ter. This skin keeps the Euro­pean space probe safe so that it can op­er­ate con­tin­u­ously at a dis­tance of 60 so­lar radii. Al­though this is six times fur­ther away than Parker So­lar Probe’s clos­est ap­proach, there is a par­tic­u­lar rea­son for choos­ing this dis­tance. “It goes as close as you can go and still use tele­scopes to look at the Sun,” ex­plains Hor­bury. Parker So­lar Probe’s only tele­scope looks to the side to take im­ages of the so­lar wind rush­ing by.

So­lar Or­biter’s tele­scopes will study the Sun’s sur­face with a va­ri­ety of in­stru­ments over a wide range of dif­fer­ent wave­lengths so that as­tronomers can de­ter­mine the sur­face gas’s den­si­ties, tem­per­a­tures and the mag­netic field. It then con­tains a sec­ond suite of in­stru­ments that mea­sure the same prop­er­ties for the so­lar wind as it passes the space­craft. Parker So­lar Probe is de­signed to fly through the ex­act re­gion of the Sun’s at­mos­phere where it breaks its con­nec­tion to the so­lar sur­face and be­comes the so­lar wind. So by shar­ing their data the mis­sion sci­en­tists can make the con­nec­tion be­tween events on the so­lar sur­face, the launch­ing of the so­lar wind, and the down­stream con­di­tions. This is the stuff of dreams for the peo­ple in­volved in un­der­stand­ing space weather.

“So­lar Or­biter is all about mak­ing the con­nec­tion be­tween what hap­pens on the Sun and what hap­pens in the so­lar wind,” says Hor­bury.

EARLY WARN­ING

Beyond mere cu­rios­ity – which would be rea­son enough to launch these mis­sions – there is an im­por­tant prac­ti­cal ap­pli­ca­tion: safe­guard­ing the tech­nol­ogy we rely on ev­ery day.

As well as cre­at­ing the au­rora, the in­ter­ac­tion of the so­lar wind with Earth’s mag­netic field can be se­verely dam­ag­ing to im­por­tant tech­nol­ogy. The Car­ring­ton Event, which took place in 1859, is the great­est of these so-called so­lar storms on record. The au­rora was seen across two-thirds of the planet, the global tele­graph net­work went down and com­passes spun use­lessly. To­day, the same could hap­pen with sat-navs, telecom­mu­ni­ca­tions and power sta­tions – all the tech­nol­ogy that so­ci­ety re­lies on to func­tion. Yet we get only 30 to 60 min­utes warn­ing from a NASA space­craft called ACE (Ad­vanced Com­po­si­tion Ex­plorer).

Once these two mis­sions have per­formed their work, the hope is that this warn­ing time will rise to a day or two. That’s be­cause so­lar storms are sparked by flares on the Sun that trig­ger a sud­den ejec­tion of ma­te­rial from the corona into the so­lar wind. It takes a day or two for this erup­tion to cross space, so know­ing the way in which the so­lar wind is launched is crit­i­cal if we are go­ing to cal­cu­late the sever­ity of any in­com­ing so­lar storms. It could also give us more time to pre­pare and pro­tect any im­por­tant electrics.

“The data we are sup­ply­ing will be used to make trans­for­ma­tional im­prove­ments to the mod­els. A few years from now when we see a big event, the model is go­ing to ac­cu­rately tell us what is com­ing to the Earth,” says Fox. “It is ex­tremely for­tu­itous that we have the two mis­sions go­ing up in a sim­i­lar time frame. They are so syn­er­getic, that I couldn’t be more ex­cited that they will be up to­gether. It’s per­fect.”

“THE AU­RORA WAS SEEN ACROSS TWO- THIRDS OF THE PLANET, THE TELE­GRAPH NET­WORK WENT DOWN AND COM­PASSES SPUN USE­LESSLY”

ABOVE: The so­lar ar­ray of the Parker So­lar Probe un­der­go­ing ther­mal tests

The Sun at the mo­ment of an erup­tion

Dr Stu­art Clark is an astron­omy writer with a PhD in astro­physics. He is the au­thor of The Sun Kings

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