鈥淚MAGINE going to a star,鈥� says Jim Randolph. 鈥淭hat鈥檚 probably one of the most exciting missions that anyone will ever fly.鈥� And Randolph should know. He is an engineer at NASA鈥檚 Jet Propulsion Lab in Pasadena, California, and for decades, he has been dreaming of a tiny craft tough enough to skim above the seething surface of our very own star: the Sun.
This probe would fly through loops of incandescent plasma stretching millions of kilometres into space. It could watch waves of searing gas slosh across the solar surface and witness fiery tornadoes the size of Earth that may help create the solar wind. It could even snatch a sample of star. Of course, it would have to survive temperatures soaring to millions of degrees-yet this isn鈥檛 impossible. In fact, after decades of planning, Randolph is finally realising his dream.
This month, scientists at NASA are gathering to select the instruments and sensors that will equip just such a craft-named Solar Probe-for the mission ahead. The budget is approved, and barring accidents, Solar Probe will blast off in six years鈥� time on a long journey that will take it just a few million kilometres away from the Sun鈥檚 surface.
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This trip will open up one of the most mysterious regions of the Solar System. It should tell us how the Sun spews blobs of plasma Earthwards, and help predict the magnetic storms that can wreck communications satellites and bring down power grids. The measurements it makes will help astronomers understand not just the physics of our own star, but billions of other stars shining in alien solar systems throughout the galaxy.
Randolph first got involved with Solar Probe completely by chance. In 1975 he was working at JPL on NASA鈥檚 upcoming Voyager mission. At the same time, Giuseppe Colombo, an astronomer at the University of Padua in Italy, published a paper suggesting a way to use Jupiter鈥檚 gravity to slingshot a probe into a precise trajectory toward the Sun.
The paper caught the eye of William O鈥橬eil, manager of JPL鈥檚 advanced study group. His job was to pick outlandish ideas and see what JPL engineers would make of them. 鈥淗e came over and said, 鈥榯his is a far-out thing and it鈥檚 probably impossible to do, but why don鈥檛 you take a look at it?'鈥� Randolph recalls.
Randolph was immediately hooked, but knew that any craft designed to study a star up close would need fantastic protection against the extreme temperatures it would encounter. The obvious answer would be a shield that would cast a protective shadow, or umbra, over the science instruments on the craft behind. Even when the shield glows red-hot, the sensors would have to remain cool. 鈥淓verything has to stay in that umbra,鈥� says Solar Probe project scientist Bruce Tsurutani. 鈥淥therwise it gets vaporised.鈥�
Tough customer
By the late 1970s, Randolph had convinced NASA to sponsor durability tests on potential materials for the heat shield. His favourite was a tough black composite material called 鈥渃arbon-carbon鈥�. This is made by laying a cloth of carbon fibres into a mould, wetting it with epoxy resin and then burning away the epoxy in an oven. What鈥檚 left is an amorphous matrix of carbon particles binding the fibres together.
Carbon-carbon is extremely stiff, light and laughs in the face of roasting temperatures, so it forms the tips of nuclear warheads, which must hold their shape despite intense heating as they re-enter the Earth鈥檚 atmosphere. It also protects the nose and wings of the space shuttle as it comes back to Earth.
But protecting the Solar Probe is something else. A carbon-carbon shield would have to last for hours in the intense heat of the Sun. Randolph and his colleagues had to find the best kind of carbon-carbon for the job. But first they needed to recreate the awesome power of the Sun right here on Earth.
They got off to a shaky start. In the early 1980s, American and French engineers installed a piece of carbon-carbon inside a vacuum chamber at the French solar furnace that nestles in a valley in the Pyrenees at Odeillo. This furnace focuses sunlight into a vacuum chamber through a quartz window. 鈥淯nfortunately,鈥� says Randolph, 鈥渢here were some accidents at that facility during these tests.鈥� The quartz window on the furnace kept breaking, and the team from NASA left in disgrace. 鈥淭hey kind of told us to go home,鈥� Randolph recalls. Next stop was Lockheed Martin鈥檚 Vortek Facility in Littleton, Colorado, which uses artificial lights to simulate sunlight.
Meanwhile, Randolph began to mull over the shape the shield should take. Since the spacecraft was more or less cylindrical, wouldn鈥檛 a circular shield be the obvious choice? But the large radio antenna the craft would need to beam data back to Earth during the mission was going to be a problem. To keep the antenna in the shade, the team kept fiddling with the probe鈥檚 design. 鈥淲e were struggling with that silly antenna being down in the umbra,鈥� Randolph recalls.
Then, about eight years ago, Randolph had a flash of inspiration: why not use the heat shield as the radio antenna? Here he hit a snag-Randolph knew that Solar Probe had to point the shield toward the Sun to shade the sensors. But if the shield was turned Sunwards it couldn鈥檛 be beaming data back to Earth.
One day over lunch Randolph was explaining his dilemma to a colleague. On the back of a napkin he had sketched a cone to represent the shadow thrown back by the heat shield. Then an idea hit him. He drew a line that sliced through the cone at an angle and drew a front view of the slice he had just created. 鈥淲hat you get is an ellipse. That鈥檚 what we had on the back of this napkin,鈥� Randolph recalls.
He realised that if he gave the shield an elliptical outline, instead of a circular one, it could still cast a circular shadow if installed on top of the probe at an angle, providing the craft kept the Sun straight ahead as it approached. With more tinkering, Randolph converted the ellipse into a three-dimensional paraboloid, much like the communications dishes on many geosynchronous television satellites. In the end he figured out that a 2.4-metre-tall paraboloid could bounce data sideways toward Earth while still casting a perfectly circular shadow across the instruments on Solar Probe (see Diagram). 鈥淭he sun shield is one of the weirdest things you鈥檝e ever seen. Nobody鈥檚 ever done anything like this,鈥� Randolph says.FIG-mg22743901.jpg
Randolph鈥檚 team are maximising the shield鈥檚 ability to handle heat by giving it a hard, smooth finish. To apply the coating, they put the carbon-carbon in an oven and fill it with methane gas. 鈥淭he carbon from the methane deposits itself inside the carbon-carbon,鈥� says Randolph. 鈥淚t fills the pores.鈥� Despite more tests last month inside a solar furnace at the University of Alabama in Huntsville, it鈥檚 still a mystery why this finish keeps the shield cooler. 鈥淭o be honest with you, we don鈥檛 know the physics going on there at a molecular level,鈥� he says.
Blinded by gas
If they get the design wrong, it could jeopardise the whole mission. As it approaches the Sun, the heat will increase until the craft is weathering 3000 times more solar energy than we receive on Earth. If this intense heat vapourises even the top layer of the shield, the gas particles this creates could disrupt the craft鈥檚 sensors.
Luckily they鈥檝e got plenty of time to get it right. The shield won鈥檛 be built for two years or so and NASA isn鈥檛 planning to launch Solar Probe until February 2007. It will head out towards Jupiter and swing back round for its first pass at the Sun in October 2010, when sunspot activity is near a peak. Then, in 2015, it will return at a sunspot minimum. These two visits will give scientists back home a chance to study the way the Sun鈥檚 atmosphere or corona changes during the solar cycle.
Despite the long delay before launch, George Withbroe, science director for NASA鈥檚 Sun-Earth Connection programme, says NASA is committed to this project. The space agency has kept it ticking over for decades while the technology was developed, but this year the mission鈥檚 budget is $1.5 million, he says, and it will increase to $2 million in 2001, with big money promised later. 鈥淭he money is there now,鈥� says Ed Weiler, NASA鈥檚 associate administrator for space science. And this month, a team of engineers has begun to finalise Solar Probe鈥檚 scientific payload.
Researchers at NASA began thinking seriously about the mission鈥檚 science instruments back in 1996. Then early last year, in response to a NASA announcement, scientists began sending NASA proposals for specific instruments they would like to include in the mission, such as magnetometers, spectrographs and particle detectors. The agency is now reviewing the proposals at its headquarters in Washington DC. The details are confidential.
But the probe will certainly need a plasma collector to grab some 鈥渟tar dust鈥�-charged particles from the Sun鈥檚 corona. When the periscope-like collector extends slightly from the body of the craft, plasma particles will pour in through a minute pinhole and hit a wire mesh which has a voltage applied across it. This behaves like a mirror for charged particles: when they enter the mesh, they are deflected sideways into the craft鈥檚 body where a spectrometer will measure their energy and determine their composition.
Solar Probe will also take pictures of the Sun鈥檚 surface with small cameras. To prevent them from burning out, the cameras will be behind the shield-Randolph plans to punch two small holes in it so that a tiny amount of light will leak through. He will attach narrow tubing to these holes and run them through two secondary heat shields that will be the last line of defence before the Sun鈥檚 energy reaches photosensitive charge-coupled devices much like the detectors in video cameras. Although the images they record will be small, they should show the roiling surface in unprecedented detail.
These and other sensors should be able to explain much of the Sun鈥檚 most puzzling behaviour (New 杏吧原创, 1 May 1999, p 44). How, for example, does the Sun鈥檚 magnetic field behave near the poles? What does the fine structure of the star鈥檚 corona tell us about what鈥檚 going on inside the Sun? And where does the solar wind come from?
This wind is actually a cloud of electrons, ions and other particles streaming away from the Sun at high speed. It has two components, a fast wind that blows steadily at speeds of up to 750 kilometres per second and a more complex, slower wind, whose speed varies, and whose elemental content changes unpredictably. Some astrophysicists believe the fast wind is formed from huge vortices of gas that spin on the surface of the Sun. The slow wind may originate from long plumes of plasma that twirl outwards from the Sun鈥檚 poles. To clear up the mystery, Solar Probe will sample these plumes, and look for clues to how they are formed. It will even fly through the top of some plumes to record the conditions directly. 鈥淵ou can measure the solar wind near Earth, but you鈥檙e seeing it days after it has left the Sun. Lots of things are lost. So you really want to go to the source,鈥� says Withbroe.
Astrophysicists hope Solar Probe will answer another question: why is the surface of the Sun thousands of times cooler than the star鈥檚 outer corona? The data will also help them interpret measurements of the Sun made by other craft such as SOHO and TRACE that watch the Sun from a great distance. 鈥淵ou can do all the remote sensing you want,鈥� says Weiler, 鈥渂ut until you do in situ measurements, you don鈥檛 really understand what鈥檚 happening.鈥�
The ultimate goal would be to learn how to predict the Sun鈥檚 massive belches of plasma particles, called Coronal Mass Ejections. These huge blobs of plasma shoot outwards and churn up the Earth鈥檚 placid magnetic field into dangerous magnetic storms that can silence communications satellites and knock out the power supply to millions of people, as happened in Quebec in 1989. 鈥淲hen we understand the solar wind better, where it comes from and why, then we should be able to predict its variations and when magnetic storms will occur,鈥� says Tsurutani.
For the mission to succeed, the craft must weather the storms it will face as it passes close to the Sun. Its trajectory must be perfect, says Randolph, with the Sun directly in front of the shield and Earth at right angles to its course. With just a slight wobble or tilt, delicate instruments will roast and communication with NASA鈥檚 Deep Space Network will be impossible.
The unusual shape of the heat shield won鈥檛 help keep the probe on course. Since it must be installed at an angle, photons of sunlight will strike it and act like wind on a sail, pushing the craft sideways. 鈥淥ne of the issues that we have always been concerned with is that the photon pressure would be so high that the spacecraft would want to tip over,鈥� Randolph says.
Engineers thought about adding a long tail on the opposite side of the craft to balance the force of the photons on the shield. But the tail would have been ridiculously long, Randolph says. Instead, they plan to fire thrusters to keep the craft from tipping as it passes the Sun, though they haven鈥檛 sorted out the exact details. 鈥淲e鈥檙e working on that issue,鈥� says Randolph. 鈥淚t鈥檚 just so different from anything ever done before.鈥�
So could Solar Probe turn out to be a modern day Icarus? Randolph doesn鈥檛 think so. 鈥淲e鈥檝e been working for 20 years to prevent the Icarus problem,鈥� he says. With the major challenges solved, the time is finally ripe to realise his dream. And according to Weiler, even Congress and the public are warming to solar physics. Other missions to observe the Sun such as SOHO and TRACE have brought solar physics into people鈥檚 living rooms, he says. 鈥淭hey showed them that the Sun is not just a white ball of gas. The Sun drives everything: the weather, the heat, the cold. Understanding what affects your daily life ought to be important.鈥�