[Air & Space Magazine] Eugene Parker’s Journey to the Sun

[Orionid]

Eugene Parker’s Journey to the Sun (1)
By Rebecca Boyle Air & Space Magazine June 2018

He coined the term “solar wind.” Now a spacecraft will brave its source.


The Earth-orbiting Solar Dynamics Observatory takes spectacular portraits of the sun, like this ultraviolet view of active regions captured in 2015. (Solar Dynamics Observatory/ NASA)

In July, humanity will dispatch its first emissary to our star. A Delta IV Heavy rocket with an added upper stage will boost NASA’s Parker Solar Probe away from Earth and, whipped by Venus’ gravity, it will soon become the fastest spacecraft ever flown. At its top speed, the spacecraft will scream through space at 430,000 mph, fast enough to travel from New York to Los Angeles in 25 seconds.

During seven years of carefully choreographed swoops, it will draw closer to the sun, like a matador dancing inward toward a glowering bull. It will ultimately settle into an elliptical orbit that, at perihelion, comes within 3.9 million miles of the sun’s visible surface, more than seven times closer than any spacecraft has dared venture. It will not touch the surface. That would be too dangerous—and impossible, because the sun is a roiling furnace of plasma, a state of matter unable to form what could be considered a surface. Instead, the probe will fly directly into the only region of the solar system so far unexplored by spacecraft: the corona. What happens here, in the sun’s atmosphere stretching out millions of miles, affects this planet and every other place in our neighborhood, but its dynamics remain mysterious.

We know that the sun’s atmosphere is much hotter than its outer plasma layer, a fact that seems to defy thermodynamics. Solar wind from the corona drifts outward, crossing 93 million miles of space to create the aurora in Earth’s magnetosphere. From Earth, we can’t see the corona, unless we watch during a total solar eclipse. The Parker Solar Probe will sample this layer of the sun directly, and for the first time we will be able to retrieve information about the realm that connects Earth and our star.

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Comet tails were the first evidence that the sun was more than a static ball in the sky. In 1607, an apparition appeared at night: a comet that would eventually be named after Edmond Halley, the astronomer who predicted its 75-year orbit. German astronomer Johannes Kepler was among its many observers, and he wrote to Galileo Galilei wondering whether it was sunlight that caused the comet’s tail to smear across the sky. Perhaps one day, he speculated with grand vision, voyagers could use this solar power as propulsion on trips across the stars: “Provide ships or sails adapted to the heavenly breezes, and there will be some who will brave even that void.”

Not until the 20th century did astronomers propose that along with light, the sun emits a steady stream of particles, which pushes a comet’s tail around. In the early 1950s, Eugene Parker, an astrophysicist at the University of Chicago, wanted to study why the sun’s atmosphere is so hot. He read some of the papers on solar particles and started connecting the dots. Parker, now 90, says that during his research he discovered that “the corona is mostly static near the sun. There is some slight motion, but it is not flowing at any noticeable rate.” But when you study this radiation much farther out, “it is busy blowing the comet tails away,” he says. “That says you are dealing with a gas—a hydrodynamic flow of gas.” In other words, the sun was emitting not just electromagnetic radiation but low-density gales of particles.


The probe named for Eugene Parker (in 1977) is about to get an up close portrait of the sun. (University of Chicago Photographic Archive)

If this flow from the sun was gas, Parker could use familiar physics equations to further describe what is happening in the corona. His computations revealed that at the extremely high temperatures of the corona’s outermost layers, the gas has to be flowing away from the sun extremely quickly. In fact, by the time they reached Earth, the gales would still be supersonic. Parker coined a term for the outflow: the solar wind.

“It was something most people couldn’t seem to swallow. They expressed stern disbelief,” he recalls. “I told them, you know the corona is static at the sun, and you know from the comet tails that it is moving very fast farther from the sun. You put in the temperatures that are observed [in the corona], a million degrees, and it cannot help but be a solar wind. That’s just how the dynamics turn out.”

In 1958, Parker published a paper with what his calculations revealed: that the phenomenon is made up of a complex system of plasma flow, magnetic fields, and high-energy particles. He argued that it affects all the planets and space throughout the solar system, and correctly predicted the twisted shape—now called the Parker spiral—that the rotating sun’s magnetic field would take as the solar wind carried it to the outer solar system. His theory was largely ignored until 1962, when Mariner 2 became the first probe to travel beyond Earth’s magnetic field. The spacecraft observed the supersonic solar wind (and the fact that our magnetosphere largely shields us from it), and Parker was vindicated.

The Parker Solar Probe, officially named last summer, is the first spacecraft NASA has dedicated to a person still living. It’s a tribute to the significance of Parker’s contribution to science, but also an indication of just how young the field of solar research is, and how far it still has to go.

“I have been able, in my career, to watch heliophysics go from a curiosity to an applied science,” says O. C. St. Cyr, a solar scientist at NASA’s Goddard Space Flight Center in Maryland. The Parker Solar Probe will fill in many gaps in the knowledge about the sun, helping scientists understand why the star behaves the way it does. Though we generally understand what creates the sun’s magnetic dynamo—hot, charged gases flowing in the sun’s interior create electrical charges, which generate the powerful field—nobody seems to know why it flip-flops roughly every 11 years between a state of relative tranquility and one of fury. No one knows why the sun spews coronal mass ejections, gargantuan eruptions of energetic particles, and no one can reliably predict them. This means no one can predict when solar storms from CMEs will wallop Earth, with the potential to fry telecommunications equipment on the ground and in space. Still no one knows precisely how, or why, the solar wind is generated. No one knows how it produces violent, short-lived, 6,000-mile-long jets of material, called spicules. And no one knows why the corona gets so hot.

“We don’t have the power to model the full sun in all its complexity,” says Nicholeen Viall, an astrophysicist at Goddard. It’s the interconnectedness of various solar activities that makes it so difficult to discern individual characteristics. “One spicule could launch a wave, and that triggers a magnetic reconnection event, and that heats the plasma. But this is all speculative. These are just fundamental plasma physics questions that we have to go to the sun to really answer.”

The coronal heating problem, as it’s called, remains one of the most contentious issues in solar science, because it seems to flout the rules of basic physics. The sun’s photosphere—its visible surface of plasma—is about 10,000 degrees Fahrenheit, but the wispy corona reaches millions of degrees. It’s as if you were sitting close to a campfire, and the air on your face was a hundred times hotter than the flames themselves. The scientific literature is rife with competing ideas for how the corona gets superheated: plasma waves rising up from the sun’s deep interior; magnetic braids that twist and tense, eventually snapping like rubber bands; “heat bombs” or “nanoflares,” which Parker proposed 30 years after his solar wind paper.

“Storms in the solar wind form somewhere in the vicinity of where Solar Probe is going,” says Parker. “One can make theoretical models of these storms, but you have to make assumptions as to where the energy is introduced, and right now we don’t know the answer to that.” The spacecraft will dip its instruments into the solar wind, the way a mariner might submerge her fingers in the water to feel the current. It will measure the direction and strength of plasma waves. It will measure the speed and density of a vast range of particles, from the lowest-energy solar emanations to the most energetic protons associated with solar flares, and will watch the solar wind rev up to supersonic. Teasing out these speed differences could reveal the processes that form the solar wind.

The probe will also measure magnetic fields and how they change in the presence of a shock, like that of a CME. Finding out how these clouds of charged plasma originate and stream outward is among the mission’s most important goals. Though scientists study them from Earth and from other solar-observing spacecraft, nothing compares to going there, says Nicola Fox, the mission’s project scientist and a heliophysicist at the Johns Hopkins University Applied Physics Lab in Laurel, Maryland. “With the Parker Solar Probe, we’re not just sitting back and taking pictures,” she says. “We’re not just sitting back in the comfortable region just outside the Earth. We’re plunging into the sun’s corona 24 times in the lifetime of the mission.”

A journey to the sun is of course a dangerous one. The probe will experience 475 times the solar radiation that Earth does. It won’t get close enough to experience the corona’s highest temperatures, but it will still get torched at more than 2,500 degrees Fahrenheit. The probe will be protected by specially designed heat shields, cooling pumps, and radiators, making it the toughest spacecraft ever flown. Juno, which has been orbiting Jupiter since July 2016, is enclosed in a radiation-shielding carapace, but the Parker Solar Probe’s 4.5-inch-thick carbon-composite sunshield is unprecedented in space exploration. The shield’s surface will get blasted with 2.8 million watts of solar energy, and only about 20 watts of that will get through to the instruments, nearly all of which stay tucked behind the shield, sampling the environment as it streams behind them.


The spacecraft needs a powerful cooling system. This radiator is one of two that will enable the probe to withstand thousands of degrees. (NASA/Johns Hopkins APL/Ed Whitman)

Most interplanetary probes are solar-powered; solar panels are relatively inexpensive to build and launch, and the sun’s energy is free and persistent. But the Parker Solar Probe will have too much of a good thing. Like Juno, the probe’s elliptical orbit is one protection measure, affording it a bit of a break from the intensity as it lopes around. On most spacecraft, the solar panels permanently reach out like wings to capture as much energy as possible, but the solar probe’s were designed with an unusual feature: articulated arms that can tuck the panels behind the heat shield. An onboard computer continuously forecasts energy needs and determines what percentage of the solar arrays to expose and how much to turtle away. To prevent the edges from overheating, water flows through vein-like chambers in the array’s interior, which is made from titanium but resembles corrugated cardboard. The water flows into four cone-shaped radiators, which dissipate heat into space. It’s a perpetual system, however, so when it’s not grazing the sun, the probe also carries heaters to thwart the chill of space.

At a distance of 89 million miles, communications between the probe and Earth will take several minutes, so many of the probe’s tasks are performed autonomously. The spacecraft is programmed with a litany of commands it can access as its situation changes—the most important of which is making absolutely sure the heat shield is covering what it needs to cover. “The huge level of independence the spacecraft has is a big challenge,” Fox says, “because you have to test every single one of those commands: ‘If this, we must do this.’ ”

The probe uses star trackers and an inertial measurement unit to sense its position—the latter can navigate for awhile by itself if the star trackers are blinded by, for instance, a solar outburst. Seven solar sensors mounted around the spacecraft can also give off alerts. Says Mary Kae Lockwood, spacecraft system engineer at APL, “If the spacecraft starts off-pointing a little bit from the sun, one of the solar limb sensors would become illuminated and say, ‘I’m seeing sun over here, so put me back behind the thermal protection system.’ ”

The science instruments are also packaged in a cooling system, operating at around room temperature, around 78 degrees Fahrenheit. But sticking out like whiskers from behind the heat shield are four 1/8-inch-diameter antennas and a solar probe cup that will be directly illuminated by the sun. By scrutinizing the corona so close to its source, the probe will be able to provide better data for models used to forecast space weather, and may be able to pinpoint the causes of CMEs.

[Orionid]

Eugene Parker’s Journey to the Sun (2)


We get a glimpse of the sun’s corona only during a total solar eclipse, but soon we’ll get to know it. The Parker Solar Probe will swim through this complex current of solar wind and storms to study how they form. (Steve Albers, Boulder, Co; Dennis Dicicco, Sky and Telescope; Gary Emerson, E.E. Barnard Observatory)

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Understanding space weather is a crucial step for safeguarding Earth’s economy, and for future missions to other planets. Material from coronal mass ejections usually takes several hours or days to traverse the distance between the sun and Earth. Once they reach Earth, CMEs can interfere with satellites, ground communications, and power networks. They can cause sweeping blackouts, and they can shower airplanes and spacecraft with dangerous radiation. Severe events can be catastrophic for spacecraft, and more importantly, any humans in orbit. In 1972, between Apollo missions 16 and 17, the sun unleashed a furious storm of high-energy protons, packing enough energy to burrow through four inches of water. A spacesuit would have conferred little protection. Had astronauts been on the moon at the time, they might have been exposed to deadly radiation levels, exceeding 400 times the dose of a typical CT scan. Without medical treatment, about half of people exposed to that level of radiation die within a month or two.

The metal used to build human-rated spacecraft, like the Apollo capsules and the International Space Station in orbit today, blocks much of that radiation, so astronauts can stay safe as long as they’re positioned properly in their metal shelter. Still, early warning could be a lifesaver for any missions in low Earth orbit, on the moon, on Mars, or at distant asteroids. Yet beyond their basic relation to the solar activity cycle, no one really understands why solar storms happen. Sunspots are carefully monitored, and sun-observing spacecraft can provide notice when a CME is bubbling up, but no one can yet predict them.

“There are processes that we don’t fully understand that lead to a collection or accumulation of magnetic energy in the solar atmosphere,” says Antti Pulkkinen, an astrophysicist at Goddard. “Once that accumulation of magnetic energy reaches some critical threshold, then—boom. We have significantly improved our understanding in just a couple decades, using the missions up there now and heliophysics models. But the devil is in the details…. If you want to ultimately predict these things, you need to get the details right.”

Beyond giving scientists the ability to predict hazardous sun activity, the Parker probe mission is critical for understanding the physics of our star. Just as a dearth of missions to anywhere but Mars has left little raw data for planetary scientists, a lack of missions to the sun has been challenging for heliophysicists. The ability—or inability—to gather new data affects the whole pipeline of scientific research: Without incoming observations, major research programs become more difficult to sustain, and existing programs have a harder time recruiting postdoctoral researchers and graduate students. Another upcoming mission will help the Parker probe sustain the field of heliophysics: the Solar Orbiter, a joint NASA-European Space Agency mission. After its launch next year, it will fly close to the sun to study its interior and provide close-up views of its polar regions. Goddard’s St. Cyr—his participation in both the Solar Orbiter and Parker probe missions is indicative of how small a field heliophysics is currently—says both of these spacecraft will provide much-needed new perspectives. Some scientists, he says, are still trying to squeeze new information out of data from the twin Helios spacecraft, a pair of probes launched by the United States and Germany in the 1970s.

But when the Parker Solar Probe arrives at its destination, it’s going to change the relatively quiet field of heliophysics. “[The mission] has the potential to blow the doors off solar science,” St. Cyr says. “We don’t have any data like it.”


The only living person to have a spacecraft named for him, Eugene Parker (front) visits the probe during construction at the Johns Hopkins Applied Physics Laboratory. Soon his namesake will greatly advance the field he helped pioneer. (Jean Lachat/University of Chicago)

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When Parker, the scientist, first predicted the existence of the solar wind, the Space Age was still neonatal. The first human-built satellite, Sputnik 1, had awed the world just a few months earlier. It would be another five years before the first spacecraft to observe the sun launched, the first of eight small satellites called the Orbiting Solar Observatory. It would be another 12 years before the sun was the primary target of a space mission, the twin Helios probes. In the decades that followed, missions like the Solar Dynamics Observatory, the twin Solar Terrestrial Relations Observatory, the Hinode spacecraft, and others opened a window into the sun’s behavior—but they have all been from a distance.

Until the Parker Solar Probe reaches 3.9 million miles from the sun, the Helios spacecraft will remain the record holders for closest approach. In 1975 and 1976, they flew within 29 and 27 million miles of the sun. The MESSENGER spacecraft, which visited Mercury, took some solar measurements too, from 28.8 million miles. Just beyond the edge of the solar system, in the 13-billion-mile barrens, Voyager 1 can fill in data on the tail end of the solar wind’s influence. From Earth, 93 million miles away, astronomers can measure the sun’s spectra—a way to take measure of its ingredients. They did this in droves in August 2017, when a total solar eclipse crossed the continental United States for the first time in a century. These observations help estimate the density of the corona and the speed at which the particles are traveling. But the biggest hole in our knowledge is what is happening at the star itself. “That is why these measurements are so groundbreaking and revolutionary,” Fox says.

For all that heliophysicists hope to learn about coronal science, the most promising data may be the kind no one can predict. There will be information no one knows how to use; entire careers will be built on interpreting data from the Parker Solar Probe and incorporating it into space weather prediction models. “There’s physics occurring in this region that we haven’t measured yet,” Fox says. “People have different theories, but until we actually go up and test them, we can’t say which one is correct. Or maybe they are all wrong, and there is a brand-new one no one has thought of yet.”

Parker himself says he hopes this will be the case. “The formation of the storms in space, I think, is going to provide a lot of information about things where we know very little, and the effects on Earth are going to be very serious,” he says. “I think there will be plenty to see. But don’t ask me what, because I don’t know.” This spring, he is just happily waiting to see his namesake lift off.

The launch window for the Parker Solar Probe opens on July 31 and lasts 20 days. During this time, Earth and Venus are aligned, a positioning crucial for the spacecraft’s trajectory. By September, the probe will sail past the second planet. Using this gravity assist, it will gain speed and a refined orbital path. Over the next few years it will spiral six more times around Venus, then finally angle in toward the sun. The probe will point its protective face toward the seething orange disk, whiskers to the wind. Braving unfathomable heat and radiation, the spacecraft will reach out for material forged in the star’s deepest heart. Then it will touch it, and send a message home, connecting humanity even closer to the source of everything that has ever lived, or ever will, at least in this corner of the galaxy.

Source: Eugene Parker’s Journey to the Sun

[Orionid]

Space probe to plunge into fiery corona of the sun
By Robert Sanders, Media relations | AUGUST 6, 2018




UC Berkeley physicist Stuart Bale discusses the FIELDS instruments aboard the Parker Solar Probe. Designed and built at the Space Sciences Laboratory, the instruments will measure electric and magnetic fields in the outer atmosphere of the sun to understand the corona and solar wind. (Applied Physics Laboratory video, Johns Hopkins University)

On August 11, NASA plans to launch Earth’s first spacecraft to venture inside the orbits of Venus and Mercury to touch the very edge of the sun’s fiery corona.

Outfitted with instruments designed and built at the University of California, Berkeley, the Parker Solar Probe will achieve a goal that space scientists have dreamed about for decades: to get close enough to the sun to learn how the turbulent surface we see from Earth dumps its energy into the corona and heats it to nearly 2 million degrees Fahrenheit, spawning the solar wind that continually bombards our planet.

“This is a piece of heliophysics science we all really wanted for a long time, since the 1950s,” said Stuart Bale, a UC Berkeley professor of physics, former director of the campus’s Space Sciences Laboratory and one of four principal investigators for the instruments aboard the mission. “For me personally, I’ve been working on the probe since it was approved in 2010, but I really spent a large part of my career getting ready for it.”

The solar probe will travel faster than any spacecraft in history, at its peak reaching 430,000 miles per hour, and will be only four-and-a-half solar diameters, or 3.8 million miles, above the solar surface at its closet approach to the sun around 2024. The probe is equipped with a heat shield to protect its sensors from the sun’s heat, which could reach 2,500 degrees Fahrenheit, nearly hot enough to melt steel.


Stuart Bale (left) and Keith Goetz of the University of Minnesota in the clean room last month conducting final checks to make their instrument ready for flight aboard the Parker Solar Probe.

At this distance, the solar probe will be within a region where electrons and ionized atoms – mostly hydrogen ions, or protons, and helium ions, called alpha particles – are accelerated and shot out toward the planets at high speed.

When these ions, called the solar wind, hit Earth, they interact with Earth’s magnetic fields and generate the northern and southern lights as well as storms in the outermost atmosphere that interfere with radio communications and satellite operations. Accelerated to higher speeds, so-called “solar energetic” particles can pose a hazard to astronauts.

Scientists still do not know how the solar wind ions are accelerated, or why the ions and electrons in the corona are so much hotter, about 1.7 million degrees Fahrenheit, than the surface of the sun, which is a relatively cool 10,000 degrees Fahrenheit. The Parker Solar Probe could answer those questions, and help scientists on Earth forecast the large eruptions from the sun that pose the greatest peril to our spacecraft and communications systems.

Follow the magnetic fields

FIELDS, a suite of instruments built at UC Berkeley’s Space Sciences Laboratory, is one of four instrument packages aboard the probe. With the help of a six-foot boom projecting in the direction the spacecraft is moving, it will measure the electric and magnetic fields in the corona, which will tell scientists the total energy streaming outward from the sun.




In defiance of all logic, the sun’s atmosphere gets much, much hotter farther from its blazing surface. Learn how astronomers first discovered evidence for this mystery during an eclipse in the 1800s, and what scientists today think could explain it. (NASA Godddard video)

These measurements will test one theory of how the sun heats the corona: by jiggling the magnetic field lines. The strong magnetic field of the sun stretches out far into space, but the magnetic field lines are anchored in surface regions that constantly move around because of convection below, like boiling water. The constant movement of the base of the magnetic field lines creates waves that travel outward along the lines, just as jiggling the end of a long rope sends waves to the other end. Somehow, these so-called Alfvén waves accelerate particles to high speeds and fling them into space.

“If the wave-driven model is correct, then I think our measurements will be the fundamental measurements on the mission,” Bale said.

The other popular theory is that tiny flares called nanoflares all over the surface of the sun produce magnetic fields that cross, reconnect and fling disconnected loops of magnetic field into space, accelerating ions along with it. This was first proposed in 1987 by Eugene Parker, after whom the solar probe is named. Now 91, Parker predicted the existence of and named the solar wind in the 1950s.

Radio antennas on the FIELDS package will look for radio waves created by nanoflares, which have yet to be detected, while another package of instruments, SWEAP (Solar Wind Electrons Alphas and Protons), will record the speed of solar wind electrons, protons and alpha particles as they whiz by the probe. Correlating nanoflare or microflare activity with the flux of particles streaming from the sun could confirm the magnetic reconnection theory. SWEAP is led by the University of Michigan and the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, though much of the instrument was designed and built at the Space Sciences Laboratory at UC Berkeley.


The surface of the sun, or photosphere, is about 10,000 degrees Fahrenheit, but the region through which the solar probe flies, the corona, is 2 million degrees. Scientists want to know why. (NASA image)

Two other instrument packages will be aboard the probe. WISPR, the Wide-Field Imager for Parker Solar Probe, was built at the Naval Research Laboratory and will capture visible-light images of the sun’s corona directly in front of the orbiting probe. ISʘIS (pronounced E-sis) – short for Integrated Science Investigation of the Sun, and including ʘ, the symbol for the Sun, in its acronym – is led by Princeton University and will measure the energy and identity of energized electrons and ions, including ions heavier that hydrogen and helium, in order to find out how they are sometimes accelerated to nearly light speed close to the sun.

Together, these instruments should be able to record the speed-up of the solar wind from subsonic to supersonic and the birth of the highest-energy solar particles.

“Plasma physics is really hard to study in the laboratory,” said Bale, who focuses on the role of magnetic fields and ionized plasma in space, in particular around stars like the sun. “Sticking a spacecraft right in the hot plasma makes an ideal laboratory.”

Looping around Venus

This probe is the chance of a lifetime for Bale. Though his team will deploy booms and test instrument functions one day after launch, most of the instruments will then be turned off and won’t begin taking real measurements of the corona until the probe reaches its first close approach to the sun in November.


NASA’s Parker Solar Probe shown mated to its third stage rocket motor in July. The third stage rocket allows the spacecraft to gain the speed needed to reach the sun, which takes 55 times more energy than reaching Mars. (Photo by NASA/Johns Hopkins APL/Ed Whitman)

After a loop around Venus to slow down, the probe will get the closest any spacecraft has ever been to the sun, a distance from the center of the sun equal to 36 times the sun’s radius (36 solar radii). Venus orbits at 155 solar radii and Mercury at 83 solar radii.

Over the next six years, the probe will loop around Venus six more times, gradually working its way to approximately 9.8 solar radii from the center of the sun. There, it will be well within the corona, at the outer edge of which particles exceed the speed of sound – the Alfvén speed, which is about 200 miles per second – and no longer call the sun home.

“The goal of the mission is to get inside that transition region, so we get into the real corona where the flow is subAlfvénic,” Bale said. “We think that boundary is at about 15 solar radii, so we probably won’t start hitting it until 2021.”

Once inside the corona, the probe may see the jiggling magnetic field lines, or Alfvén waves, bouncing back and forth between the sun’s surface and the edge of the corona, a turbulent cascade that may be the feedback loop that accelerates particles to the high speeds seen in the solar wind.

“In early December, I am counting on having that first pass of data at 35 solar radii, and I am sure it will be revolutionary. There will be great new stuff in there, from what we know about previous missions,” Bale said.

Over its seven-year mission lifetime, the probe will dip into the sun’s inner atmosphere 24 times. As part of NASA’s outreach efforts, more than 1.1 million people submitted their names to be recorded on a memory card that will accompany the spacecraft around the sun.

The probe is scheduled for launch in the early hours of Monday, Aug. 11, from Cape Canaveral Air Force Station in Florida, aboard a United Launch Alliance Delta IV Heavy rocket with an upper stage to boost it out of Earth orbit toward Venus.

Since the mission was approved in 2010, some 40 to 50 people at Berkeley’s Space Sciences Laboratory have worked on the solar probe. Through the formal end of the mission in 2026, and including subsequent data analysis, UC Berkeley will have received about $100 million out of a total of $1.5 billion spend on the mission.

Source: Space probe to plunge into fiery corona of the sun

[Orionid]

NASA launches Parker Solar Probe mission to study the sun up close
by Jeff Foust — August 12, 2018


A Delta 4 Heavy carrying NASA's Parker Solar Probe lifts off from Cape Canaveral, Florida, Aug. 12. Credit: NASA/Bill Ingalls

GENOA, Nev. — A NASA mission to travel closer to the sun than any previous spacecraft is on its way after a successful launch from Cape Canaveral Aug. 12.

The United Launch Alliance Delta 4 Heavy carrying NASA’s Parker Solar Probe mission lifted off from Space Launch Complex 37 at Cape Canaveral Air Force Station in Florida at 3:31 a.m. Eastern. Controllers scrubbed a launch attempt the previous day because of technical issues late in the countdown. The spacecraft separated from its kick stage 43 minutes after launch.

The 700-kilogram spacecraft required not only a Delta 4 Heavy but also a Star 48BV kick stage from Northrop Grumman in order to counteract the Earth’s rotational speed around the sun, allowing it to fall closer to the sun. The spacecraft will also perform a series of Venus flybys, starting in early October, to bring it closer to the sun.

Parker Solar Probe will perform its first close approach to the sun in November, coming within about 24.8 million kilometers. Future flybys will bring it even closer, eventually coming as close as 6.1 million kilometers, far closer than any previous spacecraft. During those later close approaches, the spacecraft will be traveling at up to 695,000 kilometers per hour.

Those close approaches are needed for Parker’s instruments to study the solar wind and the corona. By taking measurements from within the corona, scientists hope to better understand how it is heated to temperatures of millions of degrees. The up-close observations may reveal new information about solar eruptions and the solar wind.

“We know the magnetic field is the real key. We know this is why we’re making this daring mission,” said Nicola Fox of the Johns Hopkins University Applied Physics Laboratory, project science for the mission, during a pre-launch briefing Aug. 9. “We’re going to go into the transition from where the magnetic field is dominant to where that coronal material dominates the magnetic field.”

Development of Parker Solar Probe — previously known as Solar Probe Plus — dates back a decade, but the idea of sending a spacecraft close to the sun is far older. Concepts for missions to travel to the sun date back to the late 1950s, around the time that University of Chicago physicist Eugene Parker, for whom the mission is named, first proposed the solar wind.

“For the science community, and some of the engineering community, it’s really been 60 years,” said Andy Driesman, project manager for the mission, during the pre-launch briefing. “You can trace back papers and read engineering reports from the ’60s and ’70s about this mission, about different concepts, different ways to get this environment.”

What made the mission feasible was a set of technologies, including a heat shield that protects the spacecraft during its close approach to the sun as well as an active cooling system for the solar panels. The spacecraft also needed to be small enough that it could be launched on the desired trajectory.

“Certainly, finding the right materials” was key, said Fox. “It isn’t just a case of surviving the incredible heat when we’re close to the sun. We come out around Venus and it’s cold there, which means these materials have to withstand heating and cooling, very extreme changes in temperatures.”

Parker Solar Probe has a primary mission of 24 orbits around the sun through the middle of 2025. Driesman, though, said he was optimistic that the spacecraft could operate far longer. “As long as we have propellant on board, we’re going to continue to take science data,” he said.

The mission will ultimately end when the spacecraft runs out of attitude control propellant. “It will lose attitude control and those sensitive bits of the spacecraft, which we worked so hard to protect, will eventually transition to the sun,” he said. “The way I like to think about it is that, hopefully in a long, long period of time, 10 to 20 years, there’s going to be a carbon disk floating around the sun in its orbit. That carbon disk will be around until the end of the solar system.”

Source: NASA launches Parker Solar Probe mission to study the sun up close

[Orionid]

NASA dotknie Słońca. Wysłała specjalną sondę Parker Solar Probe
Przemek Berg 13 sierpnia 2018



Celem nie jest zwykła, oddalona obserwacja naszej gwiazdy, lecz dotarcie niemal do jej powierzchni. Wydaje się to niemożliwe, a jednak: w stronę Słońca wyruszyła niezwykła sonda – amerykańska Parker Solar Probe.

Dlaczego Słońce jest tak bardzo ciekawe? Po pierwsze, umożliwia istnienie życia na Ziemi, ponieważ jest typowym żółtym karłem, a więc gwiazdą niezbyt dużą, ale też nie bardzo małą, która spośród całej ogromnej menażerii gwiazd jest najlepszym kandydatem na stworzenie i utrzymanie życia na otaczających ją planetach. To niebagatelny powód. Nie ma lepszego kandydata.

Po drugie, Słońce jest dość aktywną gwiazdą – ale nie za bardzo aktywną, na szczęście – jednak bardzo wpływa na tzw. pogodę kosmiczną niemal na wszystkich planetach. Na Ziemi też, i to bardzo. Ta aktywność już nie raz zaburzyła pracę naszych, ziemskich, systemów telekomunikacji, a nawet sieci energetycznych.

Wreszcie: Słońce wciąż skrywa przed nami wiele tajemnic. Jest tworem skomplikowanym, choć na szczęście dla nas, Ziemian, w miarę bezpiecznym. Większość gwiazd nie jest taka. Musimy się o nim jeszcze wiele dowiedzieć.

W skali ziemskiej Słońce jest niewyobrażalnie wielkie

To gigantyczna kula gazu (plazmy), skupionego siłami grawitacji, której średnica równa się 109 średnicom Ziemi, a powierzchnia odpowiada prawie 12 tys. powierzchni Ziemi. Masa Słońca to masa 334 tys. mas Ziemi. Słońce w całości składa się z plazmy (w tym w 74 proc. wodoru, 25 helu i 1 proc. innych pierwiastków), jednak słoneczna plazma ma w różnych obszarach inną gęstość i temperaturę. Najgęstsze i najcieplejsze jest jądro Słońca, które zajmuje obszar równy jednej czwartej promienia całej gwiazdy. Temperatura gęstego (150 razy gęstszego od wody) jądra to ponad 13,5 mln stopni.

To tam zachodzi główna reakcja termojądrowa, w wyniku której wodór Słońca zamieniany jest w cięższy hel. Z tej przemiany bierze się energia Słońca, a więc m.in. światło i ciepło, którymi cieszymy się na Ziemi. Nad jądrem znajduje się otoczka Słońca, czyli główna, składająca się z wielu warstw o różnej temperaturze, część gwiazdy. Nad nią zaczyna się atmosfera, też wielowarstwowa i zróżnicowana: więc najpierw fotosfera, potem chromosfera i wreszcie korona słoneczna. I właśnie tej korony mamy dosięgnąć.

To, co czyni Słońce aktywnym, to jego pola magnetyczne. Słońce nie ma jednego, jednorodnego pola magnetycznego. Są ich setki i pokrywają, jak wielkie plamy, całą gwiazdę. Gdy dwa pola magnetyczne o przeciwnych kierunkach zetkną się, powoduje to nagłą zmianę energii tych pól i podgrzanie plazmy. W rezultacie, gdy pola magnetyczne anihilują, dochodzi do gigantycznej erupcji plazmy na zewnątrz. Nazywa się to rozbłyskiem słonecznym. Rozbłyski, powstające przede wszystkim w chromosferze i koronie słonecznej, są zwykle krótkotrwałe (od kilkunastu minut do półtorej godziny), ale w ich trakcie emitowane są największe ilości energii Słońca w postaci fal elektromagnetycznych oraz strumieni cząstek. W chromosferze dochodzi głównie do rozbłysków, których efektem są protuberancje oraz arkady pętli magnetycznych.

Protuberancje to wąskie łuki gęstej i stosunkowo zimnej plazmy (od kilku do kilkudziesięciu tysięcy stopni), uformowane polem magnetycznym, które wystrzeliwują ponad tarczę słoneczną. Są długie i wysokie na dziesiątki tysięcy kilometrów. Wokół protuberancji plazma ma ponad milion stopni, jednak pole magnetyczne skutecznie ją od tej wysokiej temperatury izoluje. Istnieją protuberancje łagodne i eruptywne. Te drugie są następstwem rozbłysków słonecznych, do których dochodzi zawsze w wyniku anihilacji magnetycznych pól.

Obserwuje się jeszcze inny typ aktywności, niekiedy zaliczany do protuberancji – są to tzw. arkady pętli magnetycznych, które też powstają w wyniku rozbłysków. Potrafią trwać godzinami lub nawet kilka dni. Pętle układają się jedna obok drugiej, tworząc swoisty tunel plazmowo-magnetyczny. Gdy jedne pętle zanikają, pojawiają się nowe. Dlatego Słońce jest ciekawe.

Okno startowe dla niezwykłej sondy NASA – Parker Solar Probe

Okno otworzyło się 31 lipca 2018 r. i pozostanie otwarte przez 20 dni. Sonda Parker Solar Probe została wyniesiona w przestrzeń kosmiczną na pokładzie rakiety Delta IV Heavy 12 sierpnia 2018 r. Jej wcześniejsza nazwa – „Solar Probe Plus” – została zmieniona, by upamiętnić Eugene′a Parkera, astrofizyka i badacza Słońca o niezwykłych zasługach. O wadze jego prac świadczy to, że Parker Solar Probe jest pierwszą na świecie misją kosmiczną nazwaną na cześć żyjącego naukowca. W trakcie planowanej na siedem lat misji sonda okrąży Słońce aż 24 razy i w tym czasie siedem razy przeleci też bardzo blisko Wenus, wtedy przyspieszy. Ostatnie trzy orbity będą najbliższe i wtedy sonda znajdzie się w odległości zaledwie niecałych sześciu milionów kilometrów od powierzchni Słońca. Prawie dotknie naszej gwiazdy.

Dla przypomnienia: Ziemia znajduje się w średniej odległości 150 mln kilometrów od Słońca. Większość swoich badań Parker Solar Probe przeprowadzi, pozostając już w granicach korony słonecznej, czyli bardzo rozgrzanej zewnętrznej warstwy atmosfery, w której rodzi się wiatr słoneczny. Znajdzie się siedem razy bliżej Słońca niż dotychczasowe sondy kiedykolwiek badające naszą gwiazdę, w obszarze, gdzie oddziaływanie słoneczne jest 500 razy silniejsze niż na orbicie Ziemi.

Temperatura otoczenia, z jaką Parker Solar Probe będzie musiała się zmierzyć, to ponad 1,5 tys. stopni Celsjusza, ale w takich warunkach jej instrumenty badawcze, by prawidłowo pracować, muszą pozostawać w temperaturze pokojowej. Zapewni to specjalna, 11-centymetrowa obudowa termiczna, wykonana z węglowych materiałów kompozytowych bardzo odpornych na wysokie temperatury. Sonda ma dwa rodzaje paneli słonecznych do pozyskiwania energii, pracujące w dalszej odległości od Słońca i w tej najbliższej. Te drugie to wysokotemperaturowe ogniwa specjalnie chłodzone.

Najwyższa i najgrubsza warstwa atmosfery słonecznej to właśnie korona. Temperatura w koronie wzrasta do 1,5–2 mln stopni i to w niej powstaje wiatr słoneczny. Tę wysoką temperaturę i wiatr wywołują tzw. fale Alvféna. Ich istnienie przewidział w 1942 r. słynny fizyk Hannes Alvfén, jednak obserwacyjnie zostały zidentyfikowane dopiero w 2007 r. Są to fale magnetyczne, które poruszają strugami plazmy w koronie, podobnie jak ruchy wody w oceanie poruszają wodorostami.

Ten ruch niezwykle rozgrzewa koronę słoneczną, przez co jej cząstki osiągają ogromne prędkości i mogą uciec ze Słońca pod postacią wiatru słonecznego. Sinusoidalne fale Alvféna poruszają się z prędkością ponad 40 km na sekundę i pojawiają się w interwałach 150–550-sekundowych. Przekazują też znacznie więcej energii, niż wcześniej sądzono. W koronie dochodzi też do najbardziej potężnych rozbłysków i w rezultacie do tzw. koronalnych wyrzutów masy, które kształtują tzw. pogodę kosmiczną, mającą znaczny wpływ na Ziemię. 

Parker Solar Probe ma cztery instrumenty badawcze: do analizy słonecznego pola magnetycznego, stanu słonecznej plazmy, wysoko energetycznych cząstek wiatru słonecznego oraz obrazowania przepływów tego wiatru. Jej misja potrwa do 2025 r. Wtedy właśnie Parker Solar Probe najbardziej zbliży się do Słońca.

Specjaliści z NASA uważają, że misja ta dokona przełomu w badaniach Słońca, szczególnie w badaniach mechanizmu powstawania wiatru słonecznego i przedostawania się wysokoenergetycznych cząstek w głąb naszego Układu. To z kolei powie nam wiele nie tylko o naszym Słońcu, ale też w ogóle o gwiazdach typu słonecznego, czyli o żółtych karłach, których w kosmosie jest całkiem sporo; w naszej Galaktyce prawie 10 proc.

Misja będzie kosztować Stany Zjednoczone 1,5 miliarda dolarów.

[Tekst jest aktualizowaną wersją artykułu z maja 2017 r.]

Źródło: NASA dotknie Słońca. Wysłała specjalną sondę Parker Solar Probe

[Orionid]

MISJA KU SŁOŃCU
WOJCIECH BRZEZIŃSKI 12.08.2018

Ikar zaleciałby zdecydowanie dalej, gdyby zrezygnował z wosku na rzecz izolacyjnej pianki węglowej i dwóch warstw solidnych, kompozytowych płyt. Właśnie takie zabezpieczenia pozwolą sondzie Parker Solar Probe nie tylko zbliżyć się do Słońca, ale dosłownie go dotknąć.


NASA

Wydawałoby się, że nie ma nic prostszego, niż wysłać sondę w stronę Słońca. Nasza gwiazda jest przecież tak masywna, że jej grawitacja utrzymuje na wodzy nawet obiekty tak odległe, jak Pluton. Na pierwszy rzut oka wystarczy wystrzelić sondę w z grubsza właściwym kierunku i poczekać, aż sama spadnie w stronę gwiazdy.

W rzeczywistości podróż na Słońce wymaga 55-krotnie większej energii, niż lot na Marsa. Nasza planeta okrąża Słońce z prędkością około 110 tysięcy kilometrów na godzinę. Każdy obiekt wystrzelony z jej powierzchni zaczyna swoją podróż lecąc z dokładnie tą samą prędkością względem Słońca. To oznacza, że gdybyśmy po prostu wycelowali rakietę w Słońce, nasza sonda, pędząca z ogromną prędkością w bok, ominęłaby gwiazdę szerokim łukiem. Najpierw trzeba więc wykasować większość tej prędkości.  Najprościej – wystrzeliwując sondę „do tyłu”, czyli w kierunku przeciwnym do ruchu orbitalnego Ziemi. Dopiero wtedy pojazd może zacząć „spadać” w stronę Słońca.

Jak przetrwać przy Słońcu

Parker najpierw skieruje się więc w stronę Wenus. We wrześniu minie tę planetę, wykorzystując jej grawitację do wytracenia jeszcze odrobiny prędkości, i skieruje się w stronę Słońca. 1 listopada minie gwiazdę w odległości ok. 21 milionów kilometrów. Już to będzie absolutnym rekordem: poprzedni rekordzista, sonda Helios-B z lat 70-tych była od słońca dwa razy dalej. Ale to tylko początek. Z każdą kolejną orbitą, sonda będzie zbliżać się do Słońca coraz bardziej, i lecieć coraz się szybciej. Rozpędzi się do prędkości 200 km/s i stanie się najszybszym obiektem kiedykolwiek stworzonym przez ludzkość. Ostatecznie, po około 7 latach lotu, Parker znajdzie się zaledwie 6,2 mln. kilometrów od powierzchni gwiazdy, w obszarze, w którym korona zmienia się w wiatr słoneczny. Znajdzie się wewnątrz atmosfery gwiazdy, w miejscu, w którym temperatura sięga 1400 stopni Celsjusza. Sonda będzie pracować w warunkach porównywalnych z tymi z wnętrza hutniczego wielkiego pieca.

W jaki sposób przetrwa? Pojazd ma system chłodzenia, ale jego główną obroną jest dwumetrowa, biała tarcza, która ma osłonić go przed słoneczną furią. Tarcza, zbudowana z węglowych kompozytów i grubej warstwy izolacyjnej, węglowej pianki, będzie stale zwrócona w stronę gwiazdy tak, by w jej cieniu chroniła się delikatna elektronika pojazdu. Gdyby sonda odwróciła się w inną stronę, cała elektronika usmażyłaby się w ułamki sekund. Aby nie dopuścić do takiej katastrofy, Parker będzie może najbardziej samodzielną sondą w historii – to pojazd, a nie kontrolerzy na Ziemi, będzie pilnował tego, by osłona zawsze była zwrócona we właściwym kierunku. Tarcza jest tak wytrzymała, że, po zakończeniu misji pojazdu, pozostanie na słonecznej orbicie. Nawet przez miliony lat.

Trudniejsze niż Księżyc

Sonda Parker jest pierwszym w historii pojazdem NASA nazwanym na cześć żyjącej osoby: prof. Eugene Parkera, który jeszcze w latach 50-tych był jednym z pierwszych badaczy wiatru słonecznego: strumienia plazmy i cząstek elementarnych wyrzucanych przez naszą gwiazdę z ogromnymi prędkościami. To właśnie wiatr słoneczny będzie jednym z dwóch głównych obiektów zainteresowania sondy. Drugim będzie korona słoneczna.

Naukowcy nie wiedzą dotąd, dlaczego korona Słońca jest aż tak gorąca. Powierzchnia naszej gwiazdy ma około 6 tysięcy stopni Celsjusza. Tymczasem słoneczna korona nawet 3 miliony stopni. Co ją tak podgrzewa? Może fale dźwiękowe potężnych fal przetaczających się po powierzchni Słońca? Może pola magnetyczne zmieniają koronę w rodzaj kosmicznej kuchenki mikrofalowej? Mamy hipotezy, ale dopóki nie przyjrzymy się koronie z bliska, trudno będzie o ich potwierdzenie.

Drugą zagadką jest wiatr słoneczny. Nie rozumiemy do końca, jak powstaje. A chcielibyśmy zrozumieć go jak najlepiej. Słoneczne burze mogą uszkadzać satelity, zakłócać łączność czy niszczyć sieci energetyczne.

Obie zagadki mają grubo ponad 60 lat. Misja do wnętrza korony słonecznej była jednym z najważniejszych zadań, wyliczonych w dokumentach na podstawie których, w 1958 roku, tworzono NASA. Lądowanie na Księżycu okazało się technicznie zdecydowanie prostsze.

Źródło: MISJA KU SŁOŃCU