NASA’s Parker Probe will venture closer than ever to the sun to explore its mysterious atmosphere | Science
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NASA’s Parker Probe will venture closer than ever to the sun to explore its mysterious atmosphere | Science
Ignoring the lessons of mythology, Betsy Congdon has spent the first decade of her young engineering career on a singular quest: to build something that will fly dangerously close to the sun.
On a drizzly day in May at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland, Congdon crouches next to a foil-wrapped test copy of her team’s product: a carbon-foam heat shield, a little wider and a lot thinner than a king size mattress. Another copy sits nearby, a flight-ready spare sealed in a metal drum stamped with an unintentionally ironic warning: “Do not expose to direct sunlight.”
The real one has headed south to Kennedy Space Center in Florida, where on or soon after 11 August, it will blast off, fastened to the business end of NASA’s Parker Solar Probe. Six weeks later, the probe will reach Venus. That planet’s gravity will tip the probe toward the heart of the solar system. Six weeks after that, Parker will plunge through the sun’s corona, a tenuous atmosphere of hot charged particles, or plasma, on the first of two dozen flybys between now and 2024.
During those flybys, the heat shield must keep the probe’s fragile electronics safe while temperatures on its surface soar up to a steel-melting 1370°C. The heat doesn’t come from the million-degree plasma in the corona itself, which is too thin to transfer much energy, but from the sheer glare of the sun. Yet Congdon isn’t nervous. “We’ve put it through all its paces,” she says, her voice echoing in the cavernous spacecraft assembly room. “We’ve put multiples of them through all their paces.”
If all goes well, the spacecraft—safe in the shadow of the shield—will beam back a record of the corona’s plasma and the tangled net of magnetic fields that shape it. Those data could solve fundamental mysteries. For example, what heats the plasma to more than 200 times the temperature of the sun’s surface? And how does the solar wind, a stream of plasma particles, escape into space? The solar wind has been a puzzle since solar physicist Eugene Parker, the probe’s namesake, described it in 1958. Understanding it better could help today’s researchers improve their forecasts of solar storms, the gusts of solar wind that crash into Earth’s magnetic field and, at their strongest, knock out satellites and electrical grids.
The $1.5 billion Parker isn’t the only big upcoming project aimed at the sun. On the Hawaiian island of Maui, astronomers are putting finishing touches on the Daniel K. Inouye Solar Telescope (DKIST), a $350 million project funded by the U.S. National Science Foundation. With a 4-meter mirror, DKIST is more than twice the size of the largest existing solar telescopes. It should be able to zoom in on the sun’s surface with unrivaled sharpness when operations start in June 2020. That same year, the Solar Orbiter is due to launch, with €780 million in core support from the European Space Agency. The spacecraft will observe high-energy radiation rippling through the corona from slightly farther away than Parker.
“I really think these are transformative missions,” says Howard Singer, chief scientist at the Space Weather Prediction Center in Boulder, Colorado, part of the National Oceanic and Atmospheric Administration (NOAA). Singer and his colleagues deliver forecasts of solar activity not only for satellite and grid operators, but also for astronauts and airlines that fly near the poles, where high-energy, tissue-penetrating particles more readily slip through Earth’s magnetic field.
If current schedules hold, DKIST and the Solar Orbiter will observe the corona well before Parker makes its closest solar flybys in 2024. That timing should allow heliophysicists to mix and match remote and in situ data—collected at the same moment, no less—enabling them to measure changes in the corona while watching the sun’s roiling surface for clues to the processes that stir and heat it. Earlier this year at APL, representatives of the three projects met for the first time to discuss how they could tackle the corona together. “It is absolutely a unique time for solar physics,” says Valentin Martínez Pillet, director of the National Solar Observatory in Boulder, the organization building DKIST. “There is combined science that we can do that is going to be awesome.”
Parker’s journey to the sun fulfills an ambition as old as the U.S. space program itself. In 1958, still reeling from the success of the Soviet Union’s Sputnik satellite, a National Academy of Sciences (NAS) committee chaired by early space physicists John Simpson and James Van Allen brainstormed a wish list of missions that, scientifically, could put the United States in the lead in space. One concept was a probe that would venture inside Mercury’s orbit to taste solar plasma.
For decades, the idea did not budge from the wish list. “We’ve tried it half a dozen times,” says Chris St. Cyr, project scientist for NASA’s contributions to the Solar Orbiter at Goddard Space Flight Center in Greenbelt, Maryland. “It never got the political will of the science community at the same time the funding was available.”
By the early 2000s, NASA and NAS were both pushing a solar probe as a top priority. Parker, the eventual result, will come within 0.04 astronomical units (AU) of the sun. (One AU is the average distance between the sun and Earth.) That’s 10 times closer than Mercury’s path and seven times closer than the current record holders, the Helios probes of the mid-1970s, built by West Germany and NASA. The twin probes spun once per second to evenly distribute the sun’s heat.
Even 0.04 AU represents a compromise for Parker. NASA’s previous solar probe concept, devised in 2005, would have gone at least twice as close for one or two flybys.
But it was expensive. In 2007, NASA asked APL managers to cut costs. In response, they changed the mission design, backing off from the sun and increasing the number of flybys to compensate. They also replaced a costly radioisotope generator with panels to draw solar power—all too abundant in the corona. To prevent overheating, Parker hides the panels in the shade under the heat shield as it draws closest to the sun in its elliptical orbit. The probe stretches the panels open to catch the sun’s rays when the spacecraft is farther away, while a pumping system cools them with a water bath.
Then there’s that all-important shield. In her office upstairs from the clean room where Parker was built, Congdon keeps a suitcase-size square of the black material used for testing. It’s built like a sandwich, with a thick filling of carbon foam, an airy mesh of carbon molecules, sitting between thin sheets of carbon-carbon, a material woven from carbon fibers that gets stronger, not weaker, when heated to a few thousand degrees. Thick pads of carbon-carbon adorned the nose and wings of NASA’s space shuttles.
Congdon picks up the sample and holds it out. It’s surprisingly light—the full-size shield weighs only as much as a person. At a touch, the coarse foam exposed at the edges of the sample rubs off like the lead of a soft pencil. The outside of the real shield has a white coating designed to reflect as much heat as possible, but on this unpainted sample, parts of the surface are darkened, overtoasted.
Engineers have taken pains to ensure the shield never strays from its position between Parker and the sun, including when radio contact with Earth is cut off as the probe disappears behind the sun or when the sun’s own radio emission drowns out the spacecraft’s. If sensors discover that the heat shield has rotated out of position, an automated system engages to right the craft. “We need to recover within a few minutes before something gets severely damaged,” says Jim Kinnison, Parker’s mission system engineer at APL.
Ironically, the heat shield is flammable on Earth in the presence of oxygen. One high-temperature test took a “terrifying” turn when the test chamber’s vacuum seal broke and oxygen leaked in, Congdon says. “The thing went up in flames.” But in the rarefied plasma of the solar corona, oxygen is scarce and the few atoms there have had their outer electrons torn away by bafflingly high temperatures. Parker’s science team hopes to figure out why.
The sun’s visible surface, the photosphere, simmers at about 5500°C. Grade school physics holds that because the corona is farther still from the heat source at the sun’s core, temperatures should fall. Instead, they soar to more than 1 million degrees Celsius.
Heliophysicists have battled for decades over the origin of this extra heat. On the broad strokes, at least, they agree. The energy probably starts as motion in the photosphere or just below, where astronomers see granules—seething, ever-shifting cells the size of Texas. Those are bubbles of convecting plasma, and they boil like a cauldron, carrying tremendous amounts of kinetic energy. Scientists also agree that magnetic fields transport the energy outward.
Unlike everyday materials, charged plasma responds to magnetism, flowing along field lines. The moving particles themselves create electric currents that generate additional magnetic fields. Sometimes the fields reach up through the surface of the sun and into the corona, which could establish a path for the granules’ kinetic energy to be transformed into thermal energy.
“Beyond that, if we brought in five theorists, we might get 15 theories,” St. Cyr says. But the proposed pathways of coronal heating do fall into two general branches.
In one, sudden changes in the protruding tangle of magnetic field lines pump heat into the corona. With both feet planted in the photosphere, many of those lines resemble the Gateway Arch of St. Louis, Missouri. But as the surface churns, the feet move around, tangling the lines overhead. Stress builds up. When the field lines suddenly snap into a more stable arrangement, vast amounts of energy are released into the surrounding plasma.
Missions such as NASA’s orbiting Solar Dynamics Observatory have monitored almost second-by-second changes on the sun since 2010. They have observed those abrupt changes, called magnetic reconnection, and shown that they can kick out solar flares. The events take place often enough to account for some, but not all, of the corona’s heat. Theorists have long suspected that much smaller “nanoflares” could also pop off close to the surface, too small and faint to be detected. A million such flares per second, each about as powerful as a 50-megaton hydrogen bomb, could fully account for the corona’s measured temperature.
It is absolutely a unique time for solar physics. There is combined science that we can do that is going to be awesome.
If the corona’s heat does come from swarms of undiscovered staccato explosions, freshly heated pockets of the corona should reach temperatures as high as 10 million degrees Celsius before the energy can spread around. And in recent years, satellites and suborbital rockets, observing above Earth’s atmosphere in x-rays and the ultraviolet (UV), have spotted emissions from coronal plasma at those temperatures, adding indirect support to the theory. “It’s there. That’s sort of incontrovertible,” says Goddard astrophysicist Jim Klimchuk.
Other theorists envision a different path for heat rising from the depths of the sun. The motion of the bubbling plasma cells excites waves of magnetic energy that course outward. In theory, those waves can jangle field lines in the corona like ropes in a CrossFit gym—especially lines with one foot on the sun and the other dangling into space. That wiggling heats nearby particles, which steal away thermal and kinetic energy “like a surfer on the crest of a wave,” says Kelly Korreck, a solar physicist at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts.
The trio of upcoming missions should help apportion the corona’s heat budget between reconnection and waves, and perhaps hint at specific subprocesses such as nanoflares, although Korreck sounds a note of caution: “There is no one telescope that’s definitely going to find the answer.” Parker will traverse a path where wave heating is expected to dominate. If Parker senses waves, it can check how much energy they contribute. And by measuring just-cooked plasma close to the sun—say, the gust of hot helium atoms unleashed by a nanoflare—Parker should also be able to sniff out traces of reconnection heating events.
DKIST and the Solar Orbiter, for their part, will add to the picture by studying the area beneath Parker’s path. Both observatories—DKIST, using infrared light; and the Solar Orbiter, using UV and x-rays—will map the fleeting, tangled field structures that might be sparking nanoflares.
The Parker probe will also explore the mystery that Eugene Parker, now a 91-year-old physicist emeritus from the University of Chicago in Illinois, left for his scientific heirs: What drives the gale of charged particles expanding hundreds of kilometers per second out into the solar system? Low in the corona, the solar magnetic field has a stiff hold on plasma. Somewhere above that, the particles move fast enough to shake free of the sun’s gravity and escape into the solar system. That is “where the magic happens, where the solar wind is accelerated so much that it then takes off,” says Nicola Fox, the probe’s project scientist at APL. “We’ll be in that region.”
The wind, like the corona, seems to defy basic physics: It should cool and slow down as it begins to spread into the solar system. But it doesn’t. Something keeps driving it outward—perhaps the energy emitted by particles following spiral paths or the dissipation of turbulent gusts of plasma. By recording the small-scale physics of the plasma it flies through, Parker will pinpoint where the wind takes flight and narrow the possible mechanisms that could launch it. “We all know the devil is in the details,” Fox says.
Last October, a sprightly retiree donned a hairnet, blue booties, and a lab coat to visit APL’s clean room, flanked by mission scientists. Eugene Parker had come to see his namesake, a probe devoted to studying the very wind he had described 6 decades earlier—partly from observations of comet tails pointing away from the sun like wind socks.
The idea was once controversial—two reviewers outright rejected Parker’s paper. Now, the solar wind sits at the cornerstone of an emerging applied science. Understanding the corona’s behavior on good days may prove key to predicting bad ones. Whatever physics accelerates the solar wind also launches dangerous solar storms.
Adverse space weather falls into several classes. Workaday solar wind would pose a health risk only to astronauts traveling outside Earth’s protective magnetic field, to deep-space locations such as the moon or Mars. Solar flares hurl stronger bursts of particles and radiation toward Earth that can cause problems for satellites and, funneled by the planet’s magnetic field toward the poles, create auroral light shows. The rarest and strongest events, called coronal mass ejections (CMEs), launch dense blobs of particles that can overwhelm Earth’s field and cripple communications technology. In 1967, for example, the U.S. Air Force started to prepare for nuclear war after multiple early warning radar systems appeared to be jammed. The culprit, found in time to forestall disaster, was a massive CME.
“When will they occur? How long are they going to last? How intense are they going to be?” Singer asks. “There are huge gaps in understanding how to predict some of these phenomena.”
CMEs come with little warning. The NASA and NOAA satellites that track the solar wind hover near a stable Earth-sun gravitational point that lies just 1% of the way to the sun. At solar wind speeds, a space weather event picked up there can reach Earth 15 minutes later. So learning to discern warning signs of disruptive events right at the sun from data from Parker, DKIST, and the Solar Orbiter, will lead to better predictions, Singer says.
DKIST will take a microscope to the same magnetic structures that spew flares. The Solar Orbiter will measure magnetic fields on the far side of the sun and test whether monitoring intense fields before they rotate into view could improve future predictions. And Parker should improve space weather models by measuring conditions in the corona as small flares erupt. Team members are hoping the probe may be lucky enough to dart through a CME.
But all that is work still ahead. Congdon’s own quest is almost over. The heat shield sits fastened tightly atop Parker, ready for space. She has booked her own ticket to Florida for the start of the August launch window, not to work on it, but to appreciate it as a tourist in a special viewing area for APL visitors. So has Eugene Parker, traveling with close family, who will be feted like a VIP.
“The joy on the scientists’ faces—that’s what we’re looking for,” Congdon says.