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The Biggest Explosions in the Solar System

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The Biggest Explosions in the Solar System

NASA's HESSI spacecraft aims to unravel an explosive mystery: the origin of solar flares.

Marshall Space Flight Center

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see captionJune 12, 2001 -- Astronauts love space walks. Floating weightless hundreds of kilometers above Earth, the terrain below racing by at 17,000 mph --no space traveler wants it to end. But it only takes two words to send one of those brave explorers racing back to their craft: "Solar flare!"

"Solar flares are the biggest explosions in the solar system," says Robert Lin of UC Berkeley's Space Science Lab. "They erupt near sunspots with the force of a hundred million hydrogen bombs." Astronauts caught spacewalking during a solar flare or one of their cousins, a coronal mass ejection, can absorb a radiation dose equivalent to 100 chest x-rays -- reason enough to dash for shelter.

Above: Astronaut Steven Smith floats above Earth during shuttle mission STS-103. [more]

Flares pose little direct danger to Earth dwellers because our planet's atmosphere protects us from their deadly radiation. But unpredictable solar explosions do affect our lives. They can disable satellites, scramble aircraft navigation, and interrupt high-frequency radio communications for hours.

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"One of the most amazing things about solar flares," says Brian Dennis of NASA's Goddard Space Flight Center, "is the efficient way they accelerate subatomic particles to energies exceeding 109 eV." As much as 50% of the total explosion energy emerges as electrons and atomic nuclei traveling at nearly the speed of light. "Flares operate much more efficiently than any particle accelerator we've been able to build here on Earth."

"How do flares do that?" he asks. We don't know, but terrestrial particle physicists would love to find out.

What ignites solar flares? How do they unleash so much energy so quickly? And is it possible to predict when they will happen?

Such questions have vexed astronomers since 1859 when Lord Carrington spotted a solar flare for the first time. "I was [counting sunspots on a projected image of the Sun]," he recalled, when suddenly "two patches of intensely bright and white light broke out" near a remarkably large sunspot group. "Flurried by the surprise," Carrington rushed from his telescope to call a second witness, but by the time he returned minutes later the outburst had vanished.

see captionLeft: A 4.2 MB mpeg movie shows a solar flare in action, blasting hot gas away from the limb of the Sun. [more]

Carrington knew he had glimpsed something enormously powerful, but what he saw was just the tip of the iceberg. Fast-moving particles that emerge from flares radiate mostly high-energy x-rays and gamma-rays. Lower-energy visible light isn't as important.

And therein lies the reason that flares have been able to guard their secrets for so long. The explosions are brightest at wavelengths that Earthbound observers can't see with their eyes. Telescopes are hobbled, too, because our atmosphere is opaque to x-rays and gamma-rays.

Now a new NASA satellite aims to change all that. The High Energy Solar Spectroscopic Imager (HESSI for short) will orbit Earth nearly 600 km above our planet's obscuring atmosphere, where it can record x-ray and gamma-ray emissions from flares. HESSI, which is slated for launch no earlier than June 20th, isn't the first spacecraft capable of detecting such radiation. But it will be the first to capture crisp hard x-ray and gamma-ray images of the violent explosions.

"The angular resolution of HESSI's hard x-ray images will be about 2 arcseconds or about as good as you could get from an optical telescope on the ground." says Lin, the mission's principal investigator. The gamma-ray images will be a little less detailed, with resolutions between 7 and 36 arcseconds. But, Lin notes enthusiastically, "we've never seen any gamma-ray image of a solar flare before." HESSI's will be the first.

Right: An artist's rendering of HESSI in orbit 600 kilometers above Earth. [more information]

see captionTo put these numbers into perspective, consider the following: When a solar flare erupts, it heats a region of the Sun's atmosphere many Earth-diameters across. (What Carrington saw in 1859 was the white light "bloom" from such a flare.) HESSI's hard x-ray images will reveal details only 1700 km wide -- about the distance between Los Angeles and Seattle.

That's amazing because such high-energy x-rays and gamma-rays can't be focused; they fly right through conventional lenses. Instead, HESSI forms images by looking at the Sun through finely-spaced parallel slats --like microscopic Venetian blinds-- that cast shadows across onboard radiation detectors. "We'll rotate the spacecraft every 4 seconds to create a modulation pattern from the shadows that we can analyze to form an image of the Sun," explains Lin. The process is similar to a medical x-ray, except scientists are interested in the source of the rays (the Sun), not the material that blocks them (the slats).

HESSI's cameras can make pictures of the entire Sun, but researchers will be especially interested in sunspots. "That's where flares erupt -- in the vicinity of sunspots with intense, twisted magnetic fields," says George Fisher, a colleague of Lin's at Berkeley. "Twisted magnetic fields are like rubber bands stretched taut," he explained. "They want to snap back -- violently. Reconnecting fields are probably the power source for flares."

At least that's what most solar physicists believe. The problem is, no one has ever seen it happen. "Before HESSI we couldn't locate the onset of an eruption with sufficient precision to make the connection between flares and kinks in the magnetic field," says Fisher. "I'm dying to know where flare particles are accelerated, and I think HESSI is finally going to show us."

HESSI's job is important fundamental physics, adds NASA's mission scientist Brian Dennis. Understanding how flares work could teach us how to build better particle accelerators on Earth and maybe even advance the cause of fusion power, which also involves superheated gases threaded by magnetic fields.

Left: Inside the CERN high-energy particle accelerator in Geneva. Basic research on solar flares might one day improve such devices on Earth. © CERN Geneva

HESSI's findings will also shed light on mysterious happenings far outside our solar system. "Whatever triggers solar flares could be the same mechanism that blasts jets of particles from the magnetized accretion disks of black holes and neutron stars," says Dennis. "The Sun is comparatively nearby, so it's a natural laboratory for studying such exotic processes."

Meanwhile most astronauts would be satisfied with simple timely predictions of garden-variety solar flares, a potential spinoff of the HESSI mission. If the spacecraft can accomplish that one thing, space will become a safer place ... for everyone.

Editor's note: Solar flares are closely related to coronal mass ejections (CMEs) -- billion-ton clouds of gas that billow away from the Sun and trigger geomagnetic storms when they strike Earth's magnetosphere. Scientists once thought flares propelled CMEs into space, but we've since learned that flares and CMEs can happen together or separately. Perhaps the two are different aspects of the same kind of explosion triggered by changing magnetic fields on the Sun. No one is sure. "One of HESSI's goals is to understand the relationship between solar flares and CMEs," says Dennis. [Listen to Bob Lin discuss CMEs].

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The Biggest Explosions in the Solar System
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Explosive Vocabulary:
 eV: a tiny parcel of energy called an "electron Volt." 1 eV is the energy gained by an electron when it passes through a one volt battery. An efficient 60 watt light bulb emits about 1020 eV/sec.
 arcsecond: a unit for measuring very small angles. There are 3600 arcseconds in a degree. The Sun is approximately 1800 arcseconds wide.
Web Links

HESSI Fact Sheet: The mission at a glance, from NASA's Goddard Space Flight Center.

The High Energy Solar Spectroscopic Imager: education and public outreach site from the University of California Berkeley.

The High Energy Solar Spectroscopic Imager: mission home page from NASA's Goddard Space Flight Center

es HESSI make x-ray images? It's done with shadows!

Space Spinoff: The manufacture of HESSI's image-forming grids is possible thanks to newly developed microfabrication techniques. Potential applications include x-ray imaging for high-volume baggage inspection, characterization of heavy metal deposits, and radioactive waste assessment. It's another down-to-Earth spinoff of fundamental space research.

Listen: Berkeley astronomer Bob Lin explains how solar activity affects life on Earth.

Solar Flare Alphabet Soup -- Learn how astronomers classify flares as X-, M- and C-class eruptions. From

The First Recorded Solar Flare -- Excerpt from: Description of a Singular Appearance seen in the Sun on September 1, 1859. by Richard C. Carrington, Monthly Notices of the Royal Astronomical Society, vol. 20, 13-15, 1860.

Atmospheric Transmission -- the Good News and the Bad News - Find out why you need to be above Earth's atmosphere to "see" gamma-rays and x-rays.

Why Study Solar Flares? NASA/Goddard's Solar Flare Theory web site offers some answers.

Coronal Mass Ejections -- learn more about these cousins of solar flares from NASA/Goddard's Solar Flare Theory web site.

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