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Io or Bust

Galileo braves extreme radiation as it plunges toward a close encounter with Io's volcanoes

September 16, 1999: Later today at 10:26 am PDT, NASA's Galileo spacecraft begins a daring new phase of its mission when it passes less than 670 miles above Jupiter's moon Callisto. The "daring" part isn't the close flyby of Callisto -- Galileo's done that plenty of times. What makes this flyby special is that Callisto's gravity will alter Galileo's orbit and send it hurtling toward two close encounters with Io, the innermost of Jupiter's big moons. Io is literally sizzling with fiery volcanoes that send sulfurous eruptions arcing high into the moon's tenuous atmosphere. Galileo will race over Io's surface twice, once in October and again in November, taking close-up pictures of the landscape and collecting valuable scientific data across the electromagnetic spectrum. In November Galileo might even pass through the plume of Pillan Patera, making it the first spacecraft ever to fly through an alien volcano.

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Getting to Io won't be easy. As the spacecraft nears Jupiter it will be exposed to high-energy particles trapped in the giant planet's radiation belts. Although mission planners expect the spacecraft to survive, the threat of damage is considerable.

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Galileo's "trial by fire" began two days ago when the spacecraft made its deepest approach into the Io torus since 1995. The torus is a gigantic donut of charged particles that are ejected from Io by volcanic activity. With a diameter the size of Io's orbit it spans 844 thousand kilometers and has an important impact on Jupiter's magnetic environment. As Io moves around Jupiter and through this magnetized plasma torus, a huge electrical current flows between Io and Jupiter. Carrying about 2 trillion watts of power, it's the biggest DC electrical circuit in the solar system.

"We're not doing much observing at Callisto on this encounter," noted JPL's Duane Bindschadler, the manager of Galileo's Science Planning and Operations Team, "just some radio science experiments to probe Callisto's atmosphere and its internal mass distribution. The primary science goals for this orbit are related to the Io torus and Jupiter's magnetosphere. All of the Fields and Particles instruments will be turned on and collecting data on plasma, dust, and electromagnetic fields. These are in situ instruments, which means that they sense the environment right around the spacecraft."

One of these instruments - the plasma wave detector - will be listening for eerie-sounding "chorus" emissions. Chorus signals are low frequency plasma waves that produce strange whistling and popping sounds when converted to audio frequencies (see below). Not only are they entertaining to listen to, but they also convey important scientific information. By analyzing these signatures of plasma waves, scientists can understand how energy flows between Jupiter's magnetic field and the material in the Io torus.

Below: This colorful spectrogram shows Chorus Emission from the satellite Ganymede recorded by the Plasma Wave Instrument on Galileo as it sailed past the moon in 1996. Chorus signals are plasma waves that can't be heard directly with the human ear, but they can be converted to audio frequencies. To hear chorus emission from Earth's magnetosphere recorded during a severe magnetic storm in Vancouver Island, BC, Canada on 21 Feb. 1994, click here for a RealAudio version or a WAV version.

"We haven't been this far into the torus since 1995," says Bindschadler. "In '95 the radiation levels we saw were relatively low, but apparently that's not the case now. Galileo flew through the outer edges of the torus last month [August 99] and ran into a pretty intense radiation environment. Something's happened in the inner magnetosphere of Jupiter since 1995. We're not sure what."

"One possibility is solar activity," he continued. "In 1995 the sunspot cycle was near minimum, but now we're almost on top of the solar maximum. A particle event on the sun could have injected particles into the Jovian magnetosphere and caused high radiation levels. It's also possible that Io's to blame. In July, astronomers at the Wyoming Infrared Observatory were monitoring Io and they saw a big IR outburst. That means there was a volcanic eruption that might have substantially increased the local plasma density in the torus."

If energetic particles were injected into Jupiter's magnetic field last month, will radiation levels still be high now as Galileo heads into the inner magnetosphere?

"That's a really interesting question," says Claudia Alexander, a JPL plasma physicist. "We don't know how long it takes for energetic particles in Jupiter's magnetosphere to diffuse away. On Earth, high-energy particles from the solar wind enter our magnetosphere on the Sun-facing side, and then electromagnetic forces make them flow around to the magnetotail. From there, some particles head for the poles and trigger aurora, but most are simply trapped inside the magnetosphere. They can't remain trapped forever -- otherwise the magnetic field would fill up with plasma. During geomagnetic storms these trapped particles are ejected into space in the form of giant blobs of plasma called plasmoids."

"It took 25 years of study to figure that out for Earth. We're just starting on that 25 year journey of understanding for Jupiter."

Left: A ground-based telescopic image of the Io torus, showing emissions from singly ionized sulfur. Jupiter is at the center and the Io torus is the faint ring extending to the orbit of Io at 5.9 Jupiter radii.

"Here's an example of how far we have to go," she continued. "In November, 1998 Galileo was scheduled to fly through Jupiter's magnetotail on the nightside of Jupiter, at about 90 Rj [Rj=Jupiter radii]. The center of the magnetotail contains something called the plasma sheet -- a thick layer of ionized gas that extends from Jupiter itself to beyond 150 Rj. The plasma sheet is not very massive, but it's big. At 90 Rj, it takes hours to cross through. We got ready to take the measurements, but when the spacecraft arrived the plasma sheet wasn't there! What a surprise. Imagine missing something so large."

"There was probably a coronal mass ejection that hammered Jupiter and it changed the configuration of the magnetosphere," she explained. "As it turned out the absence of the plasma sheet from where we thought it should be taught us a lot. We had a lot of missed predictions in the beginning of this mission, but Galileo is really improving our understanding of Jupiter's magnetic environment."

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Galileo's current mission is scheduled to end later this year on December 31, but depending on funding and other factors it could be extended to further study the relationship between Jupiter's magnetosphere and the Sun. Scientists would like to keep the spacecraft viable for at least another year. That's because in December 2000 the Cassini spacecraft will pass by Jupiter, providing a rare opportunity for coordinated observations of the giant planet from two spacecraft. By joining forces, Galileo and Cassini could also help researchers understand how the solar wind billows away from the sun and propagates into the outer solar system.

Below: Two sulfurous eruptions are visible on Jupiter's volcanic moon Io in this color composite Galileo image. On the left, over Io's limb, a new bluish plume rises about 86 miles above the surface of a volcanic caldera known as Pillan Patera. In the middle of the image, near the night/day shadow line, the ring shaped Prometheus plume is seen rising 45 miles above Io while casting a shadow to the right of the volcanic vent. Io is about the size of the Moon. More information.

To survive long enough for a joint rendezvous with Cassini, Galileo has to first survive the next few months. The suspense will be high in November when Galileo flies over the volcano Pillan Patera and possibly right through its active sulfurous plume. As the spacecraft flies closer and closer to Jupiter radiation levels will climb. Although the intense radiation in the vicinity of Io is strong enough to kill a human, Galileo is expected to continue operating. Nevertheless, it could be subjected to enough radiation to pepper the camera's light detector with blinding hits to many pixels, and potentially cause the computer's bits to flip in random ways, causing Galileo to enter safe mode and discontinue data taking.

"We're nervous," says Duane Bindschadler, "but not excessively so. We've had some minor radiation damage and degradation of subsystems throughout the mission. The things we begin to get concerned about now that we're approaching Io are catastrophic failures of critical subsystems. We can't absolutely predict what the radiation environment will be like and if there's an unexpected radiation storm there's not much we can do."

"Nevertheless, there are some things we can prepare for. During our last pass through the torus, for example, we saw a number of [anomalous] events in various systems on the spacecraft due to radiation. The first was a series of bus resets. Those used to be very, very serious because they would cause the spacecraft to enter safe mode, and we couldn't revive it for at least 48 hours. You lose most of the observations during an encounter when that happens. But now we've implemented a patch to the spacecraft's software to prevent unnecessary safing. That should help us out as we go deeper into the Jovian system."

With the first Io encounter just over a month away, mission scientists are excited and cautiously optimistic that Galileo will survive and complete its mission.

Left: Ron Baalke and David Seal of NASA's Jet Propulsion Laboratory have prepared several computer-generated animations of today's Callisto flyby. Click here for viewing options.

"It doesn't get much better than this," said one NASA scientist anticipating the Io images. To scientists and the public alike, October and November 1999 will be a suspenseful and exciting time for planetary exploration. Today's Callisto flyby is just the beginning.

 

Galileo has been orbiting Jupiter and its moons since December 1995. Its primary mission ended in December 1997. The spacecraft is currently near the end of a two-year extended mission that will culminate in two daring flybys of volcanoes on Io later this year. More information about the Galileo mission is available at: http://www.jpl.nasa.gov/galileo/

JPL manages Galileo for NASA' s Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology, Pasadena, CA.

Web Links

Related Stories:

Taking the Scenic Route to Io -- May 5, 1999. What's happening to the small craters on Callisto? That's the mystery scientists were contemplating as Galileo zoomed past Jupiter's pockmarked moon this morning in an orbit-changing maneuver designed to bring the spacecraft closer to volcanic Io.

Turn Left at Callisto -- May 5, 1999. Galileo heads for a daring encounter with Io's volcanoes.

Galileo buzzes Europa -- Feb. 2, 1999. Galileo executes a close flyby of Europa for the last time during the current mission.

The Frosty Plains of Europa -- Dec. 3, 1998. As Galileo returns new images of Europa, NASA scientists prepare to study samples from a potentially similar environment here on Earth.

Callisto makes a big splash -- Oct. 22, 1998. Scientists may have discovered a salty ocean and a possible ingredient for life on Jupiter's moon.

Galileo takes a close look at icy Europa -- Oct 2, 1998. The spacecraft flew within 2300 miles of the mysterious satellite last weekend.

Clues to possible life on Europa may lie buried in Antarctic ice -- Mar. 5, 1998. Exotic microbial forms turn up in ice above Antarctica's Lake Vostok.

Related Sites:

Ice, Water and Fire the Galileo Europa Mission

Galileo home page at JPL, with the latest on Europa, Callisto and Io

Jet Propulsion Laboratory home page

Io from the SEDS Nine Planets web site

Callisto from the SEDS Nine Planets web site

Jupiter from the SEDS Nine Planets web site

Io: The Prometheus Plume Aug. 18, 1997 Astronomy Picture of the Day

Close-up of an Io volcano Aug. 4, 1995 Astronomy Picture of the Day

Sizzling Io July 6, 1998 Astronomy Picture of the Day

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For more information, please contact: Dr. John M. Horack , Director of Science Communications

Author: Dr. Tony Phillips Curator: Linda Porter NASA Official: Frank Rose

Topics:

• Heliophysics