June 2, 1998
BOSTON, MASS.--A colorful new computer animation--created by Gary P. Zank of the Bartol Research Institute at the University of Delaware--shows how even a small cosmic cloud could suddenly burst the "breathing bubble" that protects life on our planet.
The simulation, presented May 28 during the American Geophysical Union's Spring meeting, also should help guide the spacecraft, Voyager 1 and Voyager 2, through a series of shock waves and a massive "wall" in space nearly two decades from now, says Zank, an associate professor at Bartol and a leading theoretical astrophysicist.
Ongoing studies of Earth's "cocoon" might someday reveal whether close encounters with cosmic clouds cause periodic extinctions, according to Zank, who earned a National Science Foundation Presidential Young Investigator Award in 1993 and a Zeldovich Medal in 1996.
"We're surrounded by hot gas," Zank notes. "As our sun moves through extremely 'empty' or low-density interstellar space, the solar wind produces a protective bubble --the heliosphere around our solar system, which allows life to flourish on Earth. Unfortunately, we could bump into a small cloud at any time, and we probably won't see it coming. Without the heliosphere, neutral hydrogen would interact with our atmosphere, possibly producing catastrophic climate changes, while our exposure to deadly cosmic radiation in the form of very high-energy cosmic rays would increase."
Zank's startling computer simulations were initially developed to support the Voyager spacecraft, deployed as part of the Voyager Interstellar Mission. Even as the sun rolls freely through wide-open space, he explains, the Earth's ever-changing bubble generates shock waves and an enormous wall of hydrogen gas. The wall, he says, will sweep past Voyager 1 around 2015--several years later than previously estimated.
Rather like a lung, the heliospheric bubble breathes, but in a highly arythmic fashion, because of an 11-year periodic cycle of solar wind properties. By simulating this breathing bubble, Zank says, he can predict the location of the boundary between the solar wind and the vast interstellar medium of space, which should help the National Aeronautics and Space Administration (NASA) prepare Voyager 1. The battery-operated vehicle is running out of power, Zank notes. To make the most of its instruments, NASA researchers must conserve energy, by switching systems on and off.
Rowdy Space Clouds
Every 66 million years or so, the solar system traces a regular path through the galaxy, oscillating up and down as it sails through "all sorts of environments," Zank reports. Over the past 5 million years, he says, "We've had incredibly smooth sailing" because the sun was lolling through an interstellar medium containing less than one atom per cubic inch of space. That's empty space, indeed: Even wispy clouds are 100 times more dense. Currently, Zank says, the solar system is in a region of space containing between 3 and 4 particles per cubic inch.
"Space," Zank notes, "is full of clouds." One particularly troublesome cloud region, located in a star-forming region towards the Aquila Rift, clearly is headed our way, according to Zank. Pushed by galactic wind, the cloud may collide with Earth's protective bubble within the next 50,000 years, he says, and some researchers think we could encounter fluffier knots of gas--containing 10 to 100 particles per cubic inch of space--far sooner. Our immediate or local interstellar environment is chock-full of gas clusters known as the Local Fluff, Zank points out, and existing instruments aren't sensitive enough to detect extremely small clouds. Consequently, Zank says, "We won't know that our heliosphere is collapsing until we see highly elevated levels of neutral hydrogen and cosmic rays, and a hydrogen wall in the vicinity of the outer planets."
Did a rogue cloud wipe out the dinosaurs? In 1939, British cosmologist Sir Fred Hoyle suggested that cosmic collisions with clouds may obliterate the heliosphere every now and then. Zank agrees. "The protective solar wind would be extinguished, and cosmic radiation might lead to gene mutations," he says. "Hydrogen would bombard Earth, producing increased cloud cover, leading perhaps to global warming, or extreme amounts of precipitation and ice ages. We can't predict every scenario at this point."
A Bon Voyage for Voyagers 1 and 2?
Using powerful new number-crunching computers at Bartol, as well as systems at national supercomputing centers, Zank created two animations to show the heliosphere in empty space some 5 million years ago, and in a dense cloud containing 10 particles per cubic inch.
In clear space, the sun blows solar wind at supersonic speeds, thereby creating the heliosphere, which Zank describes as "a funny, bullet-shaped bubble." When the interstellar medium crashes into this bubble, he explains, "it suddenly veers upward and around, like water flowing around a rock in the river." The result, he says, is a systerm of massive shock waves and a hydrogen wall, which could be 50 times thicker than the distance between the Earth and the sun.
Undisturbed by clouds, the heliosphere appears to take a breath every 11 years, as fluctuations in solar-wind speeds produce a gentle, arhythmic motion, Zank says. Flowing outward, shock waves push the wall and interstellar boundaries farther into space until at last they break and wane, allowing the boundary to contract. This shifting region between the heliosphere and its boundary may filter hydrogen through a process known as "charge exchange," in which neutral hydrogen and charged particles swap an electron, and so, change identities.
Earth's protective bubble seems to gasp spasmodically in a dense cloud, so that it collapses and reforms every 331 days, Zank says. The weight of neutral hydrogen, pressing down on the lighter solar wind, "would drive great rollups of instability," he says. "This well-defined heliosphere structure would disappear and reappear, at times obliterating the hydrogen-filtering region."
Understanding Cosmic Evolution
Zank's colorful images aren't likely to help us avoid a cloud collision, but they may spark a new appreciation for life. On Earth, he says, "These days, and the last 5 to 10 millioin years, have been extremely benign, in an astrophysical sense, and we need to make the most of them, by learning all we can about this cocoon in which we live." Moreover, Zank says, "We can't predict our future until we understand our cosmic evolutionary history."
The new Bartol simulations were obtained by solving an extremely complicated, highly nonlinear system of coupled equations. First, Zank assembled key information about conditions in interstellar space, such as the speed, density and temperature, measured by instruments on the spacecraft, Ulysses, and extrapolated from telescope data. Then, he used that information in his equations, which were fed into computers, along with a second data set describing conditions closer to Earth. Zank's research was supported by the National Science Foundation and NASA.
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