BOSTON, MASS.--A colorful new computer animation--created by Gary P. Zank of theBartol Research Institute at the University of Delaware--shows how even a smallcosmic cloud could suddenly burst the "breathing bubble" that protects life onour planet.
The simulation, presented May 28 during the American Geophysical Union's Springmeeting, also should help guide the spacecraft, Voyager 1 and Voyager 2, througha series of shock waves and a massive "wall" in space nearly two decades fromnow, says Zank, an associate professor at Bartol and a leading theoreticalastrophysicist.
Ongoing studies of Earth's "cocoon" might someday reveal whether closeencounters with cosmic clouds cause periodic extinctions, according to Zank, whoearned a National Science Foundation Presidential Young Investigator Award in1993 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 protectivebubble --the heliosphere around our solar system, which allows life to flourishon Earth. Unfortunately, we could bump into a small cloud at any time, and weprobably won't see it coming. Without the heliosphere, neutral hydrogen would interact with our atmosphere,possibly producing catastrophic climate changes, while our exposure to deadlycosmic radiation in the form of very high-energy cosmic rays would increase."
Zank's startling computer simulations were initially developed to support theVoyager spacecraft, deployed as part of the Voyager Interstellar Mission. Evenas the sun rolls freely through wide-open space, he explains, the Earth'sever-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 laterthan previously estimated.
Rather like a lung, the heliospheric bubble breathes, but in a highly arythmicfashion, because of an 11-year periodic cycle of solar wind properties. Bysimulating this breathing bubble, Zank says, he can predict the location of theboundary between the solar wind and the vast interstellar medium of space, whichshould help the National Aeronautics and Space Administration (NASA) prepareVoyager 1. The battery-operated vehicle is running out of power, Zank notes. Tomake the most of its instruments, NASA researchers must conserve energy, byswitching systems on and off.
Rowdy Space Clouds
Every 66 million years or so, the solar system traces a regular path through thegalaxy, 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 incrediblysmooth sailing" because the sun was lolling through an interstellar mediumcontaining 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, thesolar system is in a region of space containing between 3 and 4 particles percubic inch.
"Space," Zank notes, "is full of clouds." One particularly troublesome cloudregion, located in a star-forming region towards the Aquila Rift, clearly isheaded our way, according to Zank. Pushed by galactic wind, the cloud maycollide with Earth's protective bubble within the next 50,000 years, he says,and some researchers think we could encounter fluffier knots of gas--containing10 to 100 particles per cubic inch of space--far sooner. Our immediate or localinterstellar environment is chock-full of gas clusters known as the Local Fluff,Zank points out, and existing instruments aren't sensitive enough to detectextremely small clouds. Consequently, Zank says, "We won't know that ourheliosphere is collapsing until we see highly elevated levels of neutralhydrogen and cosmic rays, and a hydrogen wall in the vicinity of the outerplanets."
Did a rogue cloud wipe out the dinosaurs? In 1939, British cosmologist Sir FredHoyle suggested that cosmic collisions with clouds may obliterate theheliosphere every now and then. Zank agrees. "The protective solar wind would beextinguished, and cosmic radiation might lead to gene mutations," he says."Hydrogen would bombard Earth, producing increased cloud cover, leading perhapsto global warming, or extreme amounts of precipitation and ice ages. We can'tpredict 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 atnational supercomputing centers, Zank created two animations to show theheliosphere in empty space some 5 million years ago, and in a dense cloudcontaining 10 particles per cubic inch.
In clear space, the sun blows solar wind at supersonic speeds, thereby creatingthe heliosphere, which Zank describes as "a funny, bullet-shaped bubble." Whenthe interstellar medium crashes into this bubble, he explains, "it suddenlyveers upward and around, like water flowing around a rock in the river." Theresult, he says, is a systerm of massive shock waves and a hydrogen wall, whichcould 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, Zanksays. Flowing outward, shock waves push the wall and interstellar boundariesfarther into space until at last they break and wane, allowing the boundary tocontract. This shifting region between the heliosphere and its boundary mayfilter hydrogen through a process known as "charge exchange," in which neutralhydrogen and charged particles swap an electron, and so, change identities.
Earth's protective bubble seems to gasp spasmodically in a dense cloud, so thatit collapses and reforms every 331 days, Zank says. The weight of neutralhydrogen, pressing down on the lighter solar wind, "would drive great rollups ofinstability," he says. "This well-defined heliosphere structure would disappearand 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, butthey may spark a new appreciation for life. On Earth, he says, "These days, andthe last 5 to 10 millioin years, have been extremely benign, in an astrophysicalsense, and we need to make the most of them, by learning all we can about thiscocoon in which we live." Moreover, Zank says, "We can't predict our futureuntil 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 keyinformation about conditions in interstellar space, such as the speed, densityand temperature, measured by instruments on the spacecraft, Ulysses, andextrapolated from telescope data. Then, he used that information in hisequations, which were fed into computers, along with a second data setdescribing conditions closer to Earth. Zank's research was supported by theNational Science Foundation and NASA.
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