In a small lab on the second floor of a building at Tel Aviv University, Prof. Akiva Bar-Nun is making comets. Not the giant bundles of ice hurtling through space, but small-scale recreations of comet surfaces and the emissions of gas and minerals that form their tails.
Comets are massive chunks of ice carrying mineral dust, organic compounds and gases dating back to the emergence of the solar system 4.6 billion years ago. Most of them travel through space on elongated elliptic trajectories, at times nearing the sun and at times approaching the furthest reaches of the solar system.
When a comet comes close to the sun, it develops its trademark tail, made up of dust, ice and gas, stretching hundreds of thousands of kilometers behind a comet's core. The tail is created when a comet's frozen core warms up, releasing jets of gas that push out ice and minerals. These are sometimes carried further still by solar winds, millions of kilometers away.
Scientists believe that billions of years ago comets crashed into planets, helping create the earth's atmosphere and oceans. They also brought organic compounds that include carbon, hydrogen, nitrogen and oxygen, possibly contributing to the creation of life on our planet.
Bar-Nun and lab director Diana Laufer do simulations of comets' gas bursts, using ice surfaces created by two machines. Over several hours, a mix of gases and steam is fed onto a surface cooled down to 30 degrees Kelvin (minus 243 Celsius ), When the steam freezes, the gases are trapped.
The results are crispy ice sheets 0.001 millimeter to several centimeters thick. The larger machine, responsible for creating thicker ice sheets, is the only one of its kind in the world. It was built in the 1990s over 10 years to the lab's specific instructions.
In the second part of the experiment, the ice sheets are warmed. Soon "the gas breaks through the ice and throws out jets of ice dust, just as we see in comets," says Bar-Nun.
The simulations created by Bar-Nun allow scientists to predict the outcomes of NASA space missions.
"The great fun of planetary science is to be able to say while you wait for some spacecraft to reach its destination: 'Gentlemen, the spacecraft's findings will be so and so,'" Bar-Nun says. "A friend in the department told me that what I do is I stick my head under a guillotine and wait. That's the game, but it's a wonderful game."
Risky as playing with a guillotine might sound, Bar-Nun says that over the past 40 years, all six of his predictions came true. The last one happened in July 2005, when NASA's Deep Impact space probe shot a 300-kilogram piece of metal into the comet Tempel 1, hoping to artificially create the gas jets. Bar-Nun's simulations predicted both the thermal conductivity and the strength of the comet's core as measured by the spacecraft.
Bar-Nun laments that labs such as his are rare, as universities find it cheaper to use computer models to do research. But Prof. Dina Prialnik, who does just that further down the hall, explains that the two methods are complementary. "The lab builds the ice imitating the actual [ice] and measures its properties, but uses the physical equations that comprise the computer model," she said.
On Tuesday, February 15, at 6:40 A.M. Israel time, researchers will be able to test their simulations once again, when NASA's Stardust-Next spacecraft passes 240 kilometers from the Tempel 1 comet. Bar-Nun said the lab will investigate how the comet's surface has changed since 2005, and whether any changes match the lab's predictions.
In 2014, European Space Agency spacecraft Rosetta, which Bar-Nun helped plan, will try to trail the Churyumov-Gerasimenko comet and even land a small robot on its surface. "This will be a real celebration," he said.
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