Historic landing on ill-smelling comet may help discover how planets formed

For 25 years, scientists have dreamed of probing Churyumov-Gerasimenko’s fiery celestial body – and Tel Aviv researchers are playing an important role

View of the Churyumov-Gerasimenko comet (Photo credit: ESA)
View of the Churyumov-Gerasimenko comet (Photo credit: ESA)

If things go as planned, the first-ever spaceship to land on a comet will begin taking samples of the Churyumov-Gerasimenko comet on Wednesday – and those samples will be analyzed by a team with several Israeli members. Among them is Morris Podolak, a professor of Planetary Science at Tel Aviv University, who will help analyze the findings of the Dust Impact Monitor (DIM) experiment that the the Philae lander will be carrying when it is ejected from the Rosetta spacecraft. (A live feed of the landing can be seen at this site).

In fact, the entire Rosetta project has strong Israeli roots. Among the scientists who developed the idea of building a spacecraft to fly along a comet and send out a probe to its surface was Podolak’s colleague Prof. Akiva Bar-Nun of the Department of Geosciences at Tel Aviv University. Bar-Nun, along with numerous European scientists, was a charter member of the ROSINA (Rosetta Orbiter Sensor for Ion and Neutral Analysis) Group, which designed the craft and developed the experiments it will carry out.

Among those experiments is to boldly go where no spacecraft has gone before, by landing on the comet. Rosetta was launched in 2004, and reached the comet this August, at which time it began tailing the comet’s nucleus, taking various readings. Based on those readings, scientists have determined, among other things, what a comet smells like (bad). But determining that Churyumov-Gerasimenko gives off an odor of rotten eggs and horse-stable pee is just the appetizer; the real work will begin when Philae lands and puts a stake in the head of the comet, in order to secure itself (comets have very low gravitational pulls) while it carries out numerous experiments to determine where it came from, how it was created, and what impact it has on the planets, moons, and stars it passes by on its journey through the galaxy.

Podolak, along with students and staff members, will have a chance to participate in an experiment involving the Dust Impact Monitor, which could help researchers better understand how the planets came into being.

“Comets are small but complex bodies,” Podolak told The Times of Israel. “They are probably examples of the basic building blocks, or planetesimals, from which the planets were formed. Near the sun, where temperatures were higher, these planetesimals were composed of rocky material. Further from the sun they included ices as well.” Comets, he said, “are believed to be such icy planetesimals, and we want to understand what they are composed of, how they formed, and how they behave.”

The problem, Podolak said, is that although comets are small, they are hard to corral. As they near the sun, their ice begins to evaporate and a tail develops. “On the comet surface gas jets form and spew dust and gas into space. Solar heating may cause ‘comet quakes,’” seismic activity that could indicate deformation (a reduction in size/density) is taking place. If planets were formed in a similar manner as comets, one question for researchers would be how they made the jump from comet to planet.

Morris Podolak (Photo credit: Courtesy)
Morris Podolak (Photo credit: Courtesy)

The DIM experiment will help scientists understand how the gas jets on the comet’s surface behave. The DIM “consists of a box that will measure the impact of dust grains that are thrown out by the  gas jets.  Unfortunately we will not be able to see the grains themselves; we will only feel the impact, and then the question is what can you infer about the comet from this. Together with a former student I have developed a computer code to model the interaction of grains with gas jets. We plan to try different combinations of jets and grains to see what can best reproduce the impacts the DIM instrument will measure,” Podolak said.

Whatever the DIM and other experiments find, said Bar-Nun, the results of the Philae expedition will change the way scientists see comets – and maybe the solar system. “To date, all spacecraft have passed near comets – but none of them has yet landed on one,” said Bar-Nun. “Twenty-five years ago we thought about landing on a comet, drilling down and bringing some material back to earth. The problem was that we didn’t know at the time the mechanical strength of the surface.” With that problem resolved, Philae is ready for its moment in the sun – and Bar-Nun, Podolak, and the many other scientists working on the project can’t wait to find out what it learns.

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