Comet Eruptions Predicted by New Computer Model
This is a look at the comet Tempel 1 through a telescope. The active regions are responsible for the bright jets (left). With the help of their computer simulation the MPS-scientists can reconstruct the image seen from Earth (right).
Credit: Instituto de Astrofisica de Andalucia (Luisa Maria Lara)/Max Planck Institute for Solar System Research

A new computer model could help scientists locate active regions in comets where particles of dust spew off the surface of the comet's rock-like nucleus.

Traditionally, it has been difficult and dangerous to study comets ? particularly up close ? because the dust outbursts have the potential to damage space probes.

The new computer model, developed by scientists from the Max Planck Institute for Solar System Research (MPS) in Germany, will help researchers locate these active regions using only information available from Earth. This new method of studying comets from afar could help scientists calculate a safe flight route for the European Space Agency's space probe Rosetta, which is scheduled to arrive at the comet Churyumov-Gerasimenko in 2014.

A comet's nucleus is far more complex than an unvarying chunk of ice and dust. Under the sun's influence, volatile substances such as water, carbon dioxide and carbon monoxide are emitted from certain regions on the nucleus' surface ? the so-called active regions.

These active regions carry dust particles into space, and seen from Earth, the dust formations look like jets or spiral arms that surround the comet. These structures are normally hidden behind a veil of gas and dust called the coma.

"Pictures taken from Earth show the comet and its jets as a two-dimensional projection," said Hermann B?hnhardt from MPS.

But, without more detailed pictures or models, it is difficult for scientists to tell where the dust particles and gases originate.

To compensate, MPS researchers tried to localize a comet's active regions using an indirect approach that for the first time also accounts for the three dimensional shape of the comet.

"Until now, computer programs trying to find the active regions assumed the comet as a sphere or ellipsoid," explained Jean-Baptiste Vincent from MPS.

These assumptions are often insufficient, since in reality, comets can have unusual shapes. So, the researchers decided to take a different approach: by watching a comet for an entire rotation period, they use changes in its brightness to calculate its true form.

Next, the researchers feed the program an initial assumption of where the active regions might be located on the comet, along with estimates of the physical properties of the dust particles, including size and initial velocity upon emission from the comet's nucleus.

The resulting computer simulation delivers an image as it would be seen through a telescope on Earth. By comparing this with the actual image through a telescope, the model can be further refined step by step until the simulation and the actual image are congruent.

This new method has already passed its first test: The scientists were able to successfully apply it to the comet Tempel 1, the destination of NASA's Deep Impact Mission in 2005.

"Even though ever since this mission we know where Tempel 1's active regions are, we pretended not to," Vincent explained.

Using their computer program, the scientists only applied information that was available from Earth-based observations ? except for the nucleus shape model that was adopted from the Deep Impact mission's results. The method was successfully able to produce a simulated image showing Tempel 1's active regions that matched what had been seen through a telescope.?

Moving forward, the researchers plan to calculate the active regions of the comet Churyumov-Gerasimenko, the planned rendezvous target for the Rosetta mission. The Rosetta lander Philae is expected to touch down on the comet in late 2014.

The Rosetta mission has been en route to its destination beyond the orbit of Mars and the asteroid belt since launching in 2004. At crucial junctures of the mission, this new method could help determine a safe route for the Rosetta probe through the cometary coma, and could even assist in finding a suitable landing site.

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