 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
|
|
|
|
|
|
|
|
|
|
|
|
 |
|
|||||||||||
 |
 |
 |
 |
|||||||||
 |
 |
 |
 |
 |
 |
|
||||||
 |
|
|||||||||||
 |
 |
 |
 |
 |
|
|||||||
Why it’s So Important to Know Your Comet
Dr. Claudia Alexander, US Rosetta Mission
One of the delightful features of a comet as a solar system body is its complexity. A comet is a conglomerate of an icy material, perhaps covered with organics, that also evaporates (the correct technical term for what is does is 'sublimates') as it moves through space and approaches the Sun. In the process of sublimation, the comet nucleus will spew forth its gas, its dust in the form of grains of various sizes and compositions, and create a giant environment that is powerful enough to divert the solar wind flowing by, as well as create not one but two tails and other structures in the plasma that constitutes its atmosphere (also known as the coma). A comet is not a still object in space but changes over time as it goes around the Sun. The 'quiet' nucleus is probably not as quiet as we think, and when the comet 'turns on' when it closes in on the Sun and becomes it's fully active self, it is millions of kilometers in size.
The nature of a comet has baffled scientists for decades, ever since they were first observed with modern spectrometers. Models were first created of loosely conglomerated icy boulders in space to understand the source of their cohesion. Subsequently models were created of the way in which gas escapes from the surface while still leaving a relatively hard body in place. Models have been created of the chemistry that takes place in the atmosphere (coma), and how the solar wind and the interplanetary magnetic field, interact with the coma and affect the chemistry taking place in that environment. Models show that there may be a region of shocked solar wind near the nucleus itself, and that protons and solar wind electrons may interact in a complicated way to provide for chemistry in the environment. Models have been created to explain how the retreating gas affects the nature of the soil or the regolith, the crust of material on the surface.
Not all comets are the same, and nuanced physics and chemistry must be used to understand both the 'engine' that drives their conversion of energy from the Sun into the streaming gas that creates the comet tail, and the potential chemistry taking place on a body that may be left over star dust from the formation of the solar system. As decades have gone by, and models have failed to account for all the complexity of these baffling objects, modelers have used contemporary computing techniques to increase the range of space they can employ in the calculations, and are making use of 'adaptive grids' to define tighter cells for calculation for ranges closer to the nucleus. A million kilometers of space is a lot of ground to cover, even with a huge computer! Models allowed for the prediction, for example, that comet Wild 2 would be dustier than initially expected, but very few of the models accurately predicted the power and force with which the gas of that particular comet escaped the surface. Newer models account for the rough terrain that has been observed on the surface (Wild 2), the many jets that were seen, and the measured temperatures, among other things. On most of the models, there are still a lot of unkowns that prevent investigators from completely capturing with a mathematical model, exactly how a comet will behave. Some of these unknowns include exactly what fraction of the surface is icy versus dusty, or how compact the icy surface is. These and other parameters remain to be uncovered by a future mission to a comet.
Coming Soon > Learn How modeling is important to all NASA comet missions!
