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Cassini helps refine locations of Saturn's moons

Morgan Rehnberg, Correspondent
Jul 24, 2015, 8:17 UTC

Sen—Watching the Moon rise and fall each night, exactly on schedule, one could easily come to the conclusion that there is nothing more predictable than the orbits of our Solar System’s satellites. So why did NASA’s Cassini spacecraft, in orbit about Saturn for more than 11 years, take time on three days this week to help map this very information?

It turns out that the orbits of many satellites are not so easily predicted after all. The paths taken by nearly all objects in the Solar System change slowly with time; even our own Moon drifts away from the Earth about 3.8 centimeters every year.

The situation at Saturn is far more complicated than around our own planet. Saturn’s more than 60 moons each exert their own gravitational attraction on one another. Even the planet itself complicates things; the fact that Saturn bulges around its equator causes the orientation of a given moon’s orbit to rotate, something astronomers call precession.

The planet’s smaller moons are most susceptible, as the gravitational tugs from the outer, larger moons have a proportionately greater effect. That’s why, ever since entering Saturnian orbit, Cassini regularly pauses to capture so-called astrometric images. Astrometry is the study of the motions of celestial objects and it is perhaps the oldest field of scientific study in the world.

The basic principle is simple, but astrometry requires a level of precision rarely matched in astronomy. In each observation, scientists measure the precise location of every captured moon. In order to make this measurement, the exact position and orientation of Cassini must be known. Further complicating things, these small moons are often unresolved, meaning they appear on the camera’s sensor to be smaller than a single pixel. To gain maximum accuracy, models of how light falls on a given pixel are used to peer beneath this threshold.

Once a moon’s location in each image is charted, astronomers use software to fit an orbit to the collected data, essentially trying many possible orbits to see which one best represents their observations. Rarely do the final results resemble satellite tracks as we often envision them; instead, each moon takes a path around Saturn that (sometimes not so) subtly twists and turns.

Given all the effort required, why not simply solve the mathematical equations of motion to determine what path each object will take? For one, this would require perfectly precise knowledge of the mass of each object in the Saturn system. This simply is not achievable in the real world. But, even if we could know the precise masses, a much more fundamental barrier stands in the way of scientists: The so-called three-body problem.

Place two objects in space and you can (with knowledge of their masses) exactly compute the paths they will take. But add just one more body and the task becomes impossible. It is among the most infuriating facts in all of physics and, unsurprisingly, the 60-body problem is no more tractable.

That leaves us with astrometry, and so Cassini will continue making these observations over and over until the end of the mission, each one telling us just a little bit more about how the Saturn system works.

The Cassini-Huygens mission is a collaborative effort between NASA, ESA, and the Italian Space Agency. Launched in 1997, it reached Saturn in 2004 and has since been studying the planet, its moons, and its rings. In 2005, the Huygens probe made the first landing on Titan, Saturn’s largest moon. After completing its second mission extension in 2017, Cassini will make a series of close passes to the planet and then end its time at Saturn by plunging into the planet’s atmosphere.

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