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After Dione flyby, Cassini gets back on course

Morgan Rehnberg, Correspondent
Aug 28, 2015, 14:37 UTC

Sen—For more than the last eleven years, NASA’s Cassini spacecraft has executed an intricate dance through perhaps the most complex planetary environment in the Solar System. And like any dancer, over time tiny missteps lead the ship to drift ever so slightly off course. So, in the aftermath of the recent flyby of Saturn’s moon Dione, Cassini took the opportunity to get back on track.

These tiny course corrections are commonplace; this adjustment was the mission’s 419th opportunity for a so-called orbit trim maneuver (OTM).

But if Cassini is silently gliding through empty space, what causes the bumps in the road that require such regular correction? The answer lies in gravity. Einstein’s general theory of relativity tells us that every object warps the space around it proportionally to its mass. In addition to the Sun and Saturn, the planet’s more than sixty moons each put their own little ripples into the fabric of space that Cassini must traverse.

Since we don’t have perfect knowledge of the mass or position of any object in the Solar System, over time the spacecraft accumulates little misdirections from unexpected “bumps” in its orbit. The size of these errors is miniscule: after approaching the relatively-large Dione, Cassini required a velocity correction of just 56 millimeters per second to put it back on course. Tiny thrusters mounted around the spacecraft provided the boost.

It might not seem like very much, but OTM-417, which helped target the Dione flyby three weeks ago, added just 18 mm/s of velocity, yet moved Cassini’s flyby position by about 30 kilometers.

TheCassini-Huygens missionis 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.