After a seven-year journey half-way across our solar system, a spacecraft partly built at the University of Iowa will finally reach its destination tonight. The one-point-four billion-dollar Cassini will spend four years orbiting Saturn, studying many facets of the mysterious ringed gas giant. U-of-I space physicist Don Gurnett is at NASA’s Jet Propulsion Lab in California all week to monitor progress. The Iowa-built instrument onboard studies radioactive emissions and waves in the ionized gas that surrounds the planet called plasma. Gurnett is one of the key minds in helping unlock a newly revealed puzzle about Saturn — its rotation has slowed. Saturn is a huge ball of gas, covered with clouds, some of which are moving at a thousand miles an hour. It’s impossible to tell the rotation by looking at Saturn since it has no surface like Earth. Still, scientists have determined Saturn’s day has lengthened in the past two decades by about six minutes a day. It takes about 11 hours for Saturn to spin all the way around, compared to 24 hours on Earth. Gurnett is also anxious to get a closer look at Titan, one of Saturn’s 31-known moons. Titan’s atmosphere is nitrogen-based, like Earth’s, but it’s very cold (178 degrees below zero) and Gurnett says it’s essentially raining propane and ethane. Gurnett says it’s ironic that there’s plenty of fuel on Titan but there’s no oxygen at all, so none of it can be burned. “It’s too bad we can bring all those (gases) back to Earth somehow. We’d solve our energy problem.” Titan is about the same size as our solar system’s smallest planet, Mercury. Cassini has traveled two-point-two billion miles.