![]() Launched in 1997, Cassini took seven years to reach Saturn, and at the time of this writing is beginning its 252nd orbit since 2004. Swooping past the planet just beyond the outer edge of the main ring system, Cassini’s trajectory comes approximately 80,000 kilometers (50,000 miles) from the top of the planet’s hazy atmosphere.Īfter 20 such “ring-grazing” orbits, the spacecraft will swing near Saturn’s largest moon, Titan, on April 22, 2017, using that moon’s gravity to further modify its orbit and begin a final series of 22 even more daring loops-an audacious phase of the mission dubbed the “ Grand Finale.” These final orbits will bring the spacecraft in closer still to Saturn, threading the 2,400-kilometer (1,500-mile) gap between the planet and the inner edge of its rings-which lies less than 6,600 kilometers (4,100 miles) from the cloudtops-and scientists anticipate receiving spectacular, closest-ever images of the rings and clouds, provided that it doesn't run into a sizable chunk of ring debris in a perilous region that no spacecraft has ever visited. NASA’s schoolbus-size, Saturn-circling flagship, Cassini, has begun its next-to-last sequence of orbits around the giant, ringed planet, taking the spacecraft high above Saturn’s north pole, then punching through its equatorial plane. “We just needed to create these models and calculate the impact speeds to connect the dots.” - Molly Michelson “In retrospect, it seems obvious that you would need something like Jupiter to stir the asteroid belt up this much,” Johnson says. With this discovery, the findings also suggest that Jupiter was near its current size and sitting somewhere near the asteroid belt when the CB chondrules were formed. ![]() "These meteorites represent the first time the Solar System felt the awesome power of Jupiter,” Johnson agrees. “The speeds generated in our models are easily fast enough to explain the vaporized iron in CB chondrites.” “When we include the Grand Tack in our model at the time the CB chondrites formed, we get a huge spike in impact velocities in the asteroid belt,” co-author Kevin Walsh says. Later, the formation of Saturn created a gravitational tug that pulled both planets back out to where they are today. That change in mass density caused the planet to migrate, moving inward toward the Sun to about where the asteroid belt is today. Then, as it accreted its thick atmosphere, it changed the distribution of mass in the gassy solar nebula surrounding it. One scenario describing Jupiter’s migration, known as the Grand Tack (a term taken from sailing), suggests that the gas giant formed somewhere in the outer Solar System. The team used computer simulations to see if Jupiter’s migration within the Solar System could increase the grains’ velocities. “You need to have an impact speed of around 20 kilometers (12 miles) per second to even begin to vaporize iron, but traditional computer models of the early Solar System only produce impact speeds of around 12 kilometers (7 miles) per second at the time when the CB chondrites were formed.” ![]() “Vaporizing iron requires really high-velocity impacts,” Johnson explains. They contain metallic grains that appear to have been condensed directly from vaporized iron. Researchers knew that these meteorites were formed as objects slammed into each other with incredible speed. Johnson and colleagues went hunting for the origins of these chondrites and published their findings today in Science Advances. “But those in the CB chondrites all date back to this brief period five million years after the first Solar System solids.” “The chondrules in other meteorites give us a range of different ages,” says Brandon Johnson of Brown University. A rare subtype of those, CB chondrites, have long fascinated astronomers because their chondrules date back to a very narrow window of time in the early Solar System. Some of that evidence comes from chondrite meteorites-the most common meteorites that descend to Earth-which contain chondrules, tiny spheres of previously molten material. Scientists study clues to this formation through neighboring bodies such as comets, asteroids, moons, and planets-and even through evidence that falls to Earth. The formation of our solar system was turbulent, to put it mildly.
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