With NASA's Cassini spacecraft in its six-month flyby of Jupiter, scientists are getting a good look at the giant planet's magnetosphere (right). The Magnetospheric Imaging Instrument (MIMI) aboard the craft is providing never-before-seen images of this invisible planetary feature.

Once sequenced into a movie, MIMI's images of the particle-filled magnetosphere should enable researchers to see its compression and expansion as it is blasted by solar winds. Moreover, combined with other MIMI measurements, the images disclose the shape, dynamics and even the chemical composition of the planet's magnetosphere. "They reveal that the particles we're detecting--primarily hydrogen, but also oxygen, sulfur and sulfur dioxide--are spewed from volcanoes on the Jovian moon Io and spun out into Jupiter's magnetosphere where they are trapped, energized and accelerated to high velocities," says Stamatios Krimigis of Johns Hopkins University. "Then when collisions with other particles provide them with an electron, they become neutral and are able to escape the magnetosphere. And that's when we can detect them with our camera."

An instrument similar to MIMI is also orbiting Earth, and it recently snapped its first global shot of our planet. "By improving our ability to visualize a planet's magnetosphere--whether here or at other planets in the solar system--we are better equipped to monitor its space weather," Krimigis notes. "This will benefit science and, in the case of Earth, may lead to space weather forecasts that will give advance warning of electromagnetic storms, which in the past have disrupted communications and crippled electrical power grids."