STRANGE HALO ORBITS EXPECTED AT SATURN. Consider particles in orbit above a planet. If the particles are uncharged or have a very low charge-to-mass ratio, they will follow a conventional ("Keplerian") trajectory centered about the axis of the planet at the equator (Saturn's rings are an example of such particles). If, however, the particles are highly charged, their motions are dominated by an electromagnetic interaction with the planet's magnetic dipole (Earth's van Allen belts are an example). If the charge is somewhere in between these two cases, and gravity and electromagnetic forces are comparable, then strange orbits are possible.

Scientists at the University of Colorado (Mihaly Horanyi and Jim Howard, 303-492-6903) and Loughborough University (Holger Dullin) in the UK estimate that if conditions are just right some particles could race around a planet in orbits (stable for as long as 10 years) that never cross the planet's equatorial plane (see figure at Physics News Graphics). The dust analyzer on the Cassini craft now gliding toward Saturn might be able to detect particles in these novel orbits. (Howard et al., Physical Review Letters 10 April /pnu/2000/;Select Article.)