With the Steins encounter looming, let's keep the focus on the asteroid belt, in this case by examining a connection between that distant region and our own planet. Cosmic dust particles -- tiny bits of pulverized rock up to a tenth of a millimeter in size -- move continuously through the Solar System, a kind of micro-thin fog of micrometeorites that contributes hundreds of billions of particles to Earth's atmosphere. New research into the makeup of some 600 of these particles now reveals their chemical and mineral content, allowing an overview that points to their origin. The suspected source: A group of asteroids between Mars and Jupiter. You can see one of the Koronis asteroids in the image at left, which shows 243 Ida as photographed by the Galileo probe. What we now believe about the Koronis asteroids is that they were formed some two billion years ago by the breakup of a much larger asteroid. Within the Koronis family are the ninety or so Karin asteroids, which seem to be in a...

Get out to about 2.4 AU from the Sun (2.41 AU, to be precise) and your radio signals have a long travel time. It takes 20 minutes to cross the 360 million kilometers between Earth and the Rosetta spacecraft, and that, of course, is one-way. As we've learned from all our deep space missions, spacecraft are largely on their own for the brief and critical window of an encounter, like the one with asteroid Steins that is coming up for Rosetta. Opportunities for possible trajectory correction maneuvers exist both on September 4 and 5th, but it's on the 4th that Rosetta's controllers will have their last chance to acquire optical images for navigation. Uplink commands for asteroid fly-by mode will be sent on the morning of the 5th and then we wait for results as the vehicle flips for observation and tracking. Rosetta will close to within 800 kilometers of the asteroid, passing it at a speed (relative to Steins) of 8.6 kilometers per second. Image: The approach of Rosetta's spacecraft to...