Mars also has the largest dust storms in the Solar System. These can vary from a storm over a small area, to gigantic storms that cover the entire planet. They tend to occur when Mars is closest to the Sun. A Martian dust storm that NASA’s Mars Reconnaissance Orbiter has been tracking since last week has also produced atmospheric changes detectable by rovers on Mars. Using the orbiter’s Mars Color Imager, Bruce Cantor of Malin Space Science Systems, San Diego, began observing the storm on Nov. 10, and subsequently reported it to the team operating NASA’s Mars Exploration Rover Opportunity. The storm came no closer than about 837 miles (1,347 kilometers) from Opportunity, resulting in only a slight drop in atmospheric clarity over that rover, which does not have a weather station.