Bruised by the recent loss of the $165 million Mars Polar Lander, NASA is studying a new class of smaller, cheaper and more robust spacecraft. These probes could land on Mars to reconnoiter terrain that larger missions could later study in detail. Called appropriately enough “Scouts,” two of the 220-pound (100-kilogram) spacecraft could venture to Mars as soon as 2003, said Barry Goldstein, who is leading the study at NASA