The Union of Concerned Scientists yesterday unveiled its own redesign of the Ford Explorer sport utility vehicle that would get 50 percent better mileage, cut pollution by 75 percent, and cost only $715 more than the standard Explorer. The group emphasized that the improvements are possible with existing technology. UCS released its redesign in anticipation of congressional debate scheduled for later this month on revising the nation’s fuel economy standards, which currently require SUVs to average only 20.7 miles per gallon while standard cars must get 27.5 mpg.