U.S. Robotics compares the speed of its new x2 56K modems to that of a rocket in a new spot featuring Sally Ride, whose 1983 space shuttle flight made her the first U.S. woman astronaut in space. "Who says space is the final front ier?" Ms. Ride asks. The spot from Leap Partnership, Chicago, breaks today on network, cable and spot TV. It's the third U.S. Robotics commercial this year to feature celebrity icons of scientific or technological achievement, fol lowing spots starring physicist Stephen Hawking and Apple Computer co-founder Steve Wozniak.