NASA engineers are designing a tough, oversized beach ball that could someday roll over broad, rocky expanses of Mars. After bouncing to a landing, the ball would skid and roll around the Red Planet, propelled by Martian winds. The ball could accelerate to about 22 mph in 45 mile-an-hour gusts. As envisioned, the craft, dubbed Tumbleweed, measures about two stories high or 20 feet in diameter and weighs a mere 44 pounds. Like the big wheels on jacked-up trucks, the ball is designed to keep on rolling.