After a journey of seven months and 303 million miles, a six-wheeled NASA rover will speed toward the surface of Mars tonight and, if all goes as planned, land with a bounce. The plunge through the Martian atmosphere at 12,300 mph will mark the start of the riskiest portion of the voyage. As the unmanned spacecraft Spirit plummets to the rocky surface 80 miles below, it will rely on the precisely choreographed use of heat shields, parachutes and rockets to slow its descent. Just eight seconds before hitting the ground, the golf-cart-size Spirit is to inflate a set of air bags to cushion its impact.