THE MARS PATHFINDER spacecraft arrives July 4 at its destination where, after firing retrorockets and deploying a parachute and numerous airbags, it will bounce to a halt on what was probably an ancient floodplain. After this, its missionary rover will venture forth to taste rocks and make movies. The local weather forecast calls for blue skies and wispy clouds with a small chance of a dust storm (currently 600 miles to the south), in which case the sky will be pink. In September another craft, the Mars Global Surveyor, will take up orbit around Mars. (For the latest update, view the JPL website.)