A NASA spacecraft is preparing to land on an asteroid Tuesday to collect a sample. Spacecraft OSIRIS-REX is descending toward an asteroid named Bennu to suck up rubble as a sample for closer study back on Earth. OSIRIS-REX descended Tuesday toward the surface of the asteroid, which is 200 million miles away. The spacecraft dropped out of orbit around asteroid Bennu right on time, beginning a 4 1/2-hour plunge to the rough, boulder-covered face of the ancient space rock. Bennu’s gravity was too low for the spacecraft to land — the asteroid is just 1,670 feet (510 meters) across.