Loose Rock, Here and in Space
1In the mountain town of Copper Notch, a week of heavy spring rain loosened a slope above the highway, and a landslide covered one lane with boulders, gravel and mud. A few days later, eighth grader Thandiwe Morrell went to a talk at the town library by Dr. Joaquín Ellery, a planetary scientist who grew up in the valley. He began with the landslide. Gravity pulls every rock on a slope downhill. Dry, loose rock can rest on a slope up to about 35° steep, but on a steeper slope gravity's pull wins and the rock slides.
2Then he turned to asteroids. Pictures from spacecraft show that many asteroids are rubble piles: boulders, gravel and dust resting together with gaps between them. One of these is Bennu, an asteroid about 490 m wide. When a spacecraft touched Bennu's surface to collect a sample, its arm sank into the loose rock like a hand pushed into a ball pit. Each body also has an escape speed, the speed a rock tossed straight up needs to fly off into space and not fall back. On Bennu, that speed is only about 0.2 m/s, slower than a slow walk.
3Asteroids spin, some slowly and some fast. As a pile spins faster, the rocks at its equator need a stronger inward pull to keep them moving in their circle. Telescope surveys have measured how fast thousands of asteroids spin, and Table 1 shows the fastest spin found in each size group. Pictures and radar show that some small asteroids are single solid boulders, and the strength of the rock holds each one together.
4Gravity pulls every piece of a body toward the body's center. Dr. Ellery compared Bennu with larger bodies in Table 2. Ceres, the largest object in the asteroid belt, is round, while Bennu is shaped like a lumpy spinning top. The same force reaches much farther. It holds the planets in their orbits around the Sun and holds the Sun and billions of other stars together in the Milky Way galaxy.
5Near the center of the Milky Way, astronomers have tracked a star called S2 as it circles an unseen object once every 16 years. At its closest, S2 is about 120 AU from that object (1 AU is Earth's distance from the Sun), and it moves at about 7,700 km/s. By comparison, Mercury, the fastest planet, moves at most about 59 km/s, when it is about 0.3 AU from the Sun.